How much of a fudge factor do I have?

MarkCrCo

I usually use AA rechargeable batteries for my projects. I bought a bunch of serial homework boards when they were on sale. Most of my projects have worked fine at 7.2 volts (6 batteries). My latest creations is using all 16 pins for lights, sensors, and 3 servos, and it keeps resetting using my normal power. I can go up to 8.4 volts but the battery holders get a bit weird ( 4 + 2 + 1) or I can go to 9.6 V (4+4). My question is... is that extra 0.6 Volts going to be too much for the stamp?

Duane Degn

The regulator can take input voltages from 6.25V to 26V. You should be fine with lots of different battery options.
Are you powering the servos from the board? This could cause the regulator to get hot. 3 servos might be a bit much for the LM2940 regulator.
The higher the voltage the more power the regulator will need to dissipate as heat.
If you're pulling 500mA of current from the regulator and your input voltage is 9.6V then you'll need to dump (9.6V-5V) * .5A = 2.3 Watts of heat. 
Freshly charged NiMH batteries have a higher voltage than 1.2V. Your 8 cell pack could easily measure 11V when freshly charged. Pulling 500mA from a 11V pack would create 3W of heat.
Using higher input voltages (up to 26V) isn't a problem for the regulator but the problem arises from trying to dissipate all the heat if you pull a lot of current through the regulator.
According to the datasheet the LM2940 should be able to source up to 1A of current but this assumes you're getting rid of the heat generated by the regulator.

tomcrawford

BASIC Stamp is rated for Vin up to 15 volts. The BOE regulator is rated for up to 26 volts.  So long as neither regulator (the one on the BS or the one on the BOE) gets too hot to keep your finger on it, I think you'll be fine.
Edit: Duane explained it better.  It's all about heat in the regulator(s).  EndEdit

MarkCrCo

Ok Good. The board is open to the air although I was planning on enclosing it later. If it seems to get warm I guess I'll have to add a fan (and then may need even more batteries). Thanks

Mike Green

Remember that the regulator also has a current limit (1A).  Lights, servos, and sensors all draw current and this can add up.  Servos particularly can draw heavy peak loads when just starting to move.  This depends on the size of the servo, but can be over 1A for short periods of time.  If several servos start to move at the same time or nearly so, the peak current can easily shut down the regulator.  It's usually best to have a separate battery for the servos, typically 4 NiMH cells ... without a regulator.

Chris Savage

My empirical testing shows that with a Vin of 9V the HomeWork Board really only supplies about 500 mA reliably due to thermal consideration for the regulator. So if you're going to exceed that you might want to provide a separate power source or regulator for the servos.

ajward

I've run a lot of my smaller projects from a 9.6 volt r/c car battery pack, as the only power source, without issue. for larger things like my bipeds with several servos, I power the logic board with a 6 volt (4xAA) pack and the servos with a 7.2 volt r/c pack.
Amanda