Actually, the laser tripwire was an interesting problem to try to solve. In a standard transistor-SCR (thyristor) circuit, you often see an LED where I put the relay.
The SCR is advertised as a latch-on device--once switched on, it is supposed to stay on, but it don't work as advertised:
If you replace the relay with an LED and break the laser beam (or other light source), that is, stop the light source from shining on the LDR (light dependent resistor) the LED switches on as advertised. When you restore the light shining on the LDR, however, the LED switches back off again.
So, I decided to insert the relay to switch on another circuit that would stay on even when power was cut to the relay. Oddly enough, the relay somehow keeps the SCR switched on but I'm not sure why.
In a single stroke (and by sheer accident!) I solved the problem and can now switch on any external circuit via the relay without having to add any additional components to make the externlal circuit remain on. And it will remain on until I switch the power off to the laser tripwire circuit.
Wierd. But it may be useful to others working on any number of "electric eye" applications
For complete instructions to build the laser tripwire and alarm, go to: