School Science Lessons
Electronics 2 Experiments
2009-05-18
Please send comments to: J.Elfick@uq.edu.au
See:
UNPh39 UNESCO Electronics Teacher's Guide, School
Science Lessons
See: Electronics
1 Experiments
38.5.00 Switching circuits
Acknowledgement: Topic 38.500 Switching circuits topics were
edited by Mr Brian Clarke.
Please send comments to:
brianclarke01@optusnet.com.au
Manually operated switches
38.5.001 Switching
off
38.5.002 Switches in a motor vehicle ignition
system
38.5.002.2 Coil ignition system in motor
vehicles, "points", capacitor
across points
38.5.002.3 Effect
of switching an AC circuit
38.5.002.4 Coil ignition system in motor
vehicles, high voltage at the
opening of the contacts
38.5.002.5
Switching a car headlight bulb
38.5.003 Switches in motor vehicle ignition
system
38.5.003.1 Switches in series
38.5.003.2 Switches in parallel
38.5.01 Relays, magnetically operated
switches, "make-and-break"
38.5.02 Latching relay
circuit, bistable \
flip-flop
38.5.03 Reed switch, reed relay,
"make-and-break"
38.5.1 Heat-operated
switching circuit, fire alarm
38.5.2 Light operated switching circuit, light
dark indicator, lamp on in the dark, automatic street light
38.5.3 Moisture
detector, water indicator
38.5.4 Sound operated
switching circuit with
latching, crystal microphone, burglar alarm
38.5.4.1 High-speed
flash photography
38.5.5 Transistor
amplifier, with 1. magnetic
earphone, 2.crystal earphone
38.5.6 Time operated switching circuit
38.5.7 Flashing circuit
38.5.8 Automatic
lighting control, front steps
light
38.5.9 Time delay
38.5.9.1 Seat belt warning
39.5.9.2 Headlights
ON warning
38.5.9.3 Traffic lights
38.5.9.4 Two-tone
police siren
38.5.9.5 Room air
conditioning
38.6.00
Radio waves
38.6.01 Frequency modulated, FM,
amplitude
modulated, AM, signal
38.7.00 Computer, processor, microprocessor, central processing Unit
(CPU), peripherals, mainframe
38.7.00 PC DRAM chip, microprocessor, hard disk drive, floppy disk
drive
38.7.01.1 Logic gates
38.7.01 Computer program, integrated circuit, computer applications,
databases,
38.7.01 Hardware, disk, keyboard, peripheral devices, mouse, printer,
plotter
38.7.01 Memory unit RAM and ROM memory
38.7.01 Monitor, visual Display Unit (VDU)
38.7.03.1 Bits and bytes, kilobyte,
K,
megabyte, MB, binary counter,
38.7.03.2 Decimal and binary codes
for numbers
0 to 25
38.7.03.3 Addition of binary
numbers
38.7.03.4 ASCII (American Standard
Code for
Information Exchange)
38.8.04
Light pen
38.9.0 Modern physics
38.5.00 Switching circuits
Acknowledgement: Topic 38.500 Switching circuits topics were
edited by Mr Brian Clarke. Please send comments to:
brianclarke01@optusnet.com.au
Manually operated switches
See diagram 39.2.1: SPST and SPDT reed
switches
Switches stop and start flow of current through a circuit. The switch
is "off" when the switch terminals are not connected and "on" when a
conducting part of the switch connects the terminals. When a perfect
switch is "on", current flows, when "off" no current flows. The Morse
code key is a switch controlling the current flowing to a circuit. You
usually name a mechanical switch by its "poles" and "throws", e.g.
Single Pole Single Throw, SPST, and Single Pole Double Throw, SPDT. The
"pole" is the moving arm or lever. The "throw" is the terminal to which
the pole can connect or be "thrown". A Morse Code key is usually a SPST
switch. Multipole switches have many moving arms and terminals selected
by the position of the moving switch arm. So, a 6-PDT switch has six
poles each of which can switch between two, i.e. double, terminals,
such a switch will have 18 terminals in total, 6 for the poles and 12
for the throws. A push button is a SPST switch that has a return spring
so that when you lift your finger the switch returns to the open
position. This may be identified as an "SPST mom", meaning momentary.
Some Double Throw, called DT, switches have a central rest position
where the moving arm makes no contact. You call this a Centre-Off, or
CO, switch.
38.5.001 Switching off
38.5.001.1 Switching off a d.c. circuit connecting only resistance is
easy. Connect a car headlamp bulb in series with an open contact
switch, e.g. a knife switch, and a small car battery. Use a proper
socket for the bulb to avoid shorting the battery. Observe the very
small spark as the switch contacts open.
38.5.001.2 Switching off a d.c. circuit connecting an inductance is not
easy. The current tends to continue flowing as a high voltage builds up
at the switch contacts. You can see this as an arc at the contacts as
you open the switch.
38.5.002 Switches in a motor vehicle ignition
system
38.5.002.1 Coil ignition system in motor vehicles, "points"
See diagram 38.5.00a: Spark coil without
capacitor across points
The coil ignition system in motor vehicles relies on this high voltage
at the opening of the contacts, sometimes called "points", in the
distributor. Connect a spark coil in series with a car battery and an
open contact switch. Make sure that the secondary of the spark coil is
shorted, or connected to a spark plug with its body connected to the
battery or to one of the low voltage terminals on the coil. See a small
spark at the switch contacts when you open the switch contacts and a
small spark at the spark plug.
38.5.002.2 Coil ignition system in motor
vehicles, "points", capacitor
across points
See diagram 38.5.00b: Spark coil with
capacitor across points
Connect a 1 uF 250 V capacitor across the switch terminals in the
previous experiment. When you open the switch, you will see less
sparking at the switch, but a much bigger spark at the spark plug
because the capacitor allows much bigger build-up of voltage at the
switch contacts and much less loss of energy at the switch contacts.
Arcing of switch contacts will cause interference that you can hear as
a crackling sound on a radio. It will also erode the switch contacts
that will fail eventually. The capacitor across the distributor points
in a car prolongs the life of the points. Be careful. There will be
quite a high Voltage at the switch contacts as they open, make sure
your fingers do not touch the switch parts or the wires attached to
them. (You can use a CRO to show the difference in voltage across the
coil primary and across the switch contacts with and without the
capacitor. With no capacitor across the switch contacts, the limiting
seen on the CRO is the breakdown ionization of the air. To see it on a
CRO, use Horiz Magnify to see the short duration, high voltage pulse.
Be careful! Do not try to use the CRO on the secondary of the spark
coil!) Modern cars have capacitors across most d.c. switches to reduce
wear of the switch contacts and to reduce interference to other
electronic devices in the car, such as the radio, or worse, to the
engine management computer.
38.5.002.3 Effect of switching an AC circuit
Use a low voltage step down transformer, at least 60 W, and repeat the
experiment with a car headlamp bulb, with the switch in the secondary
side. In an AC circuit containing inductance, the rapidly changing
polarity of the AC makes it difficult to open the switch at exactly the
right moment to get either no spark or a big spark at the switch
contacts. However, you can move the switch arm very gently to a
position where sustained sparking at the switch contacts occurs.
Putting a capacitor across the contacts may reduce the sparking, but
the switch cannot actually turn the circuit off, because the capacitor
will appear to continue to carry AC current. Repeat the experiment with
a choke in series with an incandescent lamp. You can see dimming and
hear the arc buzzing. Disconnect from the AC and then look at the
blackened contacts. The choke can be likened to the wiring in a
household. So, you use snap action switches in AC circuits to avoid
contact erosion and maintenance. A suitable combination of capacitor
across the switch and circuit inductance could result in resonance.
Think about what happens to the current flowing in a series resonant
circuit. At resonance, current flow in the capacitor and inductance is
at a maximum and much larger than the current flowing in the rest of
the circuit. Better quality equipment use DPST AC mains isolating
switches. Think about what happens with a SPST switch in circuit if the
active (or live, or phase) and neutral wires are swapped around, this
happens quite frequently when untrained people put power plugs on AC
equipment or even on extension leads. Switch contact materials are
often made of copper, silver, gold or platinum because of such factors
as whether machinable, proneness to erosion, ease of oxidation,
conductivity of oxide, cost, and the required life of the equipment
38.5.002.4 Coil ignition system in motor
vehicles, high voltage at the
opening of the contacts
See diagram 32.5.2.2: Ignition system
The coil ignition system in motor vehicles relies on this high voltage
at the opening of the contacts, points, in the distributor. Connect a
spark coil in series with a car battery and an open contact switch.
Make sure that the secondary of the spark coil is shorted, connected,
to a spark plug with its body connected to the battery or to one of the
low voltage terminals on the coil. See a small spark when you open the
switch contacts and a spark at the spark plug. Connect a 1 uF 250 V
capacitor across the switch terminals. When you open the switch, you
will see less sparking at the switch, but a much bigger spark at the
spark plug because the capacitor allows much bigger build-up of voltage
at the switch contacts and much less loss of energy at the switch
contacts. Arcing of switch contacts will cause interference that you
can hear as a crackling sound on a radio. It will also erode the switch
contacts that will fail eventually. The capacitor across the
distributor points in a car prolongs the life of the points. Modern
cars have capacitors across most d.c. switches because switching off an
AC circuit connecting resistance only, is easy. (Comment: You can use a
CRO to show the difference in voltage across the coil primary and
across the switch contacts with and without the capacitor. With no
capacitor across the switch contacts, the limiting seen on the CRO is
the breakdown ionization of the air. Be careful! Do not try to use the
CRO on the secondary of the spark coil.)
38.5.002.5 Switching a car headlight bulb
Use a low voltage step-down transformer, at least 60 W, and repeat the
experiment with a car headlamp bulb, switch in the secondary side. In
an AC circuit containing inductance, the rapidly changing polarity of
the AC makes it difficult to open the switch at exactly the right
moment to get either no spark or a big spark at the switch contacts.
However, you can move the switch arm very gently to a position where
sustained sparking at the switch contacts occurs. Putting a capacitor
across the contacts may reduce the sparking, but the switch on actually
turns the circuit off, the capacitor will appear to continue to carry
AC current. Repeat the experiment with a choke in series with an
incandescent lamp. You can see dimming and hear the arc buzzing.
Disconnect from the AC and then look at the blackened contacts. The
choke can be likened to the wiring in a household. Use snap action
switches in AC circuits to avoid erosion and maintenance, A suitable
combination of capacitor across the switch and circuit inductance could
result in resonance. Think about what happens to the current flowing in
a series resonant circuit, Better quality equipment use DPST AC mains
isolating switches, why?, what happens with a SPST switch in circuit if
the active, live, phase, and neutral are swapped around?, Switch
contact materials are often made of copper, silver, gold or platinum
because of such factors as whether machinable, proneness to erosion,
ease of oxidation, conductivity of oxide.
38.5.003 Switches in motor vehicle ignition system
38.5.003.1 Switches in series
See diagram 38.5.01: Multiway light
switching, local switch and remote switch
When switches are in series, current only flows if both switches are
on. Use these circuits for isolation and safety purposes. For example,
the main switch on your mains power distribution board is in series
with all the power and lighting circuits in your house. You can use
several switches in series to guide current to a circuit. Burglar alarm
circuits may use several switches in series, and all turned on when
there is no danger. If a burglar opens a door or window that has one of
these on switches that switch turns off and signals an alarm. This
method of wiring several switches in series uses less wire than if you
wired all switches in parallel. The binary logic diagram for series
switches is a form of AND gate, for two switches in series, both
switches A and B need to be on for the circuit to be on. You can also
see it as a form of OR gate, no current can flow if either switch A or
B is open.
| Switch A |
Switch B |
Outcome |
| OFF |
OFF |
OFF |
| OFF |
ON |
OFF |
| ON |
OFF |
OFF |
| ON |
ON |
ON |
The shorthand form of truth table found in binary logic texts uses "1"
for ON and "0" for OFF:
| A |
B |
O / P |
| 0 |
0 |
0 |
| 0 |
1 |
0 |
| 1 |
0 |
0 |
| 1 |
1 |
1 |
This is the truth table for an AND gate.
38.5.003.2 Switches in parallel
See diagram 38.5.01g: Motor reversing
When switches are in parallel, current flows if any switch is on. Such
circuits can be used for signalling danger when there are several risky
events that need the power to be switched on. For instance, water
sensing switches could be placed in the bathroom and under the house to
alert the owner to possible flooding danger, or to turn on a pump.
Burglar alarms can use several switches in parallel, all turned off
when there is no danger. If a door or window that has one of these off
switches is opened by an uninvited guest, that switch turns on and
signals an alarm. This method of wiring several switches in parallel
uses more wire than if all switches were wired in series. By using
several alarm circuits, some in series and some in parallel, a burglar
cannot know in advance whether to short out or open a circuit to avoid
setting off an alarm. The binary logic diagram for parallel switches is
a form of OR gate, for two switches in parallel, either switch A or B
needs to be on for the circuit to be on. It can also be seen as a form
of AND gate, no current can flow if both switches A and B are opened.
| Switch A |
Switch B |
Outcome |
| OFF |
OFF |
OFF |
| OFF |
ON |
ON |
| ON |
OFF |
ON |
| ON |
ON |
ON |
Draw the shorthand truth table as in 38.5.003.1. When you get to
transistor based switching circuits, you will see that the output of a
transistor switch can be changed by either an ON or an OFF switching
event.
38.5.01 Relays, magnetically operated
switches, "make-and-break"
See diagram 38.5.02 | See 39.2.4: Reed relay to control a
motor | See diagram 39.2.7: Reversing
an electric motor
The relay is a switch operated by an electromagnet instead of by hand
to allow small currents to switch large currents with electrical
isolation between a low voltage and a high voltage circuit and to
improve safety. You can operate the switch from far away. A relay has a
coil energized by the low voltage circuit and switch contacts connected
to the high voltage circuit. When the relay is off, the arm is at its
rest position and the normally closed (N.C.) switch rests in contact
with the common switch contact. When a current passes through the coil,
the resulting magnetic field attracts the arm so the normally open
(N.O.) switch contact is in contact with the common switch contact.
Double pole double throw, DPDT, relays are common for controlling
several circuits with one relay. You can use one relay to control the
direction of a motor. When a relay operates you can hear a click as the
relay switches on and off and see the contacts moving. Relay parts can
wear out as the switch contacts become dirty and high voltages and
currents cause sparks between the contacts. Relays have a slow response
and the switch contacts can rapidly wear out due to the sparking. You
should place a diode across a relay coil to avoid the back emf created
when the relay coil switches off. The back emf can damage components.
If using a relay to switch a 60 W lamp, the switch contacts must be
rated for 250 mA at 240 V AC. You use gold switch contacts for low
voltages and tungsten for switching high voltages. The relay must have
a coil that can be energized by the low voltage control circuit. The
circuit powering the coil must supply enough current for the relay to
operate. Coil resistance means the range of voltage for the relay to
operate. Contact rating means current and voltage that the contacts can
switch. The number and type of contacts means whether relays have a
single switching action or more, with single throw or double throw and
number of poles, with single or double throw action. You call a single
pole double throw contact a changeover contact. For more contacts use
one relay to switch another.
38.5.02 Latching relay circuit, bistable \
flip-flop
If a relay becomes "latched" on when pressing the Turn on button
energizes the coil, the only way to turn the relay off will then be to
cut the power supply by pressing the push-to-break Reset button. You
call this type of circuit "bistable" because the circuit has two stable
states for its output, on and off. You connect the normally open
switch contact of the relay to a motor that will then run indefinitely
until the Reset button is pressed turning off the coil for the trigger
button to be pressed again. The reset button can be pressed
automatically to cut the power to the relay coil after the model has
been running for a certain time. Make a relay latch with a multicontact
relay so that one set of contacts bypasses the switching transistor
keeping current flowing through the coil although the transistor stops
conducting. When it pulls in, it stays in no matter what happens in the
rest of the circuit. The only way to make the relay drop out again is
to disconnect power.
38.5.02.1 See diagram 38.5.02a
The relay will become latched on when pressing the Turn on button
energizes the coil. This is an "SPST NO mom" switch. The only way to
turn the relay off is to cut the power supply by pressing the
push-to-break Reset or Stop button, this is an "SPSTNC mom" switch.
Connect the normally open switch contact of the relay to a motor that
will run indefinitely until the Reset button is pressed turning off the
coil, waiting for the Turn on button to be pressed again. You can
operate the Reset button automatically to cut the power to the relay
coil after the motor has been running for a certain time. This is
called a latching relay circuit bistable because the circuit has two
stable states for its output, on and off. In binary logic terms,
you
can see a latching relay as a "flip-flop".
38.5.02.2 See diagram: 38.5 02b
Make a relay "latch" with a multipole relay so that one set of contacts
bypasses the switching transistor keeping current flowing through the
coil although the transistor stops conducting. When the armature pulls
in, it stays in no matter what happens in the rest of the circuit. The
only ways to make the relay drop out again are to disconnect power to,
or to short out, the coil.
38.5.02.3 See diagram: 38.5.02c
When contacts "make and "break", they wipe over each other. This
usually clears any oxidation caused by arcing from the previous
turn-off. If the return spring becomes weak or the control current is
low, the speed of "make" may be slow, and the wiping less effective.
So, some motor starter relays have two windings, a low resistance one
for a rapid "make" which then switches over to a high resistance
"holding" coil to latch the NO contacts on. For motors running for a
long time, this high resistance coil runs at a much lower temperature
than the low resistance one would have, thus reducing risk of damage to
the coil and switching gear.
38.5.02.4 See diagram: 38.5.02d: Kill
switch
with safety RCD
Safety consideration: A good application for a latching relay is as a
"kill" switch where inexperienced people may have access to dangerous
voltages, e.g. an electrical experimentation class. At the beginning of
the year, the instructor shows the students where the "kill" switch is
and show its operation. At the start of a class, the instructor pushes
a Start button to energize all the mains circuits in the room. Under
normal circumstances, at the end of the class, the instructor pushes
the Reset button to de-energize all the mains circuits. In case of an
emergency, e.g., a student gets an electric shock, anyone can run to
the kill switch and press the Reset button. Turn off the mains BEFORE
trying to touch or move a victim. A further refinement is to use a
residual current device, RCD, or an earth leakage circuit breaker,
ELCB, as the trigger for operating the Reset button.
38.5.03 Reed switch, reed relay, "make-and-break"
See diagram 38.5.03 | See 39.2.2: Reed switch and magnet,
39.2.3 Reed switch and coil, reed relay
A reed switch has flexible metal reeds with contacts at the tips in a
sealed glass tube. The 2 reed switch has normally open (NO) contacts
which close when the switch operates. The 3 reed type has a wiper, a NO
contact and a normally closed (NC) contact, thus achieving an NO and an
NC pair. When the switch is operated both these pairs change to the
opposite state. Reed switches are actuated by the magnetic field from a
nearby magnet or an electromagnet. The magnetic field magnetizes the
metal reeds so that their ends attract, or repel, as in the 3 reed
case, each other and the contacts open or close. When the magnetic
field is removed the reeds spring back to restore the contacts to their
rest state. As reed switches have only a momentary action a latching
relay circuit is needed to keep the circuit on. Reed switches are
small, easy to install, cheap and easily obtainable. However, the
contacts and reeds are small and delicate and so cannot handle large
voltages or inductive circuits which may cause the reeds to arc when
switched. If asked to carry heavy currents the reeds may heat up and
lose their springiness. Because the mass of the reed is very small,
reed relay switches can be quite a bit quicker than normal relays. Reed
switches can be built into the fixed parts of door and window
furniture, the jambs and casements, and magnets attached to the moving
parts. These can be the basis of a security system, e.g., counting the
number of entries to a secure building or setting off an alarm.
38.5.1 Heat-operated switching circuit, fire alarm
Semiconductors: The main semiconductors used in this section are diodes
and bipolar junction transistors, BJT
Diodes: Diodes are used for protection, steering or illumination.
Protection and steering diodes can be 1N914, 1N4148, OK up to about 75
mA, 1N4001, 1N4002, OK up to about 1 A, 1N5400 series, OK up to about 3
A. These are silicon and are fairly robust and cheap. Germanium diodes
are fairly fragile and often more expensive, now. Diodes for
illumination are LEDs, almost any type numbers are suitable. All are
silicon and have about the same forward Volt drop.
Transistors: Most circuits in this section use NPN BJTs because early
transistors were made from germanium and were mostly PNP. Germanium
transistors are quite heat sensitive and can easily be damaged by
excessive heat including soldering leads. After a while these
transistors become quite leaky and often the amplification of base
current is lost in the high ICE leakage current. More modern
transistors are made from silicon because it is easier to make NPN
transistors in silicon. They are not so heat sensitive and do not
develop the same leakage and ageing effects as germanium transistors.
Suitable BJTs for most of these demonstrations are the following signal
types: BC107, BC108, BC109, BC182, BC183, BC184, BC185, BC190, BC237,
BC238, BC239, BC547, BC548, BC549, BC550, BC583, BC584 2N160, 2N332 to
2N338, 2N470 to 2N480, 2N 745 to 2N754, 2N 789 TO 2N793, 2N1267 to 2N
1272, 2N1386 to 2N1390, 2N2221, 2N2222 2SC15 to 2SC18, 2SC25, 2SC26,
2SC28, 2SC29, 2SC56, 2SC103, 2SC104, 2SC105, 2SC115, 2SC120 to 2SC124.
Check with local electronics manufacturers for end-of-run components.
Even unmarked ones or ones with specific in-house markings can easily
be identified.
38.5.1.1 See diagram 38.5.1a: Alternate
back
emf protection
To protect the bipolar junction transistor, BJT, a protection diode in
reverse bias is wired in parallel with the coil. When a high back emf
is induced in the coil it can be clamped at the forward bias voltage,
~0.6V if silicon, of this diode. The same protection can be achieved by
connecting the emitter of an NPN control BJT to the control coil, here
the base emitter diode withstands the coil back emf, in this case, the
coil still develops the full back emf. Check the reverse Vce
specification of the BJT, or experiment by measuring the collector
current through a reverse biased BJT as the reverse bias is increased,
you may destroy one BJT, so, use this as a demonstration rather than as
a class experiment. Such use of a transistor as both control and
protection reduces assembly time and cost of components.
1. Connect the bell (or a lamp) to the normally open, NO, contacts on
the relay.
2. Adjust the variable resistance to just stop the bell
ringing, or just extinguish the lamp.
3. Heat the negative temperature
coefficient (NTC) thermistor with a very small flame or focus sunlight
on it with a magnifying glass.
4. When a thermistor is heated it has a
lower resistance, so more current flows through the BJT base,
increasing the collector current and the bell rings.
38.5.1.2 See diagram 38.5.1b: Bridge fire
alarm
This is the basis for fire alarm systems. The thermistor is placed at
the highest point of the room, or in the main air exhaust duct in the
ceiling, as hot air rises, the ceiling will be the hottest part of the
room, normally. Because of the variability of NTC thermistors even from
the same batch, and the variable ageing effect, the thermistor is
usually set in a mounting receptacle, along with a small heating
element to keep the thermistor at a roughly constant temperature, above
normal room temperature, this improves the response time. The
thermistor is connected into a, Wheatstone, bridge circuit where the
other three arms are kept at a constant temperature, one arm is
variable to set the trigger point. Now, any change in the resistance of
the thermistor can be detected quite quickly and reliably and the
bridge circuit is also quite sensitive to small changes. If a fire
starts, the heated air rises even more rapidly to the ceiling or passes
through the exhaust duct, changing the resistance of the thermistor,
setting off the alarm. Now go back to the transistor, relay, variable
resistance, thermistor demonstration.
1. Swap the variable resistor
and the thermistor.
2. Leave the bell, or lamp, connected to the NO
contacts.
3. Adjust the variable resistor so the bell just starts
ringing, or the lamp starts glowing.
4. Heat the thermistor as before.
5. The bell, lamp, should stop working.
This kind of circuit can be
used for turning off a night time heating device, or a night time
insect killer. If the house is air conditioned and there is no
temperature difference between day and night, the thermistor will need
to be outside, where it can sense the usual diurnal temperature
variations. Some thermistors are positive temperature coefficient, PTC,
devices. What changes need to be made to the two demonstrations to
achieve the same outcomes, but using a PTC instead of an NTC? Note, I
did NOT say "in place of".
38.5.2 Light operated switching circuit, light
dark indicator, lamp on in the dark, automatic street light
See diagram 38.5.2
In the dark the resistance of the light dependent resistor, LDR, is
high, so less current flows through it, more current flows to the base,
so base current is high, collector current is high, and
the lamp lights. In the light, the resistance of LDR is low, so more
current flows through it, VBE falls, less current flows in the base, so
collector current is low, and the lamp does not light. If a 10 k ohm
variable resistance replaces the 10 k ohm fixed resistance, you can
adjust the level of light / darkness at which the lamp will turn on.
This is the basis of child care night lighting, automatic street
lighting and automatic factory lighting. For automatic street and
factory lighting there may be a half hour delay so that the circuit
does not respond whenever a cloud passes overhead. It may be combined
with a 24 hour clock to prevent lights being turned on in the middle of
a very overcast day, or to turn off all lighting in an office when
nobody is expected to be present, e.g., during public holidays. It may
also have an override circuit, e.g., operated by a security circuit, or
by radio or telephone line, so that lights can be turned on
automatically when there is heavy smog, or when an intruder has been
detected. To prevent positive feedback between the building or street
lighting and the LDR, which would cause the control circuit to latch
up, the LDR is aimed carefully away from the controlled lighting. For a
factory in the Southern hemisphere, the LDR may be just under the
southern eaves of the roof, opposite for Northern hemisphere, for
street lighting, the LDR may be positioned on top of the luminaire's
reflector pointing to the sky. Two LDRs may be required, one to sense
sunrise and the other, sunset. These can be wired in parallel, why? o
As LDRs also have some heat sensitivity, the LDR may be mounted in a
holder that has an infrared band stop, or RG band pass, filter.
38.5.3 Moisture detector, water indicator
See diagram 38.5.3 | See reference: "FUNWAY" | See 39.2.6: Automatic washing line
Put spaghetti insulation around the connecting wires used as probes,
leaving about 20 to 30 mm uninsulated at the sensing ends of the
probes. Why?, what is the resistance of human skin?, The operation of
this circuit is similar to that for the light detector in 38.5.2b, with
the probes replacing the LDR. Will this circuit work in distilled or
deionized water, with methylated spirit or oils? Why or why not?
In normal soil moistened with water, inorganic salts in the soil
dissolve and ionize, i.e., the positive and negative ions of the salts
separate, providing electrons for carrying current. Almost all
inorganic salts ionize to about the same degree, organic salts are much
more variable in their ionization. The probes have a voltage between
them provided by the sensing circuit. Electrons flow from the probe
connected to the BJT's base, through the ionized salts in the moistened
soil, to the other probe. If there is very little moisture, the
available ions are further apart, and hence the "resistance" of the
soil is high. If there is a great deal of moisture, ions are readily
available, and the resistance between the probes falls. Beyond a
certain increase in moisture level, no further drop in resistance
occurs. When the resistance between the probes falls, Vbe rises, base
current increases, collector current increases and the LED lights. The
automatic washing line is out of favour because the copper strips in
salt will leave marks in the clothing that are difficult to remove!
38.5.4 Sound operated switching circuit with
latching, crystal microphone, burglar alarm
See diagram 38.5.4a: Check sensitivity of a
circuit
See diagram: 38.5.1: Sound activated burglar
alarm
Tap on the crystal microphone to make it produce enough base current to
switch on the transistor. The crystal microphone can easily produce up
to 1 V AC if tapped sufficiently vigorously. The base emitter junction
of the BJT acts as a rectifier so that voltage excursions from the
crystal microphone that exceed the base bias will increase base current
and thence collector current. The collector current passing through the
transistor to the emitter provides gate current to trigger the
thyristor and current flows through the lamp. The lamp will stay on
until the battery is disconnected because the thyristor acts as a
latching switch. To make a sound activated burglar alarm, substitute a
relay for the thyristor and a bell for the lamp. A crystal microphone
is quite sensitive, i.e. its output is quite high for any defined
change in sound pressure, any NPN signal BJT running in this open loop
fashion, has very high gain. Further, the gate turn-on current of the
thyristor is not specified very accurately, and depends on temperature.
Consequently, this is really a demonstration of the principles, it
could not be a serious commercial device as it stands.
Discover how
variable is this device:
1. Insert a small resistor in the lead
between the BJT's emitter and the SCR's gate, or in the BJT's collector
lead, say, 1 k ohm.
2. Connect a CRO across the 1 k ohm resistor.
3.
Experiment with making increasingly louder sounds, perhaps an audio
oscillator fed to an audio amplifier and a loudspeaker, till the lamp
is triggered on.
4. Experiment with temperature variation by holding
the BJT and the thyristor in your fingers.
5. If a sound pressure
meter is available, plot the range of levels that provide reliable
turn-on. Your sound detector can be further developed by connecting the
crystal microphone across a 1 M ohm logarithmic potentiometer, with the
wiper connected to the BJT's base.
38.5.4.1 High-speed flash photography
See diagram 38.5.4b: Flash gun delay
Nonetheless, this demonstration is the basis for firing flashguns for
high-speed photography. Usually, the SCR is in the flashgun. This
system can be used for capturing the effect of crash testing motor
vehicles, for observing the effects of small arms projectiles on
targets or for observing the effect of a stone thrown at a glass plate.
Such experiments need to be set up in the dark because the camera
shutter is left open. The delay between your hearing a shot, pressing a
shutter control and the operation of the shutter, even in very fast
electronic cameras, is too long and variable, this delay can be between
200 and 400 ms, Murrell. Even in electronic and digital cameras, there
is always a finite delay between operating the local or remote
electronic shutter release, and operation of the actual shutter, good
sports photographers learn to anticipate this delay in each camera
used. In the more sophisticated flashgun firing circuits, a variable
delay circuit is used to allow for different transit times. For
instance, when photographing handgun projectiles, you need to allow for
the sound to travel from the hand gun to the crystal microphone and for
the speed of the projectile, sound travels at about 340 m / s and
handgun
projectiles travel at between 250 m / s and 500 m / s. For the stone
and
glass experiment, the crack in glass can travel at up to 400 m / s. Try
photographing someone sneezing violently, the material exiting from the
mouth and nose can reach the speed of sound! There is no need to allow
for the time between firing the SCR and initiating the flash, the delay
is usually less than 1 ms, Metz, Rollei.
38.5.5 Transistor amplifier, with magnetic
earphone and crystal earphone
A. See diagram 38.5.5
The size of the base current depends on value of the fixed resistor.
The capacitor stops d.c. passing through the microphone but allows the
AC
signal from the microphone to flow to the base. The diode can be seen
as a very nearly perfect switch. When the voltage applied is in the
forward bias direction, current flows.
When the diode is biased in the
opposite direction, there is almost no current flow:
1. Connect a
diode, ammeter and current limiting resistor in series.
2. Connect
this circuit to a variable voltage power supply,
3. Plot the current
flow vs the applied voltage,
4.Reverse the polarity of the power
supply,
5. Plot current vs applied voltage.
Note: do not exceed the
VRRM specification. The diode can be seen as an automatic switch, when
the polarity is correct, current flows, no human intervention is
required. This can be used as a safety device to protect polarity
sensitive devices, e.g., Zener diodes act as normal diodes when forward
biased, but their current carrying capacity is limited and the Zener
effect can fail if the forward current limit, IFM, is exceeded.
B. See
diagram 38.5.5a
It can be used as a simple two wire signalling device by connecting a
current limiting resistor in series with a pair of LEDs, e.g., one
green and one red, each in series with a reverse bias protection diode,
but the two branches of LED and diode are in reverse polarity to one
another. At the far end of the two wires, voltage of one polarity will
light the green LED, reverse the polarity and the red LED lights. A
bell, a buzzer or an incandescent lamp can be substituted for each LED.
If the ground is sufficiently conductive, one of the two wires can be
replaced by a ground spike at each end of the circuit, a higher voltage
will be required to sound the bell or buzzer or light the lamp. This is
the basis of some early telegraphic and telephonic signalling. The
beauty of the diode as a switch is that it is silent and has no moving
parts. The main drawbacks of the diode are the slight reverse current,
negligible with silicon diodes, and the unpredictable failure mode, you
cannot predict in advance whether the diode will fail short circuit or
open circuit, though a gradual build-up of current is likely to weld
the elements.
38.5.6 Time operated switching circuit
See diagram 38.5.6a: Car courtesy light
switch
A. When switch 1 closes, the lamp lights and the capacitor starts to
charge until VC > VBE, then current flows in the BJT's base to
switch on the BJT, relay contacts open, lamp switches off. Open switch
1 and close switch 2 to let capacitor discharge. Open SW2 and close SW1
to recommence. What determines how long it takes for lamp turn-off?
Time constant. Try different value capacitors to vary the turn-off
time.
B. Courtesy lighting
This was the basis for some automatic courtesy lighting circuits used
in motor vehicles, opening the door operated SW1, turning on the
ignition switch operated SW2. Thus, the lights stay on long enough for
the driver to insert the ignition key in the ignition lock. Opening the
door with the engine running may not turn on the courtesy lights unless
another circuit is used, or perhaps SW2 is operated by the starter
motor solenoid, but then the capacitor will stay charged after the door
closes, a bleed resistor, say 100 k ohm to 1 M ohm, across the
capacitor could fix that. Nowadays, an integrated circuit like the 555
would be used, with a diode feeding a strobe circuit to short the
capacitor.
38.5.7 Flashing circuit
See diagram 38.5.7 | See reference: "FUNWAY"
A. The LEDs flash alternatively, perhaps at too fast a rate to see.
When the switch is closed both capacitors start charging. Either
capacitor could start charging first. One capacitor will charge faster
than the other. If capacitor 1 charges faster some current flows from
the capacitor 1 to the base of transistor 2 which stops the capacitor 2
connected to its base from charging. Some current flows from the
capacitor 2 to the base of transistor 1 which stops the capacitor 1
connected to its base from charging. When capacitor 2 is fully charged
current stops flowing to the base of transistor 1, which no longer
stops capacitor 1 connected to its base from charging.
B. The capacitor with the least capacitance will charge faster. At
start-up, both capacitors are like short circuits, so, both bases start
at VBAT less VDIODE + V390R + VLED. This should turn both BJTs on hard,
but one BJT will turn on a bit ahead of the other, because of
component differences. Say BJT1 turns on faster, VCE falls thus
reducing VC of BJT1, this stops the capacitor, C1 connected to its
collector, from charging. Meanwhile, C2 keeps charging and when it is
fully charged, no more current flows in C2 and BJT1 turns off. VC of
BJT1 now rises allowing C1 to charge, leading to BJT2 turning on. As
long as the battery is connected and has life in it, the LEDs will keep
flashing alternately. In binary logic terms this is called a bistable,
symmetrical flip-flop. Try altering the values of C1 and C2 to see the
effect on the flash rate. By setting C1 significantly different from
C2, you can predict which LED will turn on first, but now the bistable
is no longer symmetrical. Tolerance of capacitor values If you have a
means of measuring capacitance, you can show that two capacitors with
the same markings have different values of capacitance and so you can
predict which LED will come on first.
Measuring capacitance
If you do
not have a capacitance tester, here is a method for measuring
capacitance: (You will need a high input impedance multimeter, e.g. a
DMM with 10 M ohm input on the voltage ranges, a 100 k ohm fixed
resistor, a d.c. power supply, a SPDT switch and a stopwatch.)
1.
Connect
the components as in diagram 38.5.7a,
2. Set the output of the PSU to
about 90% of the rated voltage of the device under test, DUT,
3. Set
the output of the PSU to Constant voltage and a very low current,
4.
Turn on the DMM and allow it to auto zero,
5. Turn the switch on to
"Charge" for several seconds, then "Discharge" the capacitor with the
100 k ohm resistor, repeat this charge discharge cycle several times
till when the DMM shows full voltage, as in step 2, there is no
charging current shown on the PSU meter, this should remove any
polarization effect,
6. Calculate 63% of the voltage in step 2.,
7.
At the same time as turning the switch to "Charge", start the
stopwatch,
8. When the DMM indicates the 63% value, stop the
stopwatch,
9. Calculate the value of C,
10.To be certain of the
value,
discharge the capacitor through the 100 k ohm resistor and repeat steps
7 to 9 several times., time constant for a 100 uF capacitor and a 100 k
ohm resistor is 10 s.
38.5.8 Automatic lighting control, front steps
light
See diagram: 38.5.8: Using IR
Could
use 38.5.2, above, if all that is required is that the lights come on
when ambient daylight falls below a predetermined level. For acting on
the presence of humans or warm blooded animals, a Passive Infrared,
PIR, detector, or an active IR, AIR, detector could be used. With a
PIR, or an AIR, detector, the device senses both temperature and
movement, for instance, the Perkin Elmer LHi954 and 958 detectors, PIR
devices, are most sensitive in the 0.1 Hz region and have a total
included sensing angle of 110. The detector has a FET wired as a source
follower in the same case. If the PIR detector is wired into the
circuit of 38.5.2 in place of the LDR, the resistance between the drain
and the source, RDS, increases with movement and incoming IR radiation.
Thus, it acts in the opposite manner from the LDR and needs to be wired
as in the second circuit, 38.5.2b, if the controlled circuit is to be
turned on. Warning As the PIR device is sensitive to both heat and
movement, it cannot be hand-held. For a more sophisticated detector,
use an IR light aimed at an opto transistor. A suitable IR emitter is
the Infineon SFH415-U, the Jaycar ZD-1902 or ZD-1905, this requires at
least 20 mA current flow, connect in series with an ordinary diode,
e.g., 1N4001 or 1N914, or 1N4148, a 220 ohm resistor and a 6V battery.
A suitable detector is the SLD-70 IR2A, the Infineon SFH213, both
photodiodes, or the Jaycar ZD-1946 NPN phototransistor. Each device has
only two leads, either cathode and anode, or collector and emitter,
connect the photodetector in place of the LDR in the circuit of
38.5.2b, anode / collector to the + supply and cathode / emitter to the
base of the BJT. So that when it detects emission from the IR emitter,
the circuit is turned on. These photodevices have some sensitivity to
ordinary light, they can be set off by sunlight, a car's headlight or a
torch, so, put the device into a light tight tube and fit an IR
bandpass, or an RG bandstop, filter in the end of the tube. Similarly,
put the IR emitter into a tube with a double convex lens to focus the
beam on the receiver. If an ordinary incandescent lamp is used instead
of the IR emitter, put an IR bandpass, or RG bandstop, filter before
the lens. This will make it very difficult for the casual observer to
see the beam. If you wish to make the demonstration more visually
dramatic, use a red LED pointer torch as your emitter, the kind used by
slide show hosts, and leave the IR filter off the detector, blow some
smoke across the path of the beam to reveal the beam and show the
effect of interrupting the beam. Then replace the red LED pointer with
your carefully constructed IR emitter in its tube and replace the IR
filter over the IR detector. Blowing talcum powder over the beam will
not change the frequency of the light, so, the IR beam will not be
revealed to the human eye. If fibre optic cables are being articulated
in your country, ask your local telecommunications installation firm if
it can spare any photodevices. Some devices may have the IR filter
already fitted. Some may be side viewing, and some axial.
38.5.9 Time delay
Seat belts, traffic lights, police siren, bar code, morse code,
headlights on, air conditioning, The Morse code key is a switch
controlling the current flowing to the circuit
See diagram: 38.5.9: Monostable time delay
In 38.5.01d, the idea of slowing down operation of a mechanical relay
was introduced. In 38.5.02, the idea of the flip-flop was introduced,
this is a bistable electronic delay. By removing BJT2 and leaving R2
and C1 in place, you have a monostable electronic delay. Varying the
value of the R or C will alter the time before turn on., time constant.
However, while VBE, the voltage at which the BJT will turn on, or off,
is fairly constant, as the battery ages, the time taken to turn off
will increase. If instead of a battery, a constant voltage power supply
is used, this limitation is removed. A constant voltage can be achieved
by using a Zener diode in series with a current limiting resistor as
the supply for the RC circuit feeding the base of the BJT, if the whole
circuit were powered from the Zener diode, the voltage would drop a
little when the circuit turned on and you might get some instability or
hysteresis. With modern components, e.g., tantalum capacitors, which
have very low leakage, and metal oxide resistors, which are very
stable, quite long time delays can be achieved, consistently. Note:
Select tantalum capacitors whose voltage rating is about three times
the likely voltage to be experienced in the application. Applications
for this include motor vehicle seat belts and headlights-on warnings,
traffic lights, police sirens and room air conditioners.
38.5.9.1 Seat belt warning
See diagram: 38.5.9a: Seat belt warning
A switch under each person's seat in series with a switch in the seat
belt buckle receptacle controls the current fed to the resistor, R, in
the circuit above. If the delay is set to, say, 12 seconds, this allows
enough time for each person to get seated and "belt-up". If the
driver's circuit is kept separate and operates a relay to control the
ignition, the vehicle will stop if the driver fails to belt-up in time.
All the other circuits can be used to operate a display in front of the
driver to show which passenger has not belted-up in the required time.
The time delay also allows a person to disconnect a seat belt for a
very short time, e.g. to belt-up a child. A resistor across the C can
make the turn-on and turn-off times quite unsymmetrical. Because of the
range of operating voltages of a car battery, 10 V to 15 V, the RC
chain would be fed by a Zener set at, e.g. 6.8 V, the battery voltage
may drop as low as 6V during starting, but shortly after when charging
commences, the voltage would rise quite quickly, and 6.8 V would be
available within a couple of seconds to warn of seat belts not done up.
39.5.9.2 Headlights ON warning
See diagram: 38.5.9b: Headlights ON warning
If you drive to work in the morning when it is dark or foggy and when
you arrive at your destination the sun has risen or the fog has
dispersed, you may forget to turn off your headlights. Motor vehicles
are normally fitted with about a 60 Ah battery. Normal headlamps are 60
W each. A pair of headlamps, plus the side, tail and number plate lamps
draw about 12 A. So, in about 5 hours the battery will be flat. If you
leave the car for a normal 9 hour day, not only will the battery be
flat, but also you may have damaged it permanently. Some people
actually get into a habit of turning off the headlamps BEFORE turning
off the ignition, just as they turn on the headlamps AFTER the engine
has started. These habits are actually kind to the battery. However,
some people never learn these habits. So you want a circuit that will
alert the driver within a short time of turning off the ignition that
the headlamps are still on. The headlamp circuit is normally in series
with a relay, another set of contacts on the headlamp relay, or from
the headlamp switch provides a feed to the collector of the time delay
circuit. The ignition circuit provides a feed to the Zener diode
powering the RC / base circuit, only when the ignition is off and the
headlamps are on. When headlamps and ignition are both on, the warning
LED is OFF. A small hooter or other warning device is connected to the
delay BJT's emitter and the NO pair of contacts on the headlamp relay.
If the headlamps are left on after the ignition is turned off, the
voltage on the base of the BJT will gradually rise and the BJT will
turn on, and the hooter will sound. If the headlamps are turned
off before the ignition is turned off, there will be no power to
the delay circuit or to sound the hooter.
| Ignition |
Headlamps |
Warning |
| OFF |
OFF |
OFF |
| ON |
OFF |
OFF |
| OFF |
ON |
ON |
| ON |
ON |
OFF |
38.5.9.3 Traffic lights
See diagram 38.5.9c: For outline schematic,
each time delay represents a delay circuit
Some traffic lights are set on a fixed cycle during peak commuting
times, e.g. 35 seconds green in the E-W direction, then 6 seconds of
amber followed by 49 seconds of red. The lights in the N-S direction
are set to commence their green phase 4 seconds after the end of the
E-W amber phase to allow for late clearing of the intersection and
"chancers", stay on for 35 seconds, change to amber for 6 seconds and
then to red for 49 seconds. This circuit can be achieved with six BJTs
controlling six relays, by wiring some of the relay contacts in series,
you can arrange that red and green cannot both be on at the same time.
Another pair of BJTs and relays can be inserted to achieve the British
system of giving an amber + red phase as a prelude to the green phase.
The RC circuits are all fed from one Zener diode supply. In place of
the mechanical relays, "solid state" relays can be used. These use no
moving parts and are essentially tracs. In some places, pulsed LEDs
are used in place of the usual incandescent lamps. Pulsed LEDs can be
more efficient, brighter and last longer than incandescent lamps. The
pulsing of the LEDs can be achieved by a bistable set to cycle at about
60 Hz, faster than the human eye can sense. The only drawback is that
the initial purchase price of the LED cluster and control circuitry is
greater than that for an incandescent lamp and its socket.
38.5.9.4 Two-tone police siren
See diagram 38.5.9d: (Click suppression not
shown)
To achieve the two-tone police siren sound heard in France and some
other parts of Europe and the ambulance sound heard in many parts of
the world, the bistable circuit of 38.5.7 can be used. Instead of one
of the LEDs, fit a relay selecting between the output of two
oscillators to be fed to an amplifier and speaker. The armature is
connected to the amplifier input, the NO and the NC contacts of the two
relays are connected separately to the output of each oscillator.
Choose any two tones, e.g., 400 Hz and 500 Hz, sort of Ab and B#, but
not quite C, for the oscillators. If the oscillators can be set to
square wave, the tones will be much more annoying / alerting. As the
tones are not quite in tune with standard pitch, they will annoy anyone
with any musical sensitivity. The oscillators can each be the bistable
of 38.5.7, with much smaller values of RC product. The RC product in
the RC chains of one oscillator will need to be different from those in
the other oscillator. Fit a 1 k ohm resistor in place of each LED, from
the collector of one BJT in each oscillator, connect a 100 nF capacitor
to the NO, or NC, contact of the appropriate relay. Connect a 22 k ohm
resistor from the relay side of the 100 nF capacitor to ground to avoid
loud clicks as the relays change-over.
38.5.9.5 Room air conditioning
Whenever a door to an air conditioned room is opened, the temperature
and humidity are likely to change. Even when people move around inside
an air conditioned room, closer to and then further away from the room
sensor, thermostat / hygrostat, block, it will sense a change. Such
changes can be a signal to the air conditioner to come on or go off.
This rapid turning on and off can be wearing on the moving parts of the
air conditioner and on the room occupants. Feed the signal from the
room sensor to a monostable, i.e. 38.5.7 circuit, as above, but with
the R and C swapped. Replace the LED with a mechanical or "solid state"
relay to control the main air conditioner. The timer can be set to,
say, 10 minutes. Such a delay can cover room deliveries, occasional
visits to the balcony to have a smoke, savour the smog.