OK I am definitely going to need to do this to some of my solarforce lights.
Can we see some photos of the resistor added to the driver and the switch built?
Nice…lighted tailcap…how cool is that!
Missed this sofar, pretty cool, going to do this, thanks fkr making this happen, pd68&Helios!
I have more good news for people looking to try this. 
I added resistance at the tail, and put my DMM in-line to take some draw measurements.
The resistor on the driver is 560ohm, and in-line with the tail led is 4.7kohm.
Moon mode for the main Nichia LED draws 10.6ma.
With the light off, the tailcap LED draw is only 0.29 milliamps 
So assuming you use a 2500ma battery, and assuming the tail LED is left to run 24/7, you will lose roughly 8.4% of your battery capacity every 30 days, and it will run for a full year before completely exhausting your battery.
Obviously I still wouldn’t put this in a light that is never used, because there is clearly no low voltage protection, it will drain your battery to unsafe levels if left unattended. But I think it’s pretty awesome for a bedside light or some other light that gets used and recharged regularly.
Here’s the best part: when i took the measurements above, the LED I’m using is still FAR TOO BRIGHT.
It is clearly seen glowing under the clear tailcap even in direct sunlight, and in a dark room you can see the glow on the ceiling.
I expect that the current draw at my optimum brightness level will probably be 0.1ma or even lower.
Why not try also led with size 0805?
you could draw pad SMD resistor and LED, to save space;
Helios Thanks for your great work!
The pcb is designed to use an 0805 resistor, and your choice of through-hole or 0805 Led. I used a through-hole because it’s easier to get the light up to the tailcap, whereas the light from an smd led might be blocked by the switch housing and spacer ring.
I wonder what it would take to have a red, yellow, green (or whatever color you wanted) to activate at certain voltage levels…aka tail cap tells you the voltage
I know simple is better but just brainstorming here…
Wait…if it has enough power to run the LED, why would it not be enough to toggle a MOSFET?
There is a bleeder resistor on the driver that keeps the mcu shut off. Read through the last page of the thread listed in the OP for more info.
ah…did miss that…thanks pilotdog68
Have you tried 2 red LED in series? The voltage could probably be enough for them and you would get low voltage protection this way…
P.S.: I’ve thought about this - beside 2 LED’s you could also use one or more ordinary diodes (like SMD 1N4148) in series to the tailcap LED (s) to adjust the cutoff voltage to a useful level - thus getting a signal for low voltage as a bonus (LED goes out).
Hmm that would be awesome, but I didn’t even know that was a possibility. It’s above my knowledge level.
the 805 SMD red leds that I found are all 2-2.2V, my first thought is that two in series would give a 'your battery is not completely full' warning :-) . But at the sub-mA currents the voltage may be well lower than that...
So if the light goes out, it cuts the circuit? I thought it still allowed current to flow even when the led isn’t lit
The green and blue leds i bought have a 3v Vf
There is a relation between led colour and forward voltage, the voltages for these leds below are quite typical for almost all colour leds:
That is correct, at least in theory there is no current when the voltage is too low for the led to emit light
Well hey, I’ve already got a 3v green Led installed, I’ll test it with a 2.8v battery tonight.
A typical Si diode (1N4148) would add about 0.6 Volt to the LED’s own. For the real cutoff voltage of the different options (1 or 2 LED and 0…2 diodes in series) it would be best to test it (outside the flashlight?) with a variable power supply. So you can find out when the LED goes off by decreasing the power to lower levels - the current should go to zero if the LED don’t shine anymore.
You definitely can’t discharge the cell to a lower voltage level than the sum of the forward voltage of the diodes. The forward voltage in the data sheets is taken at the nominal current, the minimum voltage to lit up is lower! Unfortunately for this application there is no abrupte transision between “on” and “off” - the led will get dimmer more and more with lower voltage until the minimum is reached.
I quickly did some measurements to find this minimum for different LED’s:
With 4k7 resistor (a bit lower value could be better if the diodes claim more of the battery voltage for their own) in series:
white 5mm LED: below ~2.5V to shut off
green 5mm LED: below ~2V to s.o.
red 5mm LED: below ~1.6V to s.o.
2x red 5mm LED in series: below ~3.2…3.3 V to shut off
As I suggested, you should try 2 red LED’s in series to get best results:
- no discharge to critical levels by the tailcap feature
- optimal use of energy by two LED’s instead of one (losses in the resistor decreased)
- signaling of “low voltage” if the LED’s go dim/out
- one LED on each side of the switch for a more uniform illumination
The red XP-E2 starts emitting at about 1.7V, perhaps this is similar for a red 805 smd led (I have a few on order, I can test that ). Adding two diodes would make that 2.9V. If there are no further significant voltage eaters in the circuit (?), that could be quite ideal as a voltage indicator.
A little bit of the battery voltage is claimed by the 560 Ohm resistor on the driver. As I said, it would be best to take measurements with the actual setup.