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I'm looking for a good way to power a single LED with a coin battery (CR2032). Here are some of the requirements/characteristics of the system I'm trying to add this LED to:

  • I don't need full brightness (low to medium is enough)
  • The longer the battery lasts, the better
  • The space is constrained, so:
    • Fewer/smaller components are favorable
    • There's not much opportunity for heat dissipation
  • The color of the LED is not important
  • If the brightness can be varied it is welcomed, but not necessary at all

What would be the best way for achieving this?

ocrdu
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Razero
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  • https://electronics.stackexchange.com/questions/482857/how-to-build-a-small-led-assembly/482888#482888 – Mattman944 Aug 20 '22 at 12:47
  • What pulse period is the lighting? Always ON? – Antonio51 Aug 20 '22 at 12:54
  • @Antonio51 From a human eye perspective it's always on, but can be driven with a PWM if that's what you're asking – Razero Aug 20 '22 at 13:27
  • "Thief oscillator" perhaps wants you to search ... https://electronics.stackexchange.com/questions/623439/how-to-reduce-npn-reverse-base-current-on-this-interesting-super-efficient-joul – Antonio51 Aug 20 '22 at 14:52
  • Have you tried a resistor? Is it the best way for you? – Justme Aug 20 '22 at 15:42
  • @Justme I imagine both a Joule Thief and just adding resistor would be a good candidate, question is which one (or some other alternative) would give longer battery life. When you add resistors, aren't they just dissipate the "extra voltage" to heat? – Razero Aug 20 '22 at 16:40
  • You need to tell use which LED you are talking about (at least the approximate current you desire). The internal resistance of the cell that others mentioned will waste power anyway. You wrote that you are PWMing the LED - this way you can trade battery life for brightness already... – Sim Son Aug 20 '22 at 17:51
  • @SimSon I don't have a specific LED, but lets say something like a standard 1.8V 3mm red LED (20mA forward current). I don't necessarily want to drive it with PWM, I'm looking for a (small, battery powered) solution that will give me longest life...would PWM IC + LED powered from battery last longer than just adding resistors in series with LED? – Razero Aug 20 '22 at 19:37
  • The challenge with using a resistor is that it wastes power. PWM is more efficient and you can alter the on/off time to keep the brightness constant as the battery voltage decreases. – Finbarr Aug 20 '22 at 22:45
  • Just put the battery in series with the resistor and LED. Use a red LED. In fact using two of them in series might work (try it). – user57037 Aug 21 '22 at 08:00

3 Answers3

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Try two CR2032 batteries in series connected across the LED. There is enough internal resistance in the batteries to limit its current. If the LED is too bright, or the batteries don't last long enough, you can always add a few hundred ohms in series. Experiment!

Phil Freedenberg
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    For red and other low voltage drop LEDs one CR2032 will work. It will last 5-10 hours, depending on how much reduced brightness you can tolerate. – Mattman944 Aug 20 '22 at 13:45
  • @Mattman944 If I understand correctly, as the voltage of the battery drops over time, the LED will draw less current and produce less light. Is there a way to limit the current draw (without wasting the energy) from the beginning, so I can get low brightness for longer time? – Razero Aug 20 '22 at 13:58
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    @Razero - Add a resistor for reduced brightness and longer life, but it still will get dimmer over time. If you want constant brightness then you need a few parts (IC, etc). That should be a separate question. – Mattman944 Aug 20 '22 at 14:35
  • I think this is not a good suggestion. Two 3V batteries in series with only internal resistance to limit current is enough to damage any standard signal LED and current will likely be out of specifications for the battery. – Justme Aug 20 '22 at 15:41
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    It's worth noting that a single low voltage (red) LED pinched across a button cell is a quick, cheap and simple way of testing such cells. The brighter the LED, the better the cell. But this doesn't drain the cell for any significant amount of time. – Peter Jennings Aug 20 '22 at 16:30
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The longer the battery last the better

With very low power op-amps (double), here is the principle of one circuit.
If you look to first column, frequency = ~ 156 Hz, period = 6.382 ms.
Pulse width = 123 us adjusted by R6.
Peak current diode = ~ 40 mA
Mean current diode = ~ 670 uA (Should add op-amps supply current 550 uA)
Lasting time = 220 mAh / (0.67 mA+ 550 uA) = ~ 180 Hours.
Ok, "many" components, but also "many" choices at low currents ...
Battery on top PCB, components on bottom, easily.
NB: forget R6=250k, errors noted.

enter image description here

Antonio51
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Here is a simulation of a simple Joule Thief that works well at 1.5V and 1.0V and even down to 0.7V or less, although it might not start up below that. It draws 17.7 mW at 1.5V with 3.9 mA average through the LED. At 1 VDC it draws 7.6 mW with 1.25 mA average through LED. And at 0.7 VDC it draws 3.79 mW with 247 uA through LED. For a 220 mAh cell (330 mWh) and 5 mW average, it should last about 220/5 = 44 hours, maybe longer as it discharges and dims.

With a 2N2219A transistor I got about 9 mW at 1.5 V and 2 mW at 12 V. So a bit of experimentation may result in much longer useful life, perhaps 100-200 hours.

Simple Joule Thief

A microcontroller like the PIC10LF322 can run on 290 uA with a 1.8V (minimum) supply, and it could be programmed to PWM an LED at a visible duty cycle for perhaps 2 mA total. This would give about 110 hours for two 1.5V coin cells or a single 3V Lithium cell.

PStechPaul
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