Light Bulb Regulator
This circuit is a basic phase controlled triac, which is what light dimmers consist of, except the
control knob is replaced by a light dependant resistor LDR1.
LDR1, along with R3, is used to control the conduction angle of SCR1 (triac). The value of R3
is chosen so that when LDR1 is shielded from ambient light with a piece of black tubing
that is clamped on both ends, the lamp just begins to glow. Under normal operating conditions
LDR1 is shielded with a piece of black tubing that is open at one end. This end faces the lamp
so that LDR1 can sense the brightness of the lamp.
When power is initially applied to the circuit, LDR1 has a high resistance which makes R3
cause SCR1 to conduct. The light from the lamp causes the value of LDR1 to drop, which
causes SCR1 to conduct longer. This in turn causes the lamp to glow brighter which makes the
value of LDR1 drop further. A complete chain reaction is started which
leads to LDR1 reaching its lowest value and the lamp its maximum brightness in less than
a second. R1 and C1 have been included in the circuit to suppress any noise generated by SCR1.
The circuit is simple and may find many practical uses where one has to replace bulbs very
often.
If a manual control is needed, then replacing LDR1 with a 100Kohm potentiometer may satisfy
your needs.