Brightness control of small lamps

Circuit diagram:

Component:

P1________________470K Linear Potentiometer

R1_________________10K 1/4W resistor

R2_________________47K 1/4W resistor (see precautions)

R3__________________1K5 1/4W resistor

C1_________________22nF 63V Polyester Capacitor

C2________________100µF 25V electrolytic capacitor

D1,D2___________1N4148 75V 150mA diodes

IC1_______________7555 or TS555CN CMOS timer integrated circuit

Q1_______________BD681 100V 4A NPN Darlington Transistor

LP1_______________1.5V 200mA Bulb (See Precautions)

SW1_______________Single-pole single-throw switch

B1__________________3V (two 1.5V AA or AAA batteries connected in series, etc.)

Circuit operation:

This device was designed to control the light intensity of four incandescent bulbs (i.e., a ring illuminator) powered by two AA or AAA batteries for close-up photography with a digital camera. Obviously, it can be used in other ways according to anyone's wishes.

IC1 generates a 150Hz square wave with a variable duty cycle. When the slider of P1 is fully turned to D1, the output positive pulse appearing at pin 3 of IC1 is very narrow. The bulb LP1, driven by Q1, turns off because the voltage on its leads is too low. When the slider of P1 is turned to R2, the width of the output pulse increases, reaching its maximum amplitude when the potentiometer is fully rotated clockwise. Thus, the bulb reaches its full brightness.

Precautions:

LP1 can be one or more 1.5V bulbs connected in parallel. The maximum allowable total output current is approximately 1A.

R2 limits the output voltage (measured on the LP1 lead) to 1.5V. Its actual value depends on the total current consumed by the bulb and should be set at full load so that approximately 1.5V is obtained on the bulb lead when P1 is fully rotated clockwise.