This circuit is from a 1978 Texas Instruments optoelectronics manual. The trimpot adjusts the LED current. Other currents can be adjusted if they are within the capacity of the FET.
This TTL circuit uses LED indicators and must be powered by a voltage of 5 V. The capacitor...
Low power RF transistors such as the BF494 or BF495 and even those in the BC series can oscillate well...
This switched power supply was found in a 70s documentation. The basic components are from that...