This circuit is based on the light comparison of two LDR sensors. We found it in a 90s Popular Electronics. The transistor must be according to the indicator or load being driven.
Found in a 90s Popular Electronics, this circuit checks if a remote-control transmitter is...
This relay circuit with a dual channel MOSFET (not very common today) was found in an old American...
This application is not modern since few computers have parallel ports. But you can use this...