This circuit is from the National Semiconductor's Linear Applications Handbook of 1994. The circuit can be implemented with equivalent operational. The detected voltage is given by the diode.
Obtained from a 1969 documentation, this circuit can operate at frequencies up to 500 Hz, counting up...
We found this circuit in a Popular Electronics of the 90s. The circuit still uses light bulbs, but...
Obtained in an Argentine publication from 1973, this circuit sends the signal from a small record...