This circuit was found in a Texas Instruments documentation from the 1980s. The circuit monitors engine temperature by warning of engine warm-up and warning when the fan is on.
This circuit was found in the April 1968 Radio Electronics magazine. The circuit uses two FETs...
This 1-transistor circuit for currents up to 50 mA was found in the August 1974 Radio Electronics...
This circuit, which can also be used with flashing LEDs as long as they have in parallel a charge...