Circuit diagram:
Parts:
P1________________470K Linear Potentiometer R1_________________10K 1/4W Resistor R2_________________47K 1/4W Resistor (See Notes) 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 IC Q1_______________BD681 100V 4A NPN Darlington Transistor LP1_______________1.5V 200mA Bulb (See Notes) SW1_______________SPST Switch B1__________________3V (Two 1.5V AA or AAA cells in series, etc.)
Circuit operation:
This device was designed on request, to control the light intensity of four filament lamps (i.e. a ring illuminator) powered by two AA or AAA batteries, for close-up pictures with a digital camera. Obviously it can be used in other ways, at anyone's will.
IC1 generates a 150Hz square wave having a variable duty-cycle. When the cursor of P1 is fully rotated towards D1, the output positive pulses appearing at pin 3 of IC1 are very narrow. Bulb LP1, driven by Q1, is off as the voltage across its leads is too low. When the cursor of P1 is rotated towards R2, the output pulses increase in width, reaching their maximum amplitude when the potentiometer is rotated fully clockwise. In this way the bulb reaches its full brightness.
Notes:
- LP1 can be one or more 1.5V bulbs wired in parallel. Maximum total output current allowed is about 1A.
- R2 limits the output voltage, measured across LP1 leads, to 1.5V. Its actual value is dependent on the total current drawn by the bulb(s) and should be set at full load in order to obtain about 1.5V across the bulb(s) leads when P1 is rotated fully clockwise.
