Gadget Freak

Photography enthusiast Nick Pagazani was hindered by the range and fixed location of his camera's flash.

Since his camera has no connector for an external flash, he needed a light-activated slave trigger to fire a remote strobe. It had to ignore the pre-flashes used for red-eye correction and fire only on the main flash.

His solution: Use a microcontroller to count pulses from a phototransistor and trigger the strobe at a switch-selectable count. Brighten up your shadowy background with this flashy accessory.

You can download the build instructions in this zip file.

This Gadget Freak originally appeared in Design News.

Circuit Description

For the microcontroller I decided to use the PIC12F629 for its small size and the fact that it has an internal oscillator to help reduce the part count. The main portion of the circuit is the sensing circuit.

The phototransistor senses the ambient light and outputs a current based on the amount of light. Since we don't care whether it's light or dark, we use capacitor C2 to filter out that signal and only allow a sudden change in voltage to be passed. The capacitor and drain resistor were carefully chosen so that only a flash would create a voltage signal and not normal ambient light changes or fluorescent lights that flicker. This small flow in current opens transistor Q1 to put a "high" signal at GP0. This is the signal that tells the PIC that a flash has been sensed. The switch SW1 simply tells the PIC whether to trigger the flash output on the first flash, or the second.

The PIC triggers an SCR to fire the strobe. The SCR can block the high voltage and conduct the current surge required to fire a conventional strobe. It will also fire a strobe with a low-voltage trigger input without latching ON. For lowvoltage trigger inputs only, a second NPN transistor (I used a 2N2222) may be used instead of the SCR (See "alternative A" on the Schematic).

The connections at J1 connect to the external flash. For the flash, I use a standard hot-shoe mounted flash from my old 35mm SLR. I bought a hot-shoe adapter that allows the flash to mount to a tripod and has an external cable for connecting the flash to an external connector. I simply cut the connector off and connected the wires directly to the board.

Code Description

The code starts off by first setting up the usual parameters, one of which is the use of the internal clock, which helps make for a smaller layout. The main program begins at "main1" and starts by looking at the port GP0 and waiting for the line to go high. When it goes high, this means that the circuit has sensed a flash of light. The code then checks the state of SW1.

If it's open, then the next line executes and jumps to the flash routine to trigger the flash output. If the switch is closed, the code then checks to make sure that the input signal has gone back to 0, and then waits for the second flash signal to arrive.

The second flash signal is generally only a few milliseconds after the first, so the sensing circuit needs to dissipate the signal fairly quickly. Once the second flash signal is sensed, the flash is triggered and the program starts back at the "main1" loop.

Parts List

Amt	Part Description
1	PIC12F629 I/P microcontroller
2	0.1uF capacitor
2	1/4W 1 k Ohm resistor
1	1/4W 4.7 k Ohm resistor
3	1/4W 10 k Ohm resistor
1	1/4W 1 M Ohm resistor
1	2N2222 (or other) general-purpose transistor
2	SPST toggle switch
1	NTE5457 SCR (see schematic for alternate version)
Additional parts required
1 NTE3120 (or similar) Photo Transistor
1 3V Lithium Battery and holder
Nuts and washers for the toggle switches
Kaiser Flash Shoe Adapter, available at www.bhphotovideo.com
Photoflash Strobe
PIC development kit & software to program the PIC: K8048 (kit), Vm111 (assembled), available at www.apogeekits.com - requires RS232 port and cable. USB versions are also available.

This picture was taken with Nick's digital point & shoot and he says you can see that the flash triggered with perfect timing!