| Make Your Own
Trigger Circuits
Want to make your own trigger circuits? Try these:
You'll also need a way to connect your
trigger to a flash unit.
Prefer to purchase all the parts you need in a kit? Go to Kits.
Building your own sound
trigger
If you would like to build your own piezoelectric sound trigger
from scratch, a list of parts and a circuit diagram are given
below. The microphone is a piezoelectric buzzer element such
as #273-073 available from Radio Shack. (Use the red and black wires
only on this part.) The SCR must be
sensitive to very small gate currents and must be able to provide up
to 400-V isolation.
List of parts
Piezoelectric buzzer element
NPN general purpose transistor, such as 2N2222
5-kW variable resistor
5-kW resistor
68-kW resistor
400-V sensitive gate SCR
9-V battery and clip
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Building your own
photogate trigger
Lists of parts and circuit diagrams are given below for two
photogate circuits. Each circuit uses an infrared emitter and
detector. (A penlight can provide a substitute for the infrared
emitter, even when using an infrared detector.) The transistor
switch has a very rapid response and is good for triggering on
fast-moving objects. The Schmitt trigger
works better for slow-moving objects such as a milk drop falling
through the photogate.
There are many infrared emitters and photodetectors that can be
used with these circuits. The ones that are given in the parts lists
are usually available from Radio Shack.
An interrupter could be used in place of separate emitter and
detector. The interrupter incorporates both emitter and detector in
a single, molded plastic package. (See photograph here.)
This is useful when the object
that breaks the infrared beam is narrow, since the gap between
detector and emitter is only about an eighth of an inch.
Transistor photogate
A photogate trigger circuit that uses a transistor is shown
below. The 2N2222 transistor acts as a switch in the gate-cathode
path of an SCR. The SCR is connected either to a flash or delay
unit. As long as the phototransistor is illuminated by the LED,
the collector-emitter path of the 2N2222 is open. When the light is
blocked, the voltage across the phototransistor rises and the 2N2222
conducts, thereby gating the SCR.
The sensitivity of the photogate is controlled with the 100-kW
variable resistor. (A variable resistor with a
smaller range could be used for finer control at high sensitivities.)
This is an important feature for detecting small, fast objects and
allowing large separations between the detector and emitter. For
maximum sensitivity, the resistance is adjusted low enough to raise
the voltage across the phototransistor to the threshold of
triggering. Whenever the detector-emitter separation is increased,
the resistance must be increased, since the phototransistor's own
resistance rises as the illumination on it decreases.
Parts list for transistor photogate
Infrared emitter (XC-880-A)
Infrared phototransistor (TIL414)
2N2222 transistor
400-V sensitive gate SCR
10-kW resistor
470-W resistor
100-kW variable resistor
9-V battery and clip
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Schmitt trigger photogate
The transistor trigger may not respond well to slow-moving
objects such as falling liquid drops. A Schmitt trigger provides an
alternative. The circuit uses a 555 timer IC, shown below. When the
light from the emitter to the phototransistor is blocked by a moving
object, the voltage across the phototransistor increases. When it
reaches a certain level, a square, 9-V output pulse is produced at
pin 3, thus gating an SCR.
The sensitivity is adjusted with the 10-kW
variable resistor. For lower values of this
resistance, the value of the phototransistor's resistance that
results in triggering will also be lower. This means that triggering
will occur with less blockage of the light beam. Or, if the
emitter-detector separation were greater, giving a lower intensity
at the detector, the variable resistance would have to be greater in
order to prevent spontaneous triggering. (A
variable resistor with a larger range may be needed as
detector-emitter separation is increased.)
Parts list for Schmitt trigger
Infrared emitter (XC-880-A)
Infrared phototransistor (TIL414)
555 timer IC
400-V sensitive gate SCR
10-kW resistor
470-W resistor
10-kW variable resistor
0.01-µf capacitor
9-V battery and clip
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Building your own
electronic delay circuit
A list of parts and a circuit diagram are given below for a delay
circuit. The circuit uses a 556 timer, which consists of two 555
timers. When the input is grounded or a negative input pulse
provided, output 1 goes high. It remains high for a length of time
that is determined by the setting of the 1-MW
variable resistor and the value of the capacitance, C. At the end of
that time, output 2 goes high, remaining high for about 10 ms. This
time, termed the reset delay, is determined by the 10-µf
capacitor and 1 kW resistor. The reset
delay is normally small enough to allow the flash unit to be
triggered repeatedly with minimal delay. If a greater delay is
desirable, the 1 kW-resistor can be
replaced with, say, a 100-kW resistor.
When used with a photogate or sound
trigger to discharge a flash unit, the output of the trigger
becomes the input of the delay circuit. Either or both of outputs 1
and 2 can be used to trigger flash units.
Parts list for delay circuit
In addition to the parts listed below, several capacitors
(0.01-1.0 µf) are needed to provide different time ranges. (This
would be the value for C in the circuit diagram.)
556 timer IC
2 400-V sensitive gate SCRs
Resistors: 1 1-kW, 1 100-kW,
1 1-MW, 4 22-kW
1-MW variable resistor
Capacitors: 2 0.005-µf, 2 0.05-µf, 1 10-µf
9-V battery and clip
*A 400-V sensitive-gate SCR (part no. EC103D)
is available from Digi-Key.
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Connecting the
trigger to the flash unit
The PC cord of the flash unit can be connected to the sound
trigger without having to sever the cord to expose the bare wires.
Here's one way:
 a) Cut a half-inch long
section from the top of a plastic ballpoint pen cartridge. Insert
this inside the collar of the PC cord as shown in the cutaway
diagram to the right. If the plastic doesn't fit, shave or file off
a little. The fit has to be snug to prevent the plastic from falling
out. It serves as the insulation between the inner, positive pin and
the outer, negative collar.
b) Insert the positive lead from the sound trigger into the
plastic sleeve surrounding the pin of the PC cord. Be sure that good
electrical contact is made. Then wrap the negative lead from the
sound trigger around the collar of the PC cord. Tape the leads
securely.
If you don't mind cutting the PC cable, here are some other connection
methods.
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