Dimmer for LED flashlight
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.
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