Figure 1.
Circuit Diagram for a 555-based Light Dimmer (PNP 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., as a
circuit whose output at pin 3 is a continuous square wave. The
values of the resistors and capacitor connected to pins 6 and 7 of the
555 determines not only the frequency of the output square wave, but its
duty cycle as well. The duty cycle is simply the percentage of
time that the output of the 555 is '1' in one period of the wave.
A 555 output
of 1 turns 'on' the medium-sized NPN transistor 2N3053, causing it to pull down
the base voltage of the output power transistor 2N2955. Since the
2N2955 is a PNP transistor, this causes the 2N2955 to conduct current
and light up the bulb. Thus, the higher the duty cycle of the 555's
output, the longer the bulb is turned 'on' and the brighter it appears.
The 2N2955 is a power transistor capable of handling the high current
required to light up the bulb.
Varying the
50K potentiometer in the circuit varies the duty cycle of the 555, 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
single NPN transistor at the output...
Learn more about the 555 Timer IC and
its applications...
Go
Back to the PROJECTS Menu
HOME
