November 2, 2019

Flashlight tailcaps

You might think this is trivially simple, but there is more to it than you might think. I am aware of 3 different types on the flashlights I own.

Normal

On this type the flashlight stays off until you make the button click. After it is on, partially depressing the button will "open" the switch. Partially depressing the button when the flashlight is off does nothing.

This is called "reverse clicky" and is the most common.

Tactical

On this type you can turn the flashlight on by fully clicking it just as above. Partially depressing the button when it is on does nothing. Partially depressing the button when it is off turns it on as long as you hold the button in the partially depressed position.

This is called "forward clicky". Apparently a true tactical switch will never click and latch itself on.

Illuminated button

This is a modified version of the normal type. The button itself is illuminated when the flashlight is off (so you can find it in the dark). When the flashlight is on, the button is off.

The wiring in such a tailcap is simple. Just a pair of LED and resistors wired across the switch. When the switch is off, they get powered (something on the PCB up near the LED must deliver current), but when the switch turns on (closes) they get robbed and go off.

That much is simple. The "driver board" (the PCB up near the flashlight emitter itself) will typically need to be modified to work with an illuminated tailcap. The necessary modification is a "bleeder resistor" that supplies current from the + end of the battery to the tailcap.

The big question is why my illuminated tailcap does not work with the biscotti firmware I flashed to this flashlight.

Another question is how much current this robs. My tailcap has a pair of 360 ohm resistors. The writeup below says that as little as 0.02 mA can be arranged.

Also, curiously enough, on the driver board, I measure 80K ohms between the + battery terminal and the flashlight housing -- yet this light lights up the tailcap nicely.

Dig into the illuminated button details

A useful trick is to limit google searches to a particular website using "site:budgetlightforums.com".

Have any comments? Questions? Drop me a line!

Tom's Electronics pages / tom@mmto.org