QTC is an acronym for Quantum Tunnelling Composite.
Thats fancy high tech terminology for what is in practical terms, a pressure sensitive variable resistance material.

In the flashlight world, the raw form of QTC is what looks like a small square of thin greyish-black rubber, approximately 4mmx4mmx1mm.

 

Below: A square tab of QTC atop the momentary switch



QTC reacts to pressure, the more pressure applied to it, the lower its resistance (and the higher its electrical conductivity). This translates into a very efficient way to develop small, variable resistance switches with high power tolerances that would be extremely challenging (and for the moment, impossible) to achieve using traditional circuitry whilst still maintaining such a small footprint.

What can I do with QTC?
The QTC offers a simple way to experiment and add variable brightness to your HF-R momentary switch.

How to use it with the momentary switch?
Remove the tail cap from the end of the momentary switch.
Place the QTC on the center contact plate.
Replace the tail cap until its firm.

As you tighten the tail cap, pressure is applied to the QTC which in turn, allows current to flow through to the LED. The more you tighten it, the more current will flow and the brighter the output of the LED.
Simply loosen the tail cap to switch it off (and prevent current flowing to the LED).

Note:
The QTC is included for experimentation purposes only and whilst its very safe to use, please keep an eye on the following points:

 

  • Be sure the tail cap is not so loose that it unscrews itself.
  • After being compressed, the QTC can take a few seconds to return to its normal decompressed size. So although the light appears OFF, you might notice its actually ON again after a couple of seconds. Loosen the tail cap just enough to ensure it is in fact OFF.