This high impedance headphone amplifier was found in American documentation from the 1990s. Any general-purpose FET can be used.
Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the...
This circuit is suitable for use in radio receivers to improve the quality of weak signal...
Using an LDR as a sensor, this circuit from a 90s Hands-On Electronics triggers a relay in the...