Making a Timeout Cable for a Sound Trigger
When using a sound trigger with a particularly loud event such as a balloon burst in a small room, one sometimes gets an echo which can create multiple discharges of the flash for a single photo. This can also result when firing a gun, as both the sound of the gun firing and the bullet hitting a target can discharge the flash twice quickly in succession. So one obtains two images, slightly-displaced from the other, in the photo. In order to prevent such double exposures, you need a timeout function (also called reset delay) for the trigger. This deadens the trigger for a long enough time to prevent a second firing.
When using a delay unit with the trigger, a timeout can be set for the delayed output. Instructions for doing this are provided with the various HiViz kits and products that include delay units. Consider, for example, the SK2-DU breadboard circuit below. (Click on the image for a larger version.) The resistor between 26C and 29C controls the amount of the timeout. For the 1kΩ resistor shown in the photo, the timeout is only about a hundredth of a second. This kit and all of the HiViz breadboard kits with delay units are provided with an extra 100kΩ resistor. Replace the 1kΩ resistor with the extra 100kΩ resistor. This will produce a timeout of about 1 second and will eliminate double exposures.
For some situations in which the fastest possible response is needed for the trigger--for example, capturing a balloon burst--a delay unit can't be used, because it will introduce a small delay even at the minimum setting. In such cases, another method may work to produce the necessary timeout. The addition of a capacitor of the appropriate value across the output of the trigger can have the effect of inactivating the trigger for a short period of timing after a flash discharge. The value of the capacitor depends on the make of the flash unit; also, this trick may not work for all flash units. For a Nikon SB800, a 2.2-μf capacitor works, but this same capacitor doesn't work with a Vivitar 283 flash unit. For the latter, we use a 100-μf capacitor.
If you're using a sound trigger on a breadboard, the addition of a timeout capacitor is simple. Refer to the photo below. First, change the connection of the red wire of the flash trigger cable from 9A to 9C. Then connect the positive leg of the capacitor to 9A and the negative leg to the ground column.
If you're using an SK3 sound trigger, which is built in a project box, the easiest thing to do is add the capacitor to the flash trigger cable. Instructions for this are given below. This method also works if you have an MT-PCB2, MT-PCB3, or an MTE-PCB and are using the direct output of the sound trigger.
Click on the images for larger versions.
That completes the timeout cable. In case you have an application where you don't want the timeout, you could have a second trigger cable without a capacitor.