Thanks for the reply. This all makes sense to me. I also thought that the A1 line would be from float to ground, that is why I put a pull up resistor on it thinking it would be high until I pushed the button. With the resistor in there, it shows about a half a volt. I pulled the PIC to make sure it wasn't causing the problem but no change. Could I have done something wrong when I substituted a P channel FET for the pass transistor? I have the source to the Battery voltage coming in and the drain to the regulator. Again, thanks to all for your help! FJ From: Russell McMahon Reply-To: "Microcontroller discussion list - Public." To: "Microcontroller discussion list - Public." Subject: Re: [PIC] one button on/off circuit problem Date: Tue, 12 Jun 2007 15:40:17 +1200 I'll copy this from below as its crucial *** The turnoff of A0 must wait until after the button is released *** or the still-pressed button will retrigger the circuit. _______________________ There's a fundamental problem with the circuit diagram AND there's almost certainly a problem with your circuit implementation. Adding a pullup on A1 makes the circuit operation defined and this may have been specified elsewhere. All Off PNP = Q1 NPN = Q2 Upper diode = D1 Lower diode = D2 Button = ground diode junction on so PNP on so power on. PIC pits A0 high so NPN on so PNP held on and all is well. Now, A1 cannot either affect or see anything useful. It is behind two opposed diodes so it cannot drive Q2_c hi or low If A1 is input and Q2 is off then Q2_c is high but D2 blocks the Q2_C high so A1 see's a floating switch. Pressing button changes A1 from float in to gnd in but this is not overly useful at this stage. With Q2 on = power on then Q2C ~~= 0 volt so D1 cathode and D2 cathode are at float. Again, closing button takes A1 from float to ground. A pullup on A1 allows the button press to be seen and then the processor can turn power off using A0. *** The turnoff of A0 must wait until after the button is released *** or the still-pressed button will retrigger the circuit. _____________ With button open and no PIC then A1 *MUST* float. If it has a 10k pullup it must pullup. If it doesn't the cct is not as shown. A P channel FET will work well as long as gate drive voltage is not too high or too low. Russell -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist _________________________________________________________________ Hotmail to go? Get your Hotmail, news, sports and much more! http://mobile.msn.com -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist