There are times when a little light inside the car
would greatly assist one of the passengers but the dome light is too bright for safe
driving. The dimmer circuit in fig. 1 may be added to an existing dome light or included
with a new passenger spot lamp.
The upper op-amp generates a 700 Hz sawtooth waveform which is compared to a setpoint
voltage by the lower op-amp. When the sawtooth voltage is above the setpoint, the
transistors turn on supplying current to the bulb.
The setting of the potentiometer determines the width of the
pulses sent to the lamp and therefore the average voltage. The lamp is dim when the
potentiometer is set near the higher voltage. Since the TIP32 switches on and off instead
of simply dropping the voltage like a power rheostat, the power it dissipates remains low
and a heat sink is not necessary.
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Many autos run power to lamps with only one wire using the car
body for the return current path so the dimmer must interrupt the positive lead as shown.
Simply cut the wire leading to the lamp and connect the lamp end to the collector of the
TIP32 and connect the battery end to the circuit power input. Run an additional ground
wire to the auto chassis from the circuit. This ground wire will not carry much current
and may be a smaller gauge.
