D.I.Y. Illuminated tailcap

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pilotdog68
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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

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

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pilotdog68
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The green and blue leds i bought have a 3v Vf

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

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djozz
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pilotdog68 wrote:
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:

Smile

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pilotdog68 wrote:
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

That is correct, at least in theory there is no current when the voltage is too low for the led to emit light

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Well hey, I’ve already got a 3v green Led installed, I’ll test it with a 2.8v battery tonight.

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

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Antenne
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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

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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 Smile). 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.

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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.

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Antenne wrote:
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

Hmm so they actually shut off a bit below what the claimed Vf is?

I had actually planned to use 2 for more uniform light, but I was planning to wire them in parallel.

I think having them shut off at around 3.2v is ideal so that you still have some usable charge in the battery left.

I don’t have a variable power supply though, so if it varies much with the different set-ups it could take a lot of trial-and-error.

I’m planning to use 2 blue led’s for my first permanent installation, I assume you don’t have any of those to test?

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

Major Projects:  Illuminated Tailcap, TripleDown/TripleStack Driver

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Blue LED’s are similar to white ones – the little voltmeter modules I just bought are shutting off below 2.7V with the blue display. You could add one 1N4148 (or LL4148 for SOD80 package https://www.google.de/url?url=https://www.fairchildsemi.com/datasheets/L...) in series with the LED’s.

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Antenne wrote:
Blue LED’s are similar to white ones – the little voltmeter modules I just bought are shutting off below 2.7V with the blue display. You could add one 1N4148 (or LL4148 for SOD80 package https://www.google.de/url?url=https://www.fairchildsemi.com/datasheets/L...) in series with the LED’s.

Are you fairly certain that would be the one I need? If so, i can order them on ebay for super cheap.

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

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Yes, these are multi-purpose for smaller currents up to 200mA. They can be used on different applications, to reduce the voltage by 0.6 Volts, as rectifier or protection diode. But you could try almost any other Si diode first if you got some in your spare parts box or salvage it from an obsolate circuit and order them later when the setup is finished.

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Antenne wrote:
Yes, these are multi-purpose for smaller currents up to 200mA. They can be used on different applications, to reduce the voltage by 0.6 Volts, as rectifier or protection diode. But you could try almost any other Si diode first if you got some in your spare parts box or salvage it from an obsolate circuit and order them later when the setup is finished.

I don’t have many spare parts. The only diodes I have are LED’s, SS34 schottkeys, zeners, and the reverse polarity diodes used on all of wight’s drivers.

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

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You could use one the zeners you have, just in the forward direction (rather than the usual reverse for zeners), its Vf will be ~0.7V just like a standard diode. Obviously the reverse voltage will be whatever the zener is rated at, but it won't matter as it won't see any reverse voltage.

Just a thought...

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Do you think the standard reverse polarity diodes would work the same way?

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Reverse polarity diodes are most likely Schottkys, too. You want a forward voltage as low as possible with these, something Schottkys deliver (~0.1..0.2V) what makes them unsuitable to get a notable drop in voltage. But the zeners should work as Crux said.

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Ok. I’ll give the zener a shot when I get the 20mm boards (oshpark marked them ‘shipped” today).

I was hoping to use the other ones because I have plenty of them, wheras I only have 2 zeners left.

EDIT: I want to save my zeners, and I’ll probably need more than one, so I just ordered these.

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

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Yep, those will work fine.

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Thanks for the tips guys. It’ll make this idea even better. Now there will be no worries about over-draining cells.

I also updated the OP to make it easier for others to try this.

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pilotdog68 wrote:
Antenne wrote:
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

Hmm so they actually shut off a bit below what the claimed Vf is?

I had actually planned to use 2 for more uniform light, but I was planning to wire them in parallel.

I think having them shut off at around 3.2v is ideal so that you still have some usable charge in the battery left.

I don’t have a variables power supply though, so if it varies much with the different set-ups it could take a lot of trial-and-error.

I’m planning to use 2 blue led’s for my first permanent installation, I assume you don’t have any of those to test?

If your planning to run two in parallel could you set them up with two different colors that go off at different voltages. Maybe blue down to 3.7V and red to 3V.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

Scott

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Hmm how do you think an rgb led would behave?

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If it’s separate IAD ( individually addressable) dies then maybe it would work but then they would be all in one place. 3 – 0805 smd’s 120 degrees apart might be nicer but all you need is one or two voltage settings. I have a bunch of SMD LEDs I bought for a similar project using a quad opamp but my ideas seem to have short expiration dates.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

Scott

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The green and blue led would die off at about the same voltage, but if you place a diode before the blue one the voltage spacing between r g and b  may be useful.

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djozz wrote:

The green and blue led would die off at about the same current, but if you place a diode before the blue one the voltage spacing between r g and b  may be useful.


Exactly, set them up so that battery decay turns them off one by one.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

Scott

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The voltages for one red/green/blue are about 1.6/2.0/2.5 V for cutoff – if they are all in parallel I would add 1-2 diodes to them in series (+1.2V for two) and you get ~ 2.8/3.2/3.7 V indicator. Or one red in series (+1.6V).

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The really useful voltages IMO are ~50% remaining, ~20% remaining, and 'please recharge' . So that would be 3.85V, 3.6V and say 3.0V according to the graph below. You'd have to play around with colour leds and diodes to find out how to get there. I might have a go at that, sounds like fun. Smile And then make a new version of the switch board with a lay-out that accommodates the circuitry, when a small Omten is used it will certainly fit.

(the picture is from battery university, this should be a typical li-ion cell, voltages are resting voltages, not under load)

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The 20mm PCB’s were delivered today. They look good, although the slots are not present in these. The extra diameter should still make it possible to use the larger omten 1217.

Build thread for my S8 should be posted later today

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

Major Projects:  Illuminated Tailcap, TripleDown/TripleStack Driver

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Will it be similar for an S3 tailcap? This is mighty cool I’d say! BLF is great for this kind of stuff.

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As far as I know the S3 switch assembly is identical to the S8

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

Major Projects:  Illuminated Tailcap, TripleDown/TripleStack Driver

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Here’s another wild idea: what about incorporating a light light sensor that only turns the led on when it senses darkness?

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

Major Projects:  Illuminated Tailcap, TripleDown/TripleStack Driver

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