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As Christmas was on a budget this year, one of the presents I came up with consisted of a fishbowl with battery operated (flat CR2032 3V) tea-light candles in the bottom, these were covered in coloured glass beads and then I put air plants in the bowl mounted on driftwood. The effect is really nice (i.e. the flickering light under the beads).

However, it is not practical to keep lifting out the plants/driftwood, emptying all the beads, and swapping the batteries in every candle every few days or so.

Therefore, I thought about sticking the candles to a cardboard base, soldering thin wires to all the negatives and to all the positives of the candles (i.e. leaving the small battery cover off each one), running the cables to the back of the bowl and attaching some form of 'battery' holder that will site above the beads so it will be easy to plug in new batteries when needed.

My initial thought (as there are 12 candles and they all require 3V CR2032 batteries) is to connect them in 3 banks of three batteries and use a square 9V battery for each.

My question is (as I have no experience of this at all) will this work or am I simplifying this too much (i.e. will the 9V battery give the same 'power' output and drive the candles or will it damage them, does the length of wire have an effect at such a short distance, do I need to measure the resistance of each candle and do some calculation based on that?).

Apologies for the long-winded question but I just wanted to give someone as much info as possible.

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    \$\begingroup\$ This sounds like an electrolysis cell to me ... or an interesting way to do chemistry experiments on your fish! \$\endgroup\$ – Brian Drummond Dec 29 '13 at 13:01
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    \$\begingroup\$ @Brian, I assure you that there are no fish involved! (or water). I'm also considering drilling a hole in the side of the bowl and running in some christmas lights under the coloured glass stones but (as I have engraved the bowl) I'm reluctant to risk drilling! \$\endgroup\$ – John T Dec 29 '13 at 13:52
  • \$\begingroup\$ Thank goodness for that! Then go ahead... Joe's answer covers the points I would have made, especially series flickering LEDs. One option there is to connect several plain LEDs in series with one flickering one, they should simply flicker in unison. \$\endgroup\$ – Brian Drummond Dec 29 '13 at 14:22
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I'm afraid that a standard 9V battery may not power your lights any longer than the 12 coin cells. I am also concerned that if you connect several lights in series they may not flicker properly.

Is there a possibility that you could use an AC-to-DC converter (a "wall wart") to provide power? I know that would mean a wire running into the aquarium but it would save a lot of batteries. If not then you might consider using 2 or 3 D-cells in series. You should be able to get a plastic holder for these batteries at a local hobby shop.

Either way, you need to have a source of low voltage (3V to 6V, say) at fairly high current. Then I think you need to connect a resistor in series with each light, and connect all of the light/resistor combinations in parallel. The resistor is needed to limit the current to each light, something that the coin cells did inherently because of their low current capability. I would suggest that you start with about 100 ohms if the supply is 3V and 500 ohms if it is 6V. If the light is too dim or doesn't flicker then gradually reduce the resistance until it behaves properly. It might be a good idea to try this with just one light until you figured out a good resistor value, but you will need one resistor per light in the end.

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  • \$\begingroup\$ Thanks for the information Joe. I tried to vote you up but apparently I need '15 reputation' in order to do so? Does the resistor need to go between the +/- of the tea light or on the positive leg or negative leg? \$\endgroup\$ – John T Dec 29 '13 at 14:12
  • \$\begingroup\$ John, the resistor should be connected between the tea light and the voltage source, and it doesn't matter whether it's from the positive leg of the light to the positive terminal of the voltage or from negative to negative. If you feel that I gave you the answer you were looking for, the best thing is to "accept" the answer. \$\endgroup\$ – Joe Hass Dec 29 '13 at 14:53
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Be careful of self-flickering or fading LEDs. We found out they burn up when being plugged into an LED tester, here's why, and why it could be a disaster for a series connection:

Most LED testers indicate the MA in various slots, and commonly 20MA is the max current for an LED. They derive the 20MA by a series resistor with a 9V battery powering all slots.

When you put a self (blinking / fading / flickering) LED in the tester, the extremely low current draw of the off or dim state, causes the LED to see most of the 9V across it, which burns out the tiny chip running the B / F / F function.

Thus running 3 x LED in series to a 9V battery COULD cause excessive voltage and burn out the LED's chip.

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  • \$\begingroup\$ This could be better stated, but yes, IC drivers, bare LEDs limited by the supply impedance, and LEDs with series resistance to limit current from a voltage source are all distinct types of devices and cannot be interchanged without taking the differences into consideration. The asker needs to either characterize one of these tea lights, or replace them with something known. \$\endgroup\$ – Chris Stratton Jun 20 '17 at 17:15

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