This slave trigger supports 2 external flash units which can be of different models (e.g. HV and LV). It outputs a pulse to trigger the external flashes once counted the 2nd pulses. It is compatible with digital cameras which use a double flash. 

The circuit is pretty simple and can be built without using PCB. 

Parts

I modified a small LED reading light to accommodate the electronics. It saved me some work on building the battery compartment and the mounting bracket of the solar cell.

The solar cell is salvaged from a dual power calculator.

I used phone jack instead of PC jack because it is much cheaper.

Other parts should be readily available.

Principle of Operation

It is basically a 4-bit counter circuit. It outputs a pulse upon receiving the 2nd pulse from the solar cell. It is important that the reset switch is pressed prior to pressing the shutter (a bit user unfriendly) or it won't work.

The solar cell needs to face the built-in flash within a few inches in order not to be miss-triggered by flashes of other cameras at the same scene. It needs to be tightly coupled to the built-in flash if the ambient light is bright.

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End of the project

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