Figure 1.
Circuit Diagram for a 555-based Light Dimmer (NPN Output)
This is a
light dimmer circuit that is based on the 555
Timer IC. This circuit is more efficient than the simpler
transistor-based
'light dimmer' circuit.
The
555 timer in this circuit is configured as an
astable multivibrator, i.e., its
pin 3 output is a continuous square wave. The
resistor and capacitor values at pins 6 and 7 of the
555 determines not only the output square wave frequency, but the wave's
duty cycle as well. The duty cycle is simply the percentage of
time that the output of the 555 is 'high' in one period of the wave.
A 555 output
of 1 turns 'on' the NPN output transistor 2N3055, causing it to light up the bulb.
The higher the duty cycle of the 555's
output, the longer the bulb is turned 'on' and the brighter it appears. Varying the
50K potentiometer in the circuit varies the duty cycle of the 555's
output wave, and
therefore also varies the brightness of the bulb.
For formulas on
how to compute for the frequency and the duty cycle of the astable 555
based on its resistor and capacitor values, read the article on the
555 astable multivibrator.
See a similar circuit using a PNP
output transistor...
Learn more about the 555 Timer IC and
its applications...
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