Found in a 1990s US publication, this alarm circuit detects when a light beam is interrupted. Optical resources may be used for the sensor and instead of the alarm a relay may be used.
We found this heart rate sensor circuit in a Popular Electronics from the 90s. The placement of...
Based on an LM317T, this circuit found in Popular Electronics works with signals of frequencies up to...
The circuit shown is from a 1972 publication. Transistors can be current types like 2N2222. The...