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 circuit of an Electronics Hobbyists Handbook uses a different configuration for a triac. The...
We found this circuit in the July 1988 Radio Electronics magazine. The frequencies are given by...
Found in an English documentation from the 90's, this circuit makes slow logical pulses audible,...