The Led torch with adjustable brightness circuit uses LM3914 as the basis of a 10 step variable brightness current-regulated white LED . This circuit has only four components in the control and regulation circuit: R1,R2, VR1 and LM3914.
1. IC – LM3914
2. LED1-10 – 10 white led
3. R1 – 620 ohm
4. R2 – 100 ohm
5. VR1 – 100 k pot
6. C 1 – 10 μf 16 v
7. Power – 3-4.5 volt
8. Suitable cabinet and a general purpose PCB.
How it works?
The LM3914 is set to operate in bar graph mode so that the LEDs light progressively as its input signal increases.This signal comes from VR1, which provides a variable voltage between 0V and 4.5 v(supply voltage) to pin 5 of the LM3914. The internal resistor ladder network of the LM3914 has its low end (pin 4) connected to ground and the high end (pin 6) connected to the supply voltage via R2. Resistor R1 (620Ω) on pin 7 of IC1 sets the current through each LED to about 20mA. Set value of R1 as per input voltage. As VR1 is rotated from the 0V position (all LEDs off) to the supply voltage position (all LEDs on), the LEDs will progressively light. When all LEDs are off, the circuit will draw about 5mA. Whereas when all LEDs are illuminated, it will draw about 205mA and dissipate 307mW with a 4.5V supply.
Reference voltage at pin5 is given by- Vout=1.25(1+R2/R1)
The device dissipation will depend entirely on the input voltage and LED forward voltage. A resistor (R3) is inserted in series with the positive supply, chosen so that the LM3914’s dissipation is limited to about 500mW.