Flash triggering circuit

Posted on Dec 24, 2012

My first project after assembling an electronic design lab was to build a flash trigger that I could use for high-speed photography. I thought it would be useful to share not only the finished product but also the reasoning that went into its design — in the hopes that others will learn from and improve upon it. When I first thought about building a flash trigger, I did some research to see if anyone had a ready-made schematic available. It turns out that Johannes Eriksson has done just that — and kindly provided a schematic and a brief overview of the circuit. I pored over the circuit until I had a tentative understanding of how it worked, then set about designing my own. Since our circuit will use 5V all over the place, our first task is to turn a 9V battery into the voltage we need. Maxim’s MAX603 linear regulator is a handy chip that will output 5V given a wide range of input voltages — with a few supporting capacitors, the chip will do all the work for us. I simply followed the suggested circuit in the datasheet.Next, I needed a circuit that would output a change in voltage when a light beam is interrupted. I turned to Jameco for an infrared LED and matching phototransistor. Fairchild Semiconductor has a good application note about designing with phototransistors — essentially, a phototransistor allows current to flow only when light is present.

Flash triggering circuit
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Flash triggering circuit - image 1
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A large resistor between the phototransistor and ground causes 5V to be output when light is present, but pulls the output to ground when the beam is interrupted.Unfortunately, there’s bound to be ambient light hitting the phototransistor even when the LED beam is interrupted. The phototransistor may only reduce the voltage at its output by 1V when the beam is interrupted — we normally output 5V, and the 555 timer won’t trigger until the voltage dips below 1.67V, so the change in voltage won’t even be noticed. However, we can divert some of the current from the output of the phototransistor to ground with a variable resistor — this allows us to subtract a constant voltage from the circuit’s output. The output from our phototransistor circuit is connected to the 555 timer’s trigger pin. When it detects a low pulse, it puts 5V on pin 3 and begins to charge the .22uF capacitor on the left. The resistor and capacitor on the left form an RC circuit — the lower the resistance of the variable resistor, the faster the capacitor charges. When the capacitor is nearly full, the 555 drops pin 3 back to ground.Since the 555 timer’s output sits at 5V for an amount of time dependent on the variable resistor to the left, we can use it to delay the time between the 555 being triggered and our flash being fired. In order to do so, we need a component which will wait for the output to go from ground to 5V and back to ground again — and that’s...

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