Im using a 3.7V 18650 Li-ion battery (Ultrafire) and the circuit calls for a 27 ohm resistor. But one battery tested at 4V and the other 3.7V.

Is the resistor too low now? Which rating should my circuit be based on, the 3.7V battery rating or the 4V actual rating?


  • \$\begingroup\$ Though you didn't state it, you're question basically boils down to how to drive an LED, as discussed here: electronics.stackexchange.com/a/55824/6494 \$\endgroup\$
    – sbell
    May 30 '15 at 0:54
  • \$\begingroup\$ What is the resistance of the force-sensitive resistor? That resistance is in series with all your LEDs and their resistors, and will serioously affect the required LED resistor value. \$\endgroup\$ May 30 '15 at 6:15

"3.7 Volts" is the nominal voltage you get from a single lithium-chemistry cell. It actually ranges from 4.2V fully charged, to around 3.0V flat (any lower than that & you'll be damaging it & shortening its life).

I doubt your circuit is gonna work as you wish. White LEDs have that high forward voltage drop that intrudes on your circuit's balance when battery voltage is low, so your LED current will be all over the place. You really need to be (a) driving each LED with a constant-current circuit, & (b) sensing that force-sensitive resistor with a circuit or microcontroller and then driving the LEDs with a low-side MOSFET accordingly, rather than using the force-resistor in series with the actual LEDs.

  • \$\begingroup\$ Im fairly new at this, so sorry for any n00bness. Do you have any links/resources i could look at how to build it? \$\endgroup\$
    – digitalism
    May 30 '15 at 5:16
  • \$\begingroup\$ That's a really big question :). Basically you need to learn some basic electronics, and perhaps dip a toe into Arduio. whilst you can learn lots of stuff here on StackExchange, it's not particularly structured. There are HEAPS of online resources for learning basic electronics. \$\endgroup\$
    – Techydude
    May 30 '15 at 8:52
  • \$\begingroup\$ I got the basic idea from this Youtube video: youtube.com/watch?v=OFA2Ehj9OlQ. Im sure this build is not completely correct, but it does seem to work. Would using a constant-current circuit and adding in a arduino/adafruit board help? Im trying to keep it simple and low-cost too. \$\endgroup\$
    – digitalism
    May 30 '15 at 19:14
  • \$\begingroup\$ That video isn't playing for me. \$\endgroup\$
    – Techydude
    May 30 '15 at 21:31
  • \$\begingroup\$ Thats weird, works for me. Would something like this work for a constant current driver? link \$\endgroup\$
    – digitalism
    May 30 '15 at 23:57

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