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I have 13 adafruit LED sequins in parallel hooked up to a CR2032 coin cell. This cell is also hooked up to an adafruit Gemma board (Arduino-like board using an Attiny85). Pads D0 & D1 on the Gemma each connect to a single LED sequin. D0 & D1 can do PWM, so the Gemma's program simply fades these two LEDs on and off. This is all connected using conductive steel thread.

When I use the coin cell, the 13 LEDs all light up like I would expect, but the Gemma stays dark (it has a green LED to indicate when it's powered up). When I power the Gemma via USB, it works as expected: the two LEDs connected to the PWM pads fade on and off, while the rest of the LEDs stay on.

What is the problem with my coin cell power source? Not enough voltage? Not enough current? Could I fix it by putting two of these cells together?

Note that the LED sequins each have their own 220 ohm resistor, and when "powered from 3.3V they draw about 5mA". The Gemma "draws only 9 mA while running" and has an "On-board 3.3V power regulator with 150mA output capability"

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Coin cell batteries generally have a voltage of 3V. Your Gemma takes in 5V. Even though your LEDs are powering on, your Gemma doesn't have enough voltage to actually run.

The voltage regulator doesn't take whatever input you give it and make the proper output, it needs either a lower OR a higher voltage. In this case, and this is the most often case, you'll need a higher voltage than what your voltage regulator will output

I would caution you regarding putting 2 CR2032 cells together because this will result in 6V. If the Gemma has a regulator, then that will be safe but your leds might not have the proper resistor for a 6V source.

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  • \$\begingroup\$ Thanks. I guess I misunderstood the capabilities of the regulator. I thought that it would work with as little as 3V. According the the Gemma page, it'll work with up to 16V. I think I'll try to redesign my circuit so that the Gemma has a separate power supply. \$\endgroup\$ – WhiteHotLoveTiger Jul 29 '14 at 15:22
  • \$\begingroup\$ Yea, it has a simple buck regulator. which means it'll bring a higher voltage down (like the 5V from the USB) down to 3.3V operating levels. It cannot boost the voltage up. \$\endgroup\$ – Funkyguy Jul 29 '14 at 15:26

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