This circuit activates a relay after a time given by C1 and R1. The circuit is from a Radio Electronics from December 1974. The relay voltage is the same as that used in the power supply.
Old multimeters used a 22.5 V battery for high resistance measurements. If you have one, you...
This circuit has an opposite action to the previous one, triggering when light falls on the...
This circuit was found in a Popular Electronics from 1991. The transistor could be the BC548. The...