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Assembly and Operating Instructions for Kits


Operating Instructions for the Multi-Trigger 3


Assembly instructions for other kits


MT3 assembled


Here is a printable version of this manual.




Sensor cables included with the MT3

Interrupter photogate cable


Individual emitter and detector photogate cable


Microphone cable


Interrupter photogate cable (parts provided with MT3i kit) Variable-width photogate cable (parts provided with MT3v kit) Microphone cable (parts provided with either MT3i or MT3v kits)



Flash trigger cable

A flash trigger cable such as one of the three shown below is needed to connect the the outputs of the MT3 to a flash unit. The cables are purchased separately from the MT3 kit. The connector for the control box is a 3.5mm mono plug.


PC-type flash trigger cable

Flash shoe cable ready to connect to trigger box

Vivitar 283 type flash trigger cable

PC type Flash shoe type Vivitar 283/285/3700 type

Camera shutter cable

A shutter cable is required to actuate a camera using the MT3. The cables are purchased separately from the MT3 kit. The following cables are available from


Canon RS-60E3 shutter cable

Canon RS-80N3 shutter cable

Nikon MC-30 shutter cable

Nikon MC-DC2 shutter cable

Canon RS-60E3 Canon RS-80N3 Nikon MC30 Nikon MCDC2



Theory of Operation


Before describing how to use the Multi-Trigger 3 (MT3), we provide this introduction to its timing functions, as this will provide some insight into how the unit works as well as familiarize the user with the terminology that will be used. We'll refer to the timeline below. The upper trace labeled Trigger Pulse in the diagram to the right represents the input from the trigger. The MT3 requires a trigger input to activate the delay circuit. The trigger simply shorts the delay unit input to ground. This is done in one of three ways with the MT3. One can use the built-in photogate or sound trigger circuit, in which case the corresponding sensor cable must be connected. Alternatively, one can connect an external trigger to the auxiliary input. Any trigger that has as its output a simple circuit closure will work.


The delay circuit responds to the trigger by producing two output pulses with a delay interval between them. Output Pulse 1 (referred to as Pulse 1 later) is shown as the middle line in the diagram below. Pulse 1 is a square pulse whose duration depends on the settings of the Coarse and Fine Delay knobs. During this interval of time, the Start LED is on. At the end of Pulse 1, a second square pulse is produced, Output Pulse 2. This second pulse remains high for a length of time determined by the setting of the Timeout knob. During this time, the Finish LED is on. The output pulses are sent to the output jacks. The Instant Flash and Camera output jacks as well as the Calibration jack are actuated as soon as Pulse 1 goes high. The Delayed Flash, Camera, and Pulse jacks are actuated as soon as Pulse 2 goes high.


  1. Input: The trigger module detects a sensor input and outputs a trigger pulse at the input of the delay module.
  2. Delay: The delay module produces 2 output pulses, one immediate and the other delayed. The delay unit also produces a timeout in order to prevent another trigger pulse from activating the timer until the timeout goes to completion.
  3. Output: The output pulses from the delay circuit are sent to components that turn output devices such as flash and camera on. The outputs to camera are optically-coupled to isolate the MT3 circuitry from the camera circuitry. The outputs to flash provide electrical isolation between MT3 and flash of up to 400 V. While this isn't necessary for modern flash units, some older units may have high-voltage synch circuits. These will not damage the MT3 circuitry when the flash outputs are used. The output pulses are also sent directly to the Calibration and Delayed Pulse jacks. Pulse 1 is sent to the Calibration jack, and Pulse 2 is sent to the Delayed Pulse jack. These pulses can be used for diagnostic/calibration purposes or to trigger devices such as flash units that require a low-voltage trigger pulse.



Photograph of the MT3 control unit with functions labeled


Refer to the following photo for the locations of the knobs, switches, jacks, and LEDs on the control box. Click on the photo for a larger image.


MT3 controls


Note: It's best to have the unit turned off when inserting plugs into or removing them from the input and output jacks.


Powering the Multi-Trigger 3

AC-to-DC 9V adapterThe Multi-Trigger3 circuit is powered by 9V DC. This can be supplied either with a 9V battery or an AC-to-DC 9V adapter. The AC/DC jack on the enclosure accepts a standard 2.1mm plug such as the one on the adapter to the right. (Center pin is positive.) When the adapter is plugged into the box, the battery is automatically disconnected internally. If you use your own adapter rather than the one available from, be sure that the maximum current rating is at least 1 A. A 12V AC/DC adapte can also be used to power the unit.


In order to install or replace a battery, remove the four screws that hold the lid in place. Then lift the lid carefully so as not to stress the wires that connect the lid to the battery holder. If necessary, the battery holder itself can be removed, as it is held to the bottom of the box with hook and look tape. After replacing the battery, lower the lid on onto the box, being careful not to pinch any wires between the lid and the box. Then screw the lid back down.


When the on-off switch is flipped to the ON position, the LED next to the switch will light. If you're operating on battery power and the light is dim, that may indicate a weak battery. When the battery is weak, the unit may still function but demonstrate erratic behavior. That's a sign to replace the battery or use an AC-to-DC adapter. If the power LED doesn't come on at all, open the box to check whether the fuse is burnt out. If so, replace it with a use having a 1A rating. This is a standard fuse available at hardware and electrical supply stores.


The Delay

As mentioned in the introduction, the delay module works with any trigger that provides a simple circuit closure to ground. The internal photogate and sound trigger circuits do that. Triggers connected to the auxiliary input will also actuate the delay circuit. The latter works the same for any trigger. In this section, we describe how to make delay settings and test them, with or without flash or camera connected to the outputs.


For many applications, the Divide Delay switch should be set to the ÷1 position. We'll assume for now that you're using that position and will describe later how you might use the ÷10 position. In the ÷1 position, the delay intervals generated are those given on the Coarse and Fine Delay scales, with this caveat: The numbers on the scale may deviate by 10-20% from the actual time intervals. This is due to the tolerances of the RC circuits used to produce the time intervals. This deviation isn't important, as the scales are primarily intended to be indicators to help insure reproducible settings. What's important is that the results are predictable from one shot to the next. If it's important for your application to know the actual time intervals more accurately and you have an oscilloscope available, you can use the Calibrate output to measure time intervals for constructing a calibration curve. As an example, a typical curve is shown to the right. The time interval measured on the oscilloscope in milliseconds (ms) is plotted versus the reading on the Coarse Delay dial. Note that the reponse is linear except for the extremes of the range (dial readings of 0-50 and 450-500). For readings of 0 to 50, the delay doesn't actually increase significantly above its minimum value. Likewise, for readings of 450 to 500, the delay changes little.


In setting the delay for a particular application, first note that the dial readings for the Coarse and Fine Delays are given in milliseconds (1 ms = 0.001 s). A good strategy is to start adjustments with the Fine Delay set at 25 ms. Then adjust the Coarse Delay until the event being photographed is captured. At this point, adjust the Fine Delay to a smaller or larger value to hone in on the time interval needed for best results.


The delay circuit can be actuated either with a trigger input or by pressing the Test pushbutton. If you're not using a sensor cable, the Input Selector must be set to the SND position in order to use the Test button. If using a microphone or photogate, set the Input Selector switch to the appropriate setting. If using an external trigger plugged into the Auxiliary Input, the Input Selector switch is bypassed, and the output of the trigger connects automatically to the input of the delay unit. In any case, when the delay unit is actuated, the Start and Finish LEDs light in sequence. The Start LED lights immediately and remains on for as long as Output Pulse 1 is high. This is the delay interval. The Finish LED remains on as long as Output Pulse 2 is high. This is the Timeout interval. If one has, for example, flash units connected to the Instant and Delayed Flash outputs, the flash connected to the instant output will discharge at the beginning of the trigger pulse with no delay. The flash connected to the delayed output will discharge when Pulse 1 goes low.


The purpose of the Timeout interval is to suppress unwanted secondary flash discharges that can result from, say, echos and secondary sounds when using the sound trigger. In order to eliminate secondary discharges, increase the Timeout until only a single flash discharge is obtained for each triggering event. Note that the Timeout only applies to the delayed outputs. If a timeout is needed for an instant output, see this article for an auxiliary circuit that will achieve the purpose.


The maximum Timeout is about 1 second and the maximum delay about half a second. If higher values are needed, capacitors C8 (for the timeout) or C3 (for the delay) on the PCB can be replaced with higher values of capacitance. (See the circuit schematic.)


With the Divide Delay switch set to the ÷1 position, the minimum delay possible is about 0.5 ms with both Fine and Coarse Delay dials set to 0. If smaller delays are needed or if you want greater precision in adjusting delay intervals, flip the Divide Delay switch to the ÷10 position. In this position, the minimum delay is about 0.05 ms. Readings on the Fine and Coarse Delay dials must all be divided by 10. Note that the maximum delay possible with the Divide Delay switch in the ÷10 position is about 50 ms.

calibration curve


The Photogate

Either the interrupter or variable-width type of photogate cable may be used. Plug the cable into the PG in jack and flip the Input Selector switch to PG. Turn the Sensitivity knob to the halfway position. This position will be fine for most applications. With the unit turned on, the PG Align LED will be lit if the infrared emitter and detector are aligned. Alignment is automatic with the interrupter cable. For the variable-width cable, you'll need to point the emitter and detector at each other several inches apart. Now, when an object passes through the beam, the PG Align LED will go out and the Start and Finish LEDs will flash in sequence as described in the previous section.


If you're using the variable-width cable, you may wish to adjust the Sensitivity in some situations. With the emitter and detector aligned, turn the Sensitivity knob clockwise until the alignment LED goes out. This happens when the sensitivity is set too high. In order to set the sensitivity at the highest it can be without going over the threshold, dial the knob back to the point where the LED comes back on. If you're running on battery power, the sensitivity may drift as the battery runs down. So if you have the sensitivity adjusted to the threshold, you may have to readjust it as the battery weakens. You also need to readjust the sensitivity when you change the separation of the emitter and detector. Whenever you position the emitter and detector for an experiment, be sure to anchor them as rigidly in place as possible in order to maintain alignment as well as the sensitivity adjustment.


The Sound Trigger

Plug the microphone cable into the Mic In jack and flip the Input Selector switch to SND. Snap your fingers or tap the microphone. The Start and Finish LEDs should go off in sequence as described previously.


The Timeout is more useful with the sound trigger than the microphone. When detecting sound, the event being photographed may produce secondary sounds that would result in multiple-exposure without the timeout. For example, a balloon burst or a rifle shot may produce echos. Shattering glass continues to produce sounds as the glass falls to the floor or table. By turning the Timeout up, all flash discharges but the first can be suppressed for a period of up to about 1 second after the initial triggering event.


The Auxiliary Input

The auxiliary input jack can be used to connect other triggers to the delay unit. When a 3.5mm mono plug is inserted into Aux In, both the photogate and sound trigger circuits are internally disconnected from the delay unit. You can connect as an auxiliary input any trigger that produces a short circuit as an output. An example is a simple contact trigger. In order to use this input, you'll need to prepare a cable from your trigger output to the external input. The Aux In jack on the MT3 uses a 3.5mm mono connector. (Assembly instructions for an auxiliary input cable are given here. Parts for the cable are not included with the MT3 kit.)


The Instant and Delayed Outputs

There are three types of outputs: flash, camera, and pulse. There are two outputs of each type, one instant and the other delayed. These are described below.


Flash outputs: The flash outputs require a 3.5mm mono plug for the jack. All that's typically required to use these outputs is to plug in the flash trigger cable and turn on the flash. The outputs are protected (up to 400 V) in case your flash unit is an older model that has a high-voltage synch circuit. Note that some modern flash units that are designed to work only with a particular camera's wireless flash system cannot be triggered with the MT3. However, the great majority of flash units will work. Occasionally, one runs across a flash unit that requires an unusual setting in order to be triggered externally through the hot shoe. Some experimentation with the settings on the flash unit may be required. Manual operation is always a good place to start. If you can set the flash power on your unit, it's typical for high-speed photography to select the lowest power available. However, there are occasions in which you need more light and where the motion is slow enough to allow the use of a higher flash power. Photographing drops and splashes is an example. Experiment with the flash power to reduce blur as desired.


Camera outputs: The camera outputs require a 3.5mm stereo plug for the jack. You'll need a shutter cable with the socket that matches your camera. It's typical in high-speed photography to set the camera for complete manual operation. With the shutter cable plugged in and the camera turned on, you're ready to take pictures. Keep in mind that all cameras have a built-in shutter lag. Even though a manufacturer may claim that a particular camera has near-zero shutter lag, keep in mind that this is a relative statement. The shutter lag may be practically imperceptible if you're shooting typical action photography involving human motions. For high-speed motions, the shutter lag may make it impossible to capture the instant that you desire. Events such as balloon bursts are an example. The burst may be over in as little as 2 milliseconds, much less than a typical shutter lag. For this reason, it doesn't make sense to trigger a camera to capture the burst of a balloon. Instead, one would trigger an external flash for this situation. On the other hand, when photographing drops and splashes, shutter lag isn't a problem, because in the time it takes for a drop to fall from the photogate sensor to a pool of liquid, the camera shutter can be opened. In this case, the camera would be connected to the Instant Camera output and the external flash to the Delayed Flash output. Thus, the shutter would have time to open before the flash discharged.


The camera outputs will also trigger flash units, but here's a caution. The camera outputs have an optically-isolated stage that is rated up to 80 V. If your flash units has a high-voltage synch circuit, it may burn out the optocoupler in the MT3. (If this were to happen, the fix is simply to replace the PS2501-4 optocoupler.) If your flash unit meets the voltage criterion, you may connect its trigger cable to a camera jack. Although the plug is a mono plug, it will work fine in the stereo jack of a camera output. One reason to use a camera output for a flash instead of a camera would be if you wanted to discharge two flash units at the same time. You could connect one flash to a flash output and the other to a camera output.


Pulse outputs: The pulse outputs are just that, the square voltage pulses from the MT3 delay timer. Output Pulse 1 goes to the Calibration jack, and Output Pulse 2 goes to the Delayed Pulse jack. The latter is 3.5mm mono, while the former is a BNC jack. The output pulses are about 7V high (when powering the MT3 with a 9V source). Note that these pulses may not be used to drive high-current devices. That's not their purpose. The pulse outputs are provided in case your flash unit requires a low-voltage pulse to trigger it. This is the case for some studio units. Also, you may use the pulses to calibrate the delay and timeout dials as described in the Delay section above.Note that if you use either of the pulse outputs to trigger a device that requires a switch input, as is the case with most flash units and cameras, you will need a buffer circuit such as the Opto Switch 2. In use, a pulse output of the Multi-Trigger 3is connected to the input of the Opto Switch 2. Then the device, camera or flash, is connected to an output jack of the Opto Switch 2.


The flash and camera outputs may also be used to trigger wireless transceivers. Keep in mind when using wireless triggering for high-speed photography that the transceivers typically introduce a time delay of their own of a few milliseconds.




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