I think it makes more sense to put a button cell and e-switch in the tail cap. Current flows through the body and tail cap but all the voltage drop has been accounted for, there isn’t any left to power another led.
I didn't get a chance to try this out tonight. Struggling trying to build a very simple board in Eagle and it's kicking my butt. I see Antenne has picked up the ball and has provided some good info.
DavidEF wrote:
Yes, the only way to effectively use the body as an electrical path is to put everything in series together – the Driver and LED for normal use would be in series with the standby LED. That isn’t going to work out very well for you, I think.
The driver wouldn't be in series when the switch is closed. When the switch is closed the body will conduct Batt- to the driver via the switch. When the switch is open, the body will conduct Batt+ to the tail cap LED via the driver or via a resistor added to the driver.
That example connects both batt+ and batt - to the body. Dead short. Would need a second switch at the head to bypass the driver and led and yet not short the battery. Since it would be necessary to make both switch changes at the same time then only one DPDT switch should be used but then you would need multiple wires to the switch from the other end. Easier to run a single wire to the tail with batt+ or just put the indicator up front where +/- are both available. Best of all would be in a multi cell light with a carrier which could bring power to the tail.
If the driver LED combination doesn’t let a trickle of current through, you can add a resistor of order KOhm across the driver from + in to - in. That won’t affect operation significantly and will allow the little LED across the tail switch to light up.
Now I want a led in the tailcap 
Without the wire to positive, the led in the front is going to be stealing/wasting power, if the tailcap has a low power always on led in it.
Fritz t. Cat & ImA4Wheelr are right (although I’ve thought I already explained and tested enough to see that it is possible the suggested way). The difficult part would be to find the right place for the LED. And the user should be aware of the small (I suggested 0,5mA) current draining the battery when not in use. You have to take the Bat out if you plan longer idle times (something you should do anyway). But at 0,5mA it would take more than 200 days to drain a 2600mAh battery if my calculation is right.
@one41: There is no more power wasted if you connect the LED directly to the positive pole or not. Either way, you have to limit the current for the tailcap LED with a resistor and it makes no difference if the driver is in parallel to it or not.
A light which adresses similar needs was offered to me today by the german retailer “Conrad”:
http://www.conrad.de/ce/de/product/1301209/LED-Taschenlampe-WHD-Jacklight-batteriebetrieben-netzbetrieben-Schwarz-Silber/?ref=detview1&rt=detview1&rb=1
an short english description on the producers page:
Although somewhat overpriced the “WHD Jacklight” has some interesting features:
- inductive charger (same kind as my toothbrush has)
- blue light for orientation at night
- automatic switch-on in case of power failure
How about running positive through a tape circuit to the tail cap? It would cause your usable cell diameter to decrease. Use a series resistor with an smd led under the switch cap.
Yep, getting positive to the tailcap is the biggest hurdle here and that would be one solution. You could also file out a groove in the battery tube to make the usable battery diameter the same as before.
If on the other hand you’re thinking of doing this with a 4s1p “soupcan” light then it’s going to be much much easier to achieve a standby tailcap light.
You can just use one of those hexagonal brass standoff pegs with a contact spring or pogo contact on top to reach up between the cells and make contact with the center positive pad on the driver contact board. I’ve done this to get power to the tail of an Apex 5t6/skyray king but in that case I was doing it to power the gate of a low resistance fet switch.
Something to consider, because a standby tailcap switch light is a cool idea. I have some transluscent switch boots that would look great with a dim led illuminating them from behind, I might add one to these lights. 
A quick search for 'watchdog timers' yielded this interesting application note from Maxim. First example.
http://www.maximintegrated.com/en/app-notes/index.mvp/id/1865
The circuit would be wired in place of the led and resistor in the OP. It might need a capacitor to store power for the LED when it blinks. Operation may also depend on which driver is used, as it is in series and with this circuit.
(The second example is also very interesting, but it will take a bit more thought as to how to implement it.)
Just some food for thought...
I seem to recall this topic from years ago on other forums, but I can't seem to find it now.
^ Interesting chip there Crux. It sure seems like it could be very useful for all kinds of purposes.
I forgot to share a thought last night when I posted. I'm pretty certain a limiting resistor will be needed in the tail cap. The moment the light is turned off (buy clicking the switch to the open position), the driver will be trying to pull current. The tail cap emitter will be overloaded as it will be the only remaining path for current flow.
Routing a Batt+ wire to the tail cap is not necessary as Antenne has reported. He/she even gave tip on a quick way to check if some positive power is flowing into the flashlight body by saying this:
" . . . So far my tests suggest that it would be best to test the light in question by simply put a LED between BAT and Body with tail cap taken off and see if it lights up. Worst case would be that it burns up
"
If a driver has voltage dividers (like for LVP or tempature sensing), it already has some resistors connecting Batt+ to Ground (flashlight body). So, when the switch is off, the light body actually is positive charged. The current from the driver voltage dividers will probably not give pilotdog68 the 2mA she wants, but it will give some of it.
What did I say that made you think I was a girl? :~
Anyways, based on what you said I could possibly just swap the LVP resistor for a different value? I would give up my LVP, but I don’t think I’ve ever needed it anyways. I always recharge at the first sign of dimming.
(EDIT: Actually, if I use a different resistor, maybe I could just tweak the LVP values in the FW so it would still work.)
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It’s interesting how divided everyone is on this topic.
It seems very logical to me that this would work pretty easily, but others seem to think it’s basically impossible without running a new bat+ wire.
I think the hardest part will be making the tail LED bright enough without having too much current draw.
I’m sure as pointed out you could get it to work as is but there are a number of caveats that could make it less than straight forward. Adjusting the voltage divider would mean losing/having to tweak the LVC, I wouldn’t run a light without an LVC just for the sake of having a standby light.
Getting strange mode behaviour/losing memory, stuff like that could also crop up since we’re at least partially powering up the driver board and MCU. I don’t know for sure what the results of that could be, and it would depend heavily on the driver and the firmware combination.
I haven’t tested it but completing a circuit where everything is in series and involves a clicky MCU that’s designed to work with on/off power, there’s potential for some unintended behavior. Right?
If it was a a driver based around a momentary configuration then that at least wouldn’t be a concern.
It just doesn’t seem like a particularly clean solution to me, whereas finding a way to power the led with it’s own connection to battery + would (if at all possible) be a cleaner solution, at least electronically.
I’d gravitate towards that myself since I don’t know enough about electronics to fully visualize the complications of the other method.
Like I said the soup can lights make the batt + approach a no brainer, for anything else it’s going to be a pain.
So it’s cool to see testing done on this anyway, only way to find out what’s possible/feasible
I have no idea why I thought that. My sincerest apology pilogdog68.
I would leave the LVP circuit alone unless you are sure your FW has not LVP code. The LVP has 2 resistors in series from Batt+ to Ground. The MCU connects between the 2 resistors. You would need to decrease the value of the second resistor (One that connects directly to Ground) in order to flow more current to the flashlight body. That would lower the voltage that the MCU reads and step your light down if it has LVP code.
EDIT: I agree with LH that it may cause mode switching or other issues in some drivers. We won't know till we try. I would think with so little current (and most of it being converted to photons) that it may not cause any issues.
EDIT2: I missed the part in your last post about tweaking the FW pilotdog68. That could work, but would probably be a bit of trial and error to get everything working right. Seems much easier to just find a positive spot near the ground ring and reflow a resistor (may need a wire or copper strip to reach)
Yes definitely potential for that. Just spitballing here, but maybe tweaking the OTC settings in STAR off-time would correct it enough that mode switch would still work?
I don’t want a wire running down the tube because the Convoy lights are already tight, and it would make battery changes a royal pain. If my idea ends up not working, I’ll just give up on this.
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Lol no worries!
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On a single sided driver you could just scrape away a little mask and use the resistor to bridge the gap. A double sided driver could be a little more tricky, but definitely doable.
Here’s an idea based upon the BLF Edition Lumintop SD10. Buy or make a battery carrier with plus and minus on both sides. You get physical reverse polarity protection and can supply power to the tailcap.
I also found this - http://www.google.com/patents/US20140091725
Haven't read it though, too legal.
RadioShack sells a blinky LED (blinky circuit built in to package). Model: 276-036
I know these draw a spike of current when they blink ON, but it's simple and may just work as drawn in the OP. I'm not sure how this or other circuits behave as the cell voltage drops toward zero. They probably continue to pull current past the lower limit of where you want to keep the cell. :(
I'll see if I can find any in my parts drawers tonight...
How much current do you need to make your smd glow and where do you get the voltage potential. Current flows through the case but between driver gnd and battery neg there is very little voltage potential. The lvp resistors drop the same voltage as the main led circuit but only pass a few tenths of a mA. Answer the first part of the question above and recalculate the value of the resistance needed to replace the lvp resistors. Lvp will be disabled since pin 7 needs to see a voltage lower than what you need to power the smd.
edit this is all screwed power to the SMD cannot come from the case. If some power is bled from the lvp circuit it has to travel through a separate wire back to the SMD + resistor and then to the case. It must remain a separate path until the paths are at equal potential.
On my way home from work I stopped at one of the remaining radio shacks and grabbed some leds and resistors. I got 3 red 5mm leds, one 3v, one 1.7v, and one 3v blinking.
I first tested them a p60 light using a single mode AX2002 driver and an XM-L.
I put the 3v led in parallel to the main switch with the positive led lead connecting to the battery tube. I did not use a resistor.
It works perfectly! With the switch open, the led lights up to a nice locator brightness. I measured with my DMM in series with the led and they pull about 0.5 miliamps. The XM-L does not light.
When when the main switch is closed, the secondary led shuts off and the main led works normally.
Next I tried it with a 4-mode solarforce drop-in with an xp-l, and a 4 mode custom dropin with one of wight’s PZL drivers and a Nichia.
With the switch open, the secondary led works as expected, the Nichia lights an almost invisible amount, but the xp-l does not light at all. The secondary led seems brighter with the PZL driver.
With the switch closed, the secondary led goes dark and both main leds come on, but neither changes modes.
So I retested these two the same way, but with a 1kohm resistor in series before the + end of the led. With the switch open they both act the same, but this time the solarforce drop-in functions perfectly, changing modes without a hitch. When I turn on the custom drop-in, it goes into lvp, a few blinks and then off.
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I’ll test with the blinky led next