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Kevin Redon kevredon at mail.tsaitgaist.infoI'm back. I was looking around for a solution for such a trivial task (power switching), and there is no easy answer. >> the other idea would be : >> >> 0.7-4.5 --- VREG 5V --->|----+ >> | >> +--- VREG 3.3V >> | >> USB 5V ---------------->|----+ > not effective. > > In fact, now that I'm thinking off it, we should use one of those jacks > which disconnnect one contact when plugging in the power supply. This > way we can actually interrupt the USB 5V once a power supply plug has > been plugged in. This should be done independent of the decision on the > voltage regulator. most dc jack can provide such a solution, but I think the USB 5v should be used if the two sources are connected. USBVCC is more stable, moreover if the user leaves it connected the battery will be drained. Another point is that the normally closed circuit uses the barrel (normally the jack is used to plug wall power, cutting the board battery line). Having +5v on the barrel is not common (if the user has an of-the-self power adaptor). > >> or another nice idea (like the arduino does for the 5v) : > > looks not that trivil to me, as you don't only need the FDN304 but also > the LM358 op-amp and external circuitry. Probably a bit overkill. > agree > So I think we should route the 5V USB via the power plug, and simply use > a low-drop-out 3.3V regulator, like in the current schematics. This way > the LDO input will be USB 5V _until_ somebody plugs in an external power > supply or battery pack, which will have to provide at least 3.4V for > stable operation. > yes, I also like this solution, but has drawbacks (see above) I found two other solutions : - using a automatic power switching mux (TPS2115). too expensive and take to much space. - using a manual switch to select the power source > >> then it would be 4xAAA/AA (3xAA will only work if fully charged) > > fine with me. I honestly don't care about the number of batteries... > and the PCB will not be smaller than 4x AAA/AA anyway. > that is the decision I took. Simple, cost effective, saves place. > that should work, but you should use something like a 1N4001 as the > diode, to cope with more current 1N5819 allows 1A, it should be enough. 1N4001 has a forward voltage of 1.0V, 1N5819 only 0.6-0.7V. here the schema : https://gsm.tsaitgaist.info/SIMtrace/v0.9/ I now an doing the BOM. I'm half-way through, but it takes time. Kevin