This 4-transistor amplifier can work from 4.5 or 6 V power supplies. We found it in a Mullard transistors manual from 1960. The transistors are from the time.
Any power MOSFET that supports the current of the controlled stepper motor can be used on this...
This QST circuit generates 4 frequency signals according to the crystals used. Transistors admit...
This circuit was found in a Popular Electronics of the 1990s. The sensor is a rotating mechanical...