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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 the WorkShop.


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-kΩ variable resistor
5-kΩ resistor
68-kΩ 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. 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-kΩ 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
Infrared phototransistor
2N2222 transistor
400-V sensitive gate SCR*
10-kΩ resistor
470-Ω resistor
100-kΩ 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-kΩ 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
Infrared phototransistor
555 timer IC
400-V sensitive gate SCR*
10-kΩ resistor
470-Ω resistor
10-kΩ 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-MΩ 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 kΩ 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-kΩ 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-kΩ, 1 100-kΩ, 1 1-MΩ, 4 22-kΩ

1-MΩ 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 or the HiViz WorkShop.


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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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