School Science Lessons
38.1 Electronics, switching circuits, switches in motor vehicle ignition
system
2012-05-05c SP
Please send comments to: J.Elfick@uq.edu.au
See: UNESCO Electronics
Teacher's Guide, School Science Lessons
See: Electronics 1 Experiments
Table of contents
38.5.0 Switching
circuits
38.5.3 Switches in motor vehicle ignition system
38.5.0 Switching circuits
Topic 38.50 Switching circuits was edited by Mr Brian Clarke. Please send
comments to: brianclarke01@optusnet.com.au
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
38.5.3 Switches in motor vehicle
ignition system
38.5.3.1 Switches in series
38.5.3.2 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
38.5.00 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: Latched relay | 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. 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: Reed switch | See 39.2.2: Reed switch and magnet
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 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
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 tracks. 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.