This automatic light dimmer circuit makes it possible to control a lighting system so that it turns on or off slowly. The circuit works this way: when switch S1 is closed, the capacitor C1 is slowly charged. Once the voltage at C1 reaches 0.6, transistor T1 begins to conduct and the LED also begins to light. If the capacitor voltage increases further, then transistor T1 conducts more current and in return the LED lights brighter.

This audio mixer circuit uses an LM3900 IC but is not a profesional audio dj mixer. The IC houses four integrated Norton amplifiers. The advantage of using the four op amps is that they only need a single power supply. Since this amplifier circuit is current controlled, the DC bias is dependent on the feedback coupling.

TDA1524A is designed as an active stereo-tone/volume control for car radios, TV receivers and mains-fed equipment. It includes functions for bass and treble control, volume control with built-in contour (can be switched off) and balance. All these functions can be controlled by d.c. voltages or by single linear potentiometers.