(Circuit from BCRA Cave Radio and Electronics Group Newsletter Vol.1 No.1
Webeditor's note: Please see the Letter to the Editor in C.U. 1990 before taking any notice of this article.
This neat little circuit avoids having to use a battery by using some neat properties of doped silicon.
The lack of a battery has the obvious advantage of size, which is a major bulk element in many slaves. It will of course never annoy by running flat and may be completely cast in resin to keep the wet out. It has been suggested that this fact makes it suitable for use under-water, and if anyone could think of a way of keeping the water out of the flash unit as well this might be an advantage.
In brief the circuit works by dint of the production of electron-hole pairs by photons which are incident on the base region of the photo-Darlington transistor (2N5777). The minority carriers (electrons) drift out of the base region and set up a potential difference which enhances the efficiency of the device. The SCR (C109) is triggered into conduction when the gate potential makes the threshold. This forms a low resistance link between the flash contacts via the bridge network.
The flash must be of the Xenon type since the low resistance state is only maintained for a very short time before the photoelectrically produced p.d. is zeroed by dissipation.