odd power issue

[laazarus]

I'm using a digispark to drive a couple of servos (p0,p1) and having them move to preset positions based on pulling p2, p3 or p4 high with an rf remote board.
power for the servos, rf board and digispark are all pulled to a single point and power fed in at that point. problem I'm having is that it works perfectly using a 5v 3a BEC with a 3 cell lipo driving it (output measured at 5.02v). This power setup is too big so i'm trying to drive it with 2x aaa bateries through a 2a buck converter (measured output 5.08v) and the result seems to be that digispark is stuck in a boot loop. also tried driving it with 4x aaa (no buck converter and voltage over 6v) but get the same boot loop. With the meter connected i don't see any voltage drop as the board boots (or reboots). also tried with a smoothing cap in place but no improvement. The servos I'm testing with are only 5g and have no loading on them at this point. I have tested with a second digispark and get identical results.

can any one sugest any thing else to try, I'm running out of ideas. this all needs to be mounted on a headband so is cant be to big.

[laazarus]

ran it after the buck converter and it is 1000uf and 6.3v

[PeterF]

I think you mean boost converter - buck converters lower the voltage. There is no way you would get 5v from a buck converter with an input of 3v (2xAAA).

Do you know what the input range is on the ubec... is it a 6v - 24v one? Since you know the setup works with a proper regulated 5v supply from the UBEC, try the 4xAA and the UBEC... it will be close (as the differential input to output is only 1v), but since it is probably a switching type UBEC due to the 3A rating. And when you ran the digispark from the 6v directly... did you connect to the VIN connection, or the 5v connection on the digispark? (hope not the latter... 6v on 5v pin may have killed it!)

[laazarus]

PeterF, your right it is a boost converter. The ubec input range is 7.2v - 22.2v. I haven't run any more than 5v into the 5v pin, all the higher voltages have been into VIN.

Ralf, I tried removing the input wires from pins 3 and 4 and still got the same results

Its starting to look like its an input current issue. any configuration of AAA batteries gets the same results. shifting up to 3x AA will just about drive a single servo though it will often restart halfway through a movement. stick a single 18650 on it and it works like a champ. in a quest to keep the size down I'm currently testing with an 14500 which seems to hold it own so far

[laazarus]

That makes sense though at this point it shouldn't be an issue. the current test servos are only 5g and have no load on them so current should be minimal (the final project will use 9g servos (if they ever arrive) so here's hoping its still ok with those). When I had the volt meter running there was no significant change in voltage from the boost converter when the servos tried to move, 0.02v at most so never dropped below 5.06v.
I could look at slowing the servos down a bit though this is not ideal and moving only one servo is not an option for the overall effect of the project (animated cat ears, I have friends who work as teachers out in asia who seem to like giving me projects like this. Last set were 2 9g servos per ear with a 5 button hand remote, though they were based round an arduino leonard usb key board and run happily for  a few hours on 4 aaa's)

[laazarus]

I 'd agree that the digispark isn't at fault, getting the power right and it working proves that. The 6v was only part of testing, it's running ok now with the boost converter and a 14500 cell so that will do me. The BOM has gone up a bit but the overall footprint has gone down so that makes the job a bit easier. I've got 3 sets of these to build and another set of the more complex ones before the next trip out in a months time. may investigate the power issue a bit more after that. the digispark is a great little board for these projects so it will be handy to know the limitations and requirements as i'm obviously quite close to the the edge of some thing that it needs to run reliably.