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I am animating a small toy using three HXT900 servos controlled by a standalone ATmega328p. Space is very tight indeed! I am considering using two li-ion or li-poly 3.7v (700mAh) cellphone batteries; one for the ATmega and the other to power the servos separately.

I can remove and charge the batteries in a cellphone, so no charger chip is necessary.

I am also aware that the Li-ion/poly batteries should not go below a certain voltage, but believe that there is a protection circuit inbuilt which obviates the need for an external protection cct.

Does the readership think this is feasible?

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  • \$\begingroup\$ What is the question? Are you asking if what you have is enough? \$\endgroup\$
    – Chris Laplante
    Commented Mar 24, 2012 at 21:03
  • \$\begingroup\$ Well yes, I suppose so. Will the battery drive the Servos continuously for about 10 mins for example? \$\endgroup\$
    – RRJ
    Commented Mar 24, 2012 at 21:08

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Servos operate at about 5 to 6 volts, so you can connect the two batteries in series to form a 7.4v battery, and use a regulator to step down the voltage to between 5 and 5.5 volts. You will want to use a switching regulator (or something similar - just not a linear regulator as this will overheat). I've used the 3 amp model of this regulator with success: http://www.dimensionengineering.com/DE-SWADJ.htm. Fairly small, pretty cheap. You shouldn't need a heat sink.

Batteries in series will increase the voltage, and in parallel it will increase the number of amp-hours. You didn't say how much current each servo will be pulling, so you might need a bigger (as in higher amp-hour rating) battery. Take a look at this one: http://www.sparkfun.com/products/10471

After being stepped down by the regulator, this 5 to 5.5 volt output can be safely connected to the microcontroller to power it.

As for additional components, I'm no expert but I think you will need a crystal to drive the clock, decoupling capacitors to smooth the power (switching regulators are noisy), and probably a few other passives. I'm assuming you don't need Arduino-style programming functionality (i.e. USB programming), so you will need to buy an AVR programmer.

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  • \$\begingroup\$ Thanks for reply SC. The servos are power hungry and may pull 500-700mA or even more! So I suspect the battery will not last that long. I also worry that the battery may get 'slightly' warm:-) By having a separate battery for the Atmega I hope to reduce interference from the servos. I am using these batteries for their slim fit. \$\endgroup\$
    – RRJ
    Commented Mar 24, 2012 at 21:53
  • \$\begingroup\$ Any interference from the servos will be mitigated by the decoupling capacitors (as long as you put them right at the power inputs of the chip). \$\endgroup\$
    – Chris Laplante
    Commented Mar 24, 2012 at 22:08

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