A "slave flash," which is also called a "speed light" or "strobe," is a flashgun that sits still until it sees a bright light. The slave flashes when it sees a flash. When you use flashguns that aren't attached to the camera, you

can get great, dynamic lighting that would be hard to get any other way. The "Optical Slave Flash Trigger" is an off-camera flash trigger accessory that is shown here. It is very useful if you use a speed light. It's just a very good light sensor that picks up light pulses and wakes up the electronics, which then turns on the speed light that's attached. Project design The main idea is to make things small, cheap, separate, and able to work on their own. The best way to do this is to make the heart of the circuitry out of cheap and easy-to-find parts. For such a simple design, I'd rather not use a microcontroller, even a cheap one (it would be overkill). The design is

Use a standard light sensor to pick up the light pulse from the master flash. Make sure the light intensity is higher than a predetermined baseline (which can be altered using a potentiometer to tweak the detection sensitivity) Use an electronic switch that doesn't make contact to turn on the save flash. The Wheatstone bridge is the idea behind how this little circuit works. When light hits the phototransistor (PT1), its conductivity (and current) go up. This causes the driver transistor (TR1) to be biased in the forward direction. To initiate the slave flash, the resulting electrical current activates the opto-isolated triac (OT1) (thanks to an optical coupling, it enables safely to use even the oldest flashgun that could otherwise damage your

circuitry by high voltage ignition). But you should know that the connected slave flash doesn't work when there's too much light around, like when it's bright outside. In the same way, it may misfire if you put a very bright artificial light source in its path. Here, the potentiometer (RP1) helps you set the right light detection threshold. Schematic Parts Selection A 5mm round phototransistor is used as the light sensor in this case. The type is a silicon NPN phototransistor that works quickly and well. Because the device is made of clear epoxy, it works with both visible light and infrared radiation. The opto-isolated triac in the circuit is a type MOC3021, which is made up of a gallium arsenide infrared emitting diode and a silicon bilateral switch that are connected optically. The whole circuit can be powered by the CR2032 Lithium Manganese Dioxide battery, which has a nominal voltage of 3V and a nominal capacity of 220mAh.

Interconnections A female hotshoe is needed to connect the slave flash trigger circuit that has been put together to the slave flashgun. When the flash is in the hotshoe, all you have to do is connect the two trigger contacts. So, just use a (built-in or freestanding) sync cable to connect the switch outputs (SW1 and SW2) of the circuit to the electrical contacts (triggers) of the hotshoe.

Flash and the Camera In the range of artificial light sources, 380-750nm, Xenon is known to come close to the "natural" colour spectrum of the sun. Xenon is used a lot in camera flash systems because of this. In an electronic flashgun, an electric charge is stored in a high-voltage capacitor until it is needed. When the flash is fired, the capacitor's stored energy is released through a flashtube. When Xenon gas is put into the flashtube, it gives off a very bright light for a very short time.