This circuit uses an MC3392 low side protected switch and an MC1455 timing circuit to form an automotive instrumentation panel lamp dimmer control. The brightness of incandescent lamps can be varied by Pulse Width Modulating the input of the MC3392. The modulating signal can be obtained directly from the MC1455 timer (or a microprocessor). The MC1455 is configured as a free-running clock having a frequency and duty cycle control. The typical timer frequency is approximately 80 Hz when the frequency potentiometer is adjusted to 1.0k. This frequency was chosen so as to avoid any perceptible lamp flicker. The duty cycle potentiometer controls the duty cycle over a range of about 3% to 97%. When at 3% the lamps are essentialy off, at 97% the lamps are essentially full on. Any number of lamps can be control, so long as the total load current is less than 1 amp. The LED is used to signal the existence of a system fault (overvoltage, current limiting, or thermal shutdown).
Source: Electronics Lab