HiViz - Guide to Trigger Selection

Assembly and Operating Instructions for HiViz Kits

Trigger Circuits for Flash and Camera -- A Selection Guide

If you're having trouble deciding what trigger to get, whether you need a delay unit, and whether to trigger a flash or a camera, this guide is a good place to start. If you're considering a Multi-Trigger, start at the bottom of the table.

In order to use a trigger with a flash or camera, you need the appropriate connection method. Here is information about connection methods.

Trigger	Product ID	What does it do best?	Is a delay unit needed?	Should I trigger a flash unit or a camera?
Contact trigger	DU	This is a pressure trigger in which two metallic contacts or plates are brought together by the force of a collision. Applications include photographing objects dropped onto or thrown against the contacts. The trigger can be camoflauged for photographing animals. You can easily build your own pressure plate or metallic contacts. For electronic circuitry, you would need a delay unit.	This depends on the application. For the collision of a ball or a dropped water balloon with the trigger, a delay unit is needed to capture the shape of the object in various stages. For animals, a delay unit may not be needed.	Whenever you need very rapid response, trigger a flash unit directly. Flash units can respond to an electronic trigger signal in microseconds, while cameras--even fast ones--take milliseconds. The difference is a factor of 1000; a millisecond is much too long for many high-speed events. Wireless transmitters typically introduce delays of 1-2 milliseconds, so don't use them if you want the fastest response. Use a flash unit for the collision of a ball or water balloon with the trigger. Use the open shutter technique in which you hold the camera shutter open in a dark room (or use a relatively long exposure time such as 1 second) and then initiate the event that triggers the flash discharge. For photographing animals, millisecond delays are typically acceptable. In such situations, the camera may be unattended for long periods of time, and photographs are taken in ambient lighting. Therefore, it's convenient to trigger the camera, which in turn sets off one or more flash units in synch with the camera shutter.
Sound trigger (piezoelectric)	SK2 or SK2-DU	This trigger responds well to loud and sharp sounds such as balloon bursts, breaking glass, and gunshots. For soft sounds or thuds, see the SK3 or amplified sound triggers below.	For capturing balloon bursts, don't use a delay unit. Instead, adjust the delay by changing the distance of the microphone from the source of sound. Moving the microphone toward or away from the source by a little more than an inch changes the delay by about 0.0001 second. Likewise, for gunshots a delay unit isn't needed, since the event of interest--the collision--occurs after the sound is produced. So the delay can easily be adjusted by moving the microphone. For glass breakage such as that of bulbs, longer delays are needed than for balloons. In this case, it may be convenient to use a delay unit so that the microphone doesn't have to be moved long distances from the source of sound.	See the note above. For all the situations described to the left, trigger the flash unit directly rather than a camera.
Sound trigger	SK3	This is a newly-designed sound trigger that is more sensitive than the SK2 but costs more and takes more time and knowhow to assemble. This trigger works well for all the same things as the SK2 as well as collisions involving such objects as tennis balls and footballs.	The same considerations apply as for the SK2.	The same considerations apply as for the SK2.
Sound trigger (amplified)	AST	This is our most sensitive sound trigger. It uses a dynamic microphone and audio amplifier and responds well to a wide range of sound intensities and frequencies. The sound of a drop of water hitting a solid surface will actuate this trigger. For a do-it-yourself version, see this page.	Similar considerations apply as those above. The collision of a racquetball with a surface lasts only about 0.005 seconds, so delays are adjusted by the position of the microphone. For a different method of triggering and delay selection, see the contact trigger above.	The same considerations apply as those for the piezoelectric sound trigger.
Photogate (Schmitt trigger)	SPG1 or SPG1-DU	This trigger initiates a triggering event when an object breaks an infrared beam between an emitter and a detector. The maximum trigger separation is about a foot when using an infrared LED but can be increased substantially if a red laser beam is used instead. (A red spot or streak may appear on the subject if a laser beam is used.) Use this trigger when you need precise triggering on the position of an object. Small objects dropped or projected through the beam will produce a triggering event as long as they are not too fast or small. Bullets and BBs may not give reliable triggering.	Delay units are typically used with photogates. For example, if a falling drop breaks the infrared beam, the event of interest--a splash--occurs below that point. With a delay unit you adjust the amount of time after the beam is broken until the flash discharges or camera shutter opens.	For photographing splashes, you may choose to trigger the camera which, in turn, triggers the flash. There may be enough time from when the drop passes through the photogate until the splash occurs to account for the shutter lag of the camera. The delay would be decreased by an amount equal to the shutter lag of the camera. You don't have to know what the latter is; you can make adjustments quickly by trial and error. Another technique takes advantage of the dual outputs of the delay unit. The camera is triggered by the instant output, and the flash is triggered by the delayed output. When the infrared beam is broken, the camera shutter opens. You would predetermine how long the exposure time needed to be so that the shutter remained open long enough for the splash to form but closed almost immediately afterwards. This would automate the process of taking photos and also minimize ambient light exposure.
Photogate (Schmitt trigger)	SPG2 or SPG2-DU	The circuitry for the SPG2 trigger is the same as for the SPG1. The only difference is that the emitter and detector are housed in a U-shaped piece of plastic with a fixed separation of about 5/8 inch. This makes this trigger particularly component for drops and vibrating strings.
Crossed-beam photogate	CBP	This circuit is especially designed for photographing birds and insects. The photogate is composed of two crossed infrared beams. The subject must break both beams simultaneously in order to create a triggering event. This means you can prefocus the camera on the intersection of the beams and obtain a greater percentage of successful photos.	A delay unit usually isn't used, because you want to get the photo at the location on which you've prefocused. You can, however, use a delay unit if the need arises.	The camera is typically set up where there is a significant amount of ambient light. Even when photographing insects at night, there may be long stretches of time without a triggering event. Therefore, it's essential to trigger the camera which, in turn, sets off the flash. For best results, use your camera in complete manual mode in order to minimize shutter lag.
Photogate (transistor)	TPG or TPG-DU	The crossed-beam photogate above uses a modified version of the transistor photogate circuit. This is used when one needs the most rapid response from the trigger.
Multi-Trigger	MT-BB MT-PCB3 MT2	This unit combines the SK2, SPG1, SPG2, and DU on a single breadboard or PCB. See entries above for information on use. Here is a comparison of Multi-Trigger options.