The circuit presented was suggested by Texas Instruments in an opto-electronics manual. The transistor can be the BC548 and the supply voltage depends on the application.
This circuit triggers a relay when the detected humidity falls below a value. The system can be...
We found this circuit in an old General Electric documentation. In fact, PUTs are not common...
This alarm circuit was found in a 1997 publication. It uses two reed-switches to arm and disarm the...