This circuit was obtained in a 1964 Electronic Design. Equivalent modern transistors can be used. The circuit requires a 6 V symmetrical power supply.
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...