12v LAMP DIMMER
Here is a 12 volt @ 2 amp lamp dimmer that can be used to
dim a standard 25 watt bulb by controlling the duty cycle of a
astable 555 timer oscillator. When the potentiometer is at the
up position, the capacitor will charge quickly through both 1k
resistors and the diode, producing a short positive interval and
long negative interval which dims the lamp to near darkness.
When the potentiometer is at the lower position, the capacitor
will charge through both 1k resistors and the 50k potentiometer
and discharge through the lower 1k resistor, producing a long
positive interval and short negative interval which brightens the
lamp to near full intensity. The duty cycle of the 200Hz square
wave can be varied from approximately 5% to 95%. The two
circuits below show how to connect the lamp to either the
positive or negative side of the supply.
But the first circuit has a mistake and some components are
not needed. The 555 will sink 300mA and it can be connected
directly to the output transistor - you don't need the buffer
transistor.
When the 555 goes HIGH, the voltage on Pin 3 is 1.5v lower
than the 12v rail and thus the transistor does not turn off. The
two diodes in the circuit below are needed to drop an additional
1v so the transistor turns off.
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