Battery powered Atmega application

[Odysseas]

Hi there,

I have built a digital alarm (from my last posting 'Schmitt Trigger for counting the time'), and an acquaintance of mine has asked me if I could build an alarm like this for her. But she'd need it powered from a battery, for mobile use.

Now a few questions: Is it realistic to use an Atmega16 for a battery powered application? The datasheet specifies the power consumption for the atmega16l to be 1.1mA when fully powered up.

Which type of battery (of course, rechargable) would be appropriate? I thought of simple NimH batteries, which could be recharged directly in the alarm if an external power supply is connected.

Thanks for any help!

 

[audioguru]

You need to measure the current of the entire circuit, not just the Atmega.
Select a low-current voltage regulator.
Make a crystal-controlled 50Hz or 60Hz oscillator to replace the opto-isolator.

A little 9V Ni-MH battery is really only 7.2V which is close to being too low for an ordinary 5V regulator. Its charge (175mA/hr) lasts only 35 hours with a load of 5mA.

Six AAA Ni-MH cells (900mA/hr) will power the thing for 180 hours.

Six AA Ni-MH cells (2500mA/hr) will power the thing for 500 hours.

 

[hjames]

When doing desigining circuits that need to have low power draw, there's a couple things you want:

1) Go for the lowest voltage that you can use - 3.3V is standard, 1.8V is available in some of the Atmel's lineup. This will drop the required power tremendously because of CMOS's V^2/R power consumption, as well as possibly simplifying your power circuitry.

2) Optimize the power required for computation - there's a tradeoff in terms of clock frequency and time spent in "sleep" mode. If you can get the processor to spend 99% of it's time in sleep mode, you can cut the average power consumption by 1-2 orders of magnitude.

3) Figure out the interface - people use bare LCD panes and microcontrollers with built-in LCD drive when they need to get months/years of lifetime out of something. You probably don't want to go this far, but if you are going to use LEDs, be sure to get high-efficiency ones that are bright at <5mA.

4) NiMH and NiCD have pretty large self-discharge rates - they'll naturally lose a couple percent of their charge per day. If this is acceptable, the use them. Rechargable Lithium types are a bit better - I think it's a couple percent per month or something. Primary lithium cells have a stupidly long shelf-life and are used when you want something to run for years at a time...

James