Found in a 90s documentation, this circuit makes use of D1, a switching diode that is uncommon today. D3 is an LED. The circuit monitors 12 V batteries.
This circuit, from Texas Instruments documentation, fires a relay when light strikes the...
This circuit was found in documentation from the 1980s. The circuit can be implemented with other...
This circuit is from an 1977 American publication. The coil consists of 30 + 70 turns of 28 AWG...