Description
5 VOLTE POWER SUPPLY MODEL
Trigger
Source - http://www.electronguide.com/circuits_pages/InstantFlashTriggerForPhotography.html
The heart of my flash trigger is an LM555 timer and a DM7473 flip-flop. The 555 provides an adjustable delay between an event being triggered and the flash being fired — for example, we want to wait for a falling drop to fall a few more inches before the flash goes off. The flip-flop triggers at the end of the delay and fires our flash.
A circuit is required that would give a change in voltage when a light beam is interrupted. A phototransistor allows current to flow only when light is present. 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.
Now, instead of GIVING 5V normally, we can divert some current to ground until we’re only GIVING 2V. A 1V drop in voltage will reduce our output to 1V, which is low enough to trigger the 555.
Circuit Diagram
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. 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. 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.
Now, instead of output 5V normally, we can divert some current to ground until we’re only outputting 2V. A 1V drop in voltage will reduce our output to 1V, which is low enough to trigger the 555.
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