This circuit is from the 1968 ARLL documentation. The MOSFET admits equivalent and the coil determines the operating frequency range being the plug type.
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...