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.
This Schmitt trigger in current mode for frequencies up to 50 MHz was found in a 1969 Motorola manual....
VR can be from 50 to 220k it is recommended to connect a 100-ohm resistor in series. The lamp may...
This circuit was published in a 1977 English magazine, but remains current, since we can assemble it...