This circuit was obtained in a Hands-On Electronics from the 90's. The circuit turns on one LED at the end of the timer, erasing the other. The timing can be increased by increasing C1.
When the sensor, an ordinary photo-transistor, receives light, the relay closes its contacts. The...
We found this circuit in a 1972 documentation. The transistor, in fact, is a unijunction that can...
This circuit is from a 2010 publication by Texas Instruments. The circuit interfaces a...