# Learning Electronics

Learn to build electronic circuits

# Brightness Controller Circuit For Small Lamps and Leds

Switch-mode driven 1.5V bulb, Portable unit - 3V battery supply

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.

Circuit diagram:

Parts:

P1 = 470K
R1 = 10K
R2 = 47K
R3 = 1.5K
C1 = 22nF-63V
C2 = 100uF-25V
D1 = 1N4148
D2 = 1N4148
Q1 = BD681
B1 = 2xAA cells in series
IC1 = 7555 or TS555CN
LP1 = 1.5V 200mA Bulb
SW1 = SPST Switch

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.

Source: Red Free Circuit Design