For this project I wanted to create a compact camera flash using LEDs apposed to typical flashes which use xenon flash tubes and bulky capacitors, requiring complex, high voltage driving circuits. The flash has to be light, powerful enough to shoot a portrait at night and function with hot shoe based cameras.

I started with the design of the electronics since they decide the size of the flash housing. The flash is triggered by the camera, which completes a circuit between two contacts protruding from the flash base and interface with the hot-shoe flash mount. The flash needs to be switched on for 1-5 milliseconds immediately after the circuit is completed and turn off regardless of when the circuit is broken.

I started experimenting with a resistor-capacitor circuit which allowed for a sudden voltage pulse. The issue with this is that the capacitor fills gradually and gives a decaying voltage at the end of the pulse (right image). Because of this I ditched the RC circuit and turned to a 555 timer, this timer can provide perfectly square pulsed which allows the flash power to be maximised for the short period it is turned on.

Above is the 555 Timer circuit I've ended with for prototyping. I used the following tutorial - www.electronics-tutorials.ws/waveforms/555_timer.html - to create a 'monostable' output from the timer, which gives a single, pulse output. The time of this pulse is found via 1.1xR1xC1, I've made it about 24 milliseconds here for testing (1.1x2200x0.00001). One thing I added to the circuit is the parallel capacitor and diode seen at the bottom of the diagram, this is because the 555 timer will not end the output pulse until the input (negative) pulse has ended. The introduction of the capacitor means this input voltage is quickly neutralised by the charge of the capacitor. After the output pulse, the diode then dissipates this charge so the flash can be once triggered again.

I made a rough CAD file for an initial prototype to test the LEDs and triggering mechanism.

Here is the crude looking prototype. It has 8x 3W LEDs which will be powered through a 12v booster and darlington transistor which is triggered by the 555 timer output. The LEDs are only in use for a short period so overheating is not an issue, I plan to overload them which may compromise their brightness overtime with electrical stress so I will replace them and adjust the power where needed.

- get booster and test final circuit, running on 2xAAA

- Measure components in final circuit and redesign shell.