Dimmer for LED flashlight

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LED flashlight dimmer prototype

A simple PWM dimmer for a single cell lithium-ion LED flashlight

This simple circuit makes it possible to adjust the brightness of a high power LED flashlight without wasting energy in large resistors. A voltage doubler is used to make sure the MOSFET gate is fully turned on even when the battery is dropping in voltage.

None of the component values are very critical.

  • C1 should be selected to give a PWM frequency of about 2 kHz
  • C2 should be significantly larger than the gate capacitance of the MOSFET
  • RV2 must be adjusted so that the MOSFET turns fully on at 2.85 V battery voltage
  • The 4093 may be replaced by a 74HC14, other CMOS inverters may work but will be more prone to instabilities.

Flashlight-dimmer.png

 U1 - 4093 quad NAND CMOS Schmitt trigger
 Q1 - MOSFET power transistor, for example IRFU020
 C1 - PWM timing capacitor
 C2 - Voltage doubler storage capacitor
 R1 - Discharges the storage capacitor to make sure the transistor does not get stuck on
RV1 - Adjusts the duty cycle
RV2 - Adjusts the gate bias for a given transistor
 D1 - Voltage doubler charge diode
 D2 - White power LED (must be white or you may need a series resistor)
 D3 - Discharges the timing capacitor
 D4 - Charges the timing capacitor


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