----- Original Message ----- From: "Artie Jones" To: "Microcontroller discussion list - Public." Sent: Monday, August 18, 2008 7:47 AM Subject: Re: [EE]battery saver question > > If I use an RC circuit (1 meg ohm and 100 uF) for timing and a comparator > with built in reference voltage (2V), am I am I violating the maximum > input > specs if I yank the power away from the chip while the IN- comparator > input > is still at 2V? > > The specs for my device say the inputs should never be more than .3v above > the Vcc or .3v below ground. This was my primary concern. Please disregard > the application information given previously-it just complicates the issue > without addressing my primary question. > Hi Artie, I don't know what part you're using but the 0.3 V above and below sound like the parasitic diodes which are an inherent part of some manufacturing processes, and also serve to protect the rest of the IC. If you were to discharge a 100 mfd cap through one it would probably destroy it, and consequently the remainder of the part. On the other hand you don't need to turn the power to the comparator off at all, if it's a nano-power part as you said. I sketched up a similar circuit using a CD4093B quad CMOS Schmitt Trigger NAND gate and calculated that a 9 volt alkaline battery should power that part for over 3400 years. The only catch is that you have to discharge your capacitor in order to reset the timer to allow the power to be turned back on. A double throw power switch which would discharge the capacitor in the 'OFF' position would work. Best of luck, Bruce VE7BRZ -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist