We found this CMOS circuit for excitation of a relay in an old documentation. The transistor is a general-purpose high voltage as used in the power supply.
With this circuit, signals in the range of 10 kHz to 150 kHz can be received at close to 4 MHz...
This limiting circuit with an operational amplifier has an adjustable cut-off point and can use...
Published by Philips in 1965 documentation, the sensor for this circuit is a common LDR. The...