Infrared Flashlight Modification

Overview

This is a very simple modification to a common 16-element Light Emitting Diode (LED) flashlight which is sold at most hardware stores.

The stock flashlight has 16 white LEDs wired in parallel and is powered from three 1.5 volt "D" batteries.  The internal resistance of the three batteries limits the total current draw, so the overall flashlight circuit design is very simple.  Just some batteries, LEDs, wire, and a switch.

To turn this device into a useful night vision aid, all you'll need to do is replace the white LEDs with ones that transmit in the infrared spectrum.  These are available from Mouser or Digi-Key, or you can scrounge them from old remote controls.  If your wallet is big enough, Radio Shack also carries them.  Be sure to get through-hole, T1-3/4 (5 mm) size, 780 nanometer wavelength (anything between 780-940 nm will work) infrared LEDs.  You can also fiddle with different optical power outputs and beamwidths.  Higher output power LEDs will draw more current, which will reduce the lifespan of the batteries.  Lower optical beamwidths will give the flashlight a narrower focus.

After replacing the white LEDs with infrared LEDs, you'll need to then add a series 5 ohm, 2 watt resistor (or two 10 ohm, 1 watt resistors in parallel) to make up for the differences in voltage drops between the white (3.6V) and infrared (1.2V) LEDs.

That should be it.  This is a handy device for taking pictures of documents covertly (with a modified CCD camera) or for adding "night vision" capabilities to a video camera.  The range isn't too great, though.  You can also use red LEDs instead of infrared LEDs if you wish to make a low-observable flashlight, like for map reading or signaling.  Increase the series resistor value to compensate for the 1.8 volts red LEDs normally use.  Battery time can be extended by "pulsing" the LEDs using a 555-timer (50 Hz, 50% duty cycle) and a MOSFET power driver controlling the LEDs in series.

When not in use, the flashlight makes a good device to bash Emmanuel Goldstein's head with.

Pictures

Overview of the stock flashlight and the new infrared LEDs which will replace the white LEDs.

Taking it apart.  The reflector just slips over the LEDs.  The RED wire is for +4.5 VDC from the batteries, and the BLUE wire is GROUND.

Solder side of the LED PC board.  The outer ring is GROUND the inner ring is +4.5 VDC.

Picture of the plastic reflector removed from the LEDs.

Removed the old white LEDs and started inserting the new infrared LEDs.  Be sure to watch the polarity on the LEDs!  The "flat" side of the LED is the GROUND.

The infrared LEDs are installed and a series 5 ohm power resistor in soldered to the + ring.

Infrared flashlight in operation.  Infrared light is suppressed in most CCD or video cameras, so the beam doesn't look very bright.







Schematic