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Author Topic: How to determine how long batteries will last for my project?  (Read 645 times)
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Hi,

I have a very simple project: ATTiny85V, a pir motion sensor, and an RF transmitter. From the transmitter's datasheet its current consumption is 8 mA and I can't find the consumption of the pir in it's datasheet (unless it is called the "Effective voltage" which is 0.2 - 1.5 - not sure if that represents it's current consumption in mA).

I'm trying to figure out how to choose the right battery(ies) and how long I can expect those batteries to last. I'd like to use coin cell batteries to save space.

I am polling the pir once every second and the transmitter will only send a signal when motion is detected (in my scenario motion will only be detected a few times a day).

I apologize if this is a basic question.

Thank you in advance,
LT

pir: https://www.sparkfun.com/products/8630?
RF transmitter: https://www.sparkfun.com/products/10534
« Last Edit: November 13, 2012, 01:39:35 pm by Latency » Logged

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Coin cells are good for a few hundred mAh?
http://www.dipmicro.com/store/BAT-CR2032
225 for example.
Can the attiny be put in sleep, to wake up when PIR pulls a pin low?
Then send out some pattern - see the virtual wire library for the kind  of sequence to send that a receiver can pick up on and interpert correctly.
Best method is to measure the current during the actual conditions, sleeping and transmitting.
Probablys see ~5-10mA while sleeping depending on the PIR, and a jump to 15-20nA while transmitting.
So 225/10 = 22 hrs.
Better off with a 1000mAh LiPo, can find them in niice small flat package.
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Thanks - I'll look into putting the tiny to sleep.

I already have all the rf stuff working - and I am currently designing an enclosure for the unit that I'll 3D print. I just need to choose the right batteries so that I can finish the enclosure.

And I'd prefer to not have to change the battery(ies) every day. smiley

Could I increase the number of batteries to increase battery life?

Thanks again!
LT
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It would also be cool to somehow detect when the batteries are low. I could send out another rf message to let me know when they need changing.
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See this great tutorial http://www.eevblog.com/2011/01/23/eevblog-140-battery-capacity-tutorial/
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Increasing the batteries in series just increases the voltage - not the mAh.
If you connect in parallel then you have to start playing games to account for then running down at different rates.

I have a 1000mAh LiPo in a remote control running an 8 MHz promini that mostly sleeps, a 434 MHz Tx modukle, and 16 button keypad. Lasts several weeks to a month between charges.  Added a Maxim 1811 charge control chip to recharge it from 5V source.

Bring the power in to an analog pin and measure it when the PIR wakes the processor up.
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You guys rock - thank you so much!

By the way, I found this cool battery life calculator: http://oregonembedded.com/batterycalc.htm

Best,
BT
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