Trigger Circuit Controls Stop-Motion Camera System

Posted on Feb 4, 2014

This version of the trigger circuit for the stop-motion camera system uses an electret microphone for a sonic input, but the designer can substitute an LED and photodiode pair to create an optical trigger. For a recent home-brew project, I built a stop-motion camera system using a timed flash and shutter system. The system required a trigge

Trigger Circuit Controls Stop-Motion Camera System
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r mechanism to set the timing in motion. The input trigger needed by the system controller is a pulse train of at least two pulses. After the system is armed,  the first incoming pulse acts as a pre-trigger, and the second is the final trigger for the system. The circuit can use either an optical trigger (a light curtain ) or a sonic trigger. With very minor modifications, it can work with both triggers. The parts are all through-hole components, chosen for availability. The trigger circuit comprises five major components: the sensor, the conditioner, the pulse generator, the opto-isolator, and the power conditioner ( Fig. 1 ). The power conditioner, which is not discussed in this article, supplies both positive and negative 9 V to all the circuitry on the board, using an LM317 and an LM337, respectively. For several reasons, the trigger circuit uses an opto-isolated output. First, due to the sensitivities of the controller, I had to ensure that no ground loops were present. Previous tinkering had shown that they could cause random firing  of the system controller. Second, the voltage levels ( ±9 V) used on this board aren`t compatible with the system controller circuitry (+5 V). Finally, the flash circuit (on another board) could potentially cause a large ground bounce (a 10- µs discharge of about 1200 V) that could damage circuitry elsewhere. Note that the opto-isolator base connection (pin 6) is left floating. The...

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