# Beginner Guidance > 27 LED clock on batteries?

Hi guys,

I'm a very new beginner in Arduino. a friend told me it might be the right solution for a clock I want to design, and I thought of asking your advice here:

I want to build a clock, which must run on batteries rather than an electricity cord.

• The Clock has 27 LED lights, of 3 different colors.
• Every four seconds, the clock updates its display, causing some of the LEDs to turn on or off.
• In average, at any given moment, only half of the LEDs are turned on.
• The device doesn't have to do anything else beside running this clock program.

Can you guide me about the right Arduino components I should buy for running such a clock?

How long do you think such a clock can run on batteries?

Thank you very much in advanced,

Eyal

Please describe the color changing part more, and the arrangement of the 27 LEDs into digits or whatever.

How long the batteries last will depend on how bright the LEDs are turned on. Many LEDs are quite bright at 5 to 10mA of current.
14 LEDs at 10mA = 140mA/hour. Thus a 2500mAH LiPo battery could last perhaps up to 17 or 18 hours (2500/140) for example (2500/140).

I would use a Sparkfun Arduino Pro Mini because it is easy to modify (remove one solder spot) to run directly on 5 Volts which can be supplied from batteries or an efficient voltage Controller.
You will Need sleep mode and the 4 seconds matches perfectly.
You donâ€™t have enough Pins to drive 27 LEDs and the power would exceed the Limits of the Arduino itself. You are going to Need additional Hardware to drive those LEDs.
If at any given time, half of 27 LEDs are lit, they are going to use a pretty good chunk of power (even small numbers times 12 is quite a bit of power). What Kind of battery are you planning on using? How Long should they last before changing?

eyal_1047: - In average, at any given moment, only half of the LEDs are turned on.[/quote] That is not a good idea. Normal practice is to ensure that, at any given moment, only one LED is turned on. Try here. I imagine it can be adapted to multi-coloured LEDs and also goes quite some way to ease the shortage of pins that you are likely to suffer.

How long do you think such a clock can run on batteries?

This depends on the battery, but the best answer is "not very", which explains why clocks on batteries use LCDs.

27 LEDs = ~ 27 * 35mA = ~1 Amp.

Half "on" = 0.5Amps

AA battery = 2Ah life.

2A / 0.5A = 4 hours.

Johnny010: 27 LEDs = ~ 27 * 35mA = ~1 Amp. Half "on" = 0.5Amps AA battery = 2Ah life. 2A / 0.5A = 4 hours.

I was going to query the 35mA figure, but forgot that they're RGB LEDs. Still, I'd assume that all three colours won't be on together very often, so the average would be lower.

27 RGB LEDs and colour-changing is a lot of pins, and will require a fairly sophisticated design if the colours are fading and not just switching.

This doesn't sound like a very practical battery-powered project, either, if the clock will be running all the time. Who wants to change batteries at least daily? A car battery maybe?