With this circuit it is possible to fire an SCR from the light that falls on a phototransistor. The circuit has a lock, since when it is triggered it remains so even after the light that hit the phototransistor no longer exists. The circuit can work with relays from 6 to 12 V and the supply must be made with a voltage at least 2 V higher than that of relay activation to compensate for SCR drops. Sensitive reed relays are recommended, and the phototransistor can be of any type. Optical features in the phototransistor such as lens and tube help achieve directivity and greater sensitivity. In the waiting condition the consumption of the circuit is extremely low. To reset, simply turn the power off and on for a moment. The SCR can be the TIC106.
