Post by thread:[PIC]: Membrane latching power switch for PIC circ

Hello I am trying to move from a toggle power switch to a NO momentary membrane switch. My circuit thoughts are :- http://www.saurel.demon.co.uk/files/Membrane.bmp (Caps not shown) where the membrane switch S1 supplies power to the circuit at startup, which boots the pic. The first thing the PIC does is sets Pin B0 high, so S1 can be released. When the PIC detects button press of another membrane switch ("OFF" - not shown) pin B0 can be set low, thus powering off the circuit. Is this a sound design concept, and anyone suggest how to do it better? Can't find anything on Google / PIClist Archive or FAQ. Thanks in advance for any help. Les -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

part 0 44 bytes his is a multi-part message in MIME format. part 1 1424 bytes content-type:text/plain; charset=us-ascii (decoded 7bit) You have the right idea. If you only want one button but can tolerate a few more discretes, use the following circuit. Pushing the button will power up the pic just like you said, and at bootup set portb.1 high. Pressing the button again momentarily, sensed by portb.2, (polled or interrupt) and with a small timing routine will shut the circuit off. Depending on your routine, if you hold the button down too long, it may restart the power. Rick Les Otter wrote: {Quote hidden}> Hello > I am trying to move from a toggle power switch to a NO momentary membrane > switch. > > My circuit thoughts are :- > http://www.saurel.demon.co.uk/files/Membrane.bmp > > (Caps not shown) > where the membrane switch S1 supplies power to the circuit at startup, which > boots the pic. > The first thing the PIC does is sets Pin B0 high, so S1 can be released. > When the PIC detects button press of another membrane switch ("OFF" - not > shown) pin B0 can be set low, thus powering off the circuit. > > Is this a sound design concept, and anyone suggest how to do it better? > Can't find anything on Google / PIClist Archive or FAQ. > Thanks in advance for any help. > > Les > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads part 2 2469 bytes content-type:image/gif; (decode) part 3 154 bytes -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Yes, the concept is good -- however there is at least one basic problem with your circuit. You'll never get more than about 4.4V at the emitter of the transistor, since the PIC can only supply ~5V of base drive to the transistor. Use a PNP transistor (or P channel FET) with NPN driver stage (or N channel FET) to supply the regulator circuit. Matt Pobursky Maximum Performance Systems On Sat, 10 Aug 2002 13:04:31 +0100, Les Otter wrote: {Quote hidden}>Hello I am trying to move from a toggle power switch to a NO momentary >membrane switch. > >My circuit thoughts are :- >http://www.saurel.demon.co.uk/files/Membrane.bmp > >(Caps not shown) >where the membrane switch S1 supplies power to the circuit at startup, >which boots the pic. >The first thing the PIC does is sets Pin B0 high, so S1 can be >released. >When the PIC detects button press of another membrane switch ("OFF" >- not shown) pin B0 can be set low, thus powering off the circuit. > >Is this a sound design concept, and anyone suggest how to do it >better? -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

I've done this (in a non-PIC circuit, but the same sort of thing) using a P-MOSFET with the gate tied high with a high-value resistor. PIC sets the pin LOW to keep power on, sets the pin as input (hi-Z) to turn off. Dale --- We are Dyslexia of Borg. Fusistance is retile. Your ass will be laminated. On Sat, 10 Aug 2002, Les Otter wrote: > Is this a sound design concept, and anyone suggest how to do it better? > Can't find anything on Google / PIClist Archive or FAQ. > Thanks in advance for any help. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Here's what I'm talking about: http://madmax.botkin.org/dale/FETpower.png On Sat, 10 Aug 2002, Les Otter wrote: > I am trying to move from a toggle power switch to a NO momentary membrane > switch. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Don't forget that in some cases you don't actually need to turn the PIC off at all - with the right voltage reg, sleep mode draw is on a par with battery shelflife. The PIC can usually pretty easily power up other parts of the system as required. e.g. with a 9V PP3/MN1604 battery (500maH), and a micropower reg like a 78LC50 (ON/Mot), the 30uA or so draw in sleep with WDT enabled equates to about 15,000 hours, or about a year and a half. On Sat, 10 Aug 2002 10:16:13 -0500, you wrote: >Here's what I'm talking about: > >http://madmax.botkin.org/dale/FETpower.png > >On Sat, 10 Aug 2002, Les Otter wrote: > >> I am trying to move from a toggle power switch to a NO momentary membrane >> switch. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Matt Pobursky said.. > You'll never get more than about 4.4V at the emitter of the transistor, > since the PIC can only supply ~5V of base drive to the transistor. Use > a PNP transistor (or P channel FET) with NPN driver stage (or N channel > FET) to supply the regulator circuit. > > Thanks for the help..but I don't understand why I will only get 4.4v. I thought that if I supply 5v to the NPN base from a logic high PIC output pin, the transistor would act as a simple switch and pass the +9v across the C-E junction. Why only 4.4v? Thanks Les -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Dale Thanks, but that link is currently broken. Will try later. Rgds Les {Quote hidden}> -----Original Message----- > From: pic microcontroller discussion list > [spam_OUTPICLISTTakeThisOuTMITVMA.MIT.EDU]On Behalf Of Dale Botkin > Sent: 10 August 2002 16:16 > To: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU > Subject: Re: [PIC]: Membrane latching power switch for PIC circuit > > > Here's what I'm talking about: > > http://madmax.botkin.org/dale/FETpower.png > > On Sat, 10 Aug 2002, Les Otter wrote: > > > I am trying to move from a toggle power switch to a NO > momentary membrane > > switch. > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > > -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

"Dale Botkin" <daleKILLspamBOTKIN.ORG> said: > Here's what I'm talking about: > http://madmax.botkin.org/dale/FETpower.png Er... Proxomitron says the connection to your server timed out. i.e. the URL is bad. This one works though: http://www.botkin.org/dale/FETpower.png Later. -- Phil. .....philpemKILLspam.....dsl.pipex.com http://www.philpem.dsl.pipex.com/ -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Whoops. Sorry about that, mere mortals can't reach madmax on port 80 thanks to my stupid damned ISP. Try this: http://www.botkin.org/dale/FETpower.png Sorry for the confusion. Dale --- We are Dyslexia of Borg. Fusistance is retile. Your ass will be laminated. On Sat, 10 Aug 2002, Les Otter wrote: > Thanks, but that link is currently broken. Will try later. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Hi Les, In response to your question (below), you're right in that applying +5V to the base of your NPN transistor will make it act like a switch; but the transistor is basically a controlled _current_ source. So you design the circuit to pass the desired current into the collector and you get almost the same current out the emitter (depending on your beta -- for your purposes, you can probably assume it's the same) when the transistor is on. BUT, the voltage difference between the base and the emitter is 0.6-0.7 volts, like a diode (there is a pn junction between the base and emitter pins after all). So, if you apply +5 volts from a PIC pin to the base then your emitter will be at 4.4 volts. Regards, Todd. {Quote hidden}> Thanks for the help..but I don't understand why I > will only get 4.4v. I thought that if I supply 5v to > the NPN base from a logic high PIC output pin, the > transistor would act as a simple switch and pass the > +9v across the C-E junction. Why only 4.4v? > > Thanks > Les __________________________________________________ Do You Yahoo!? HotJobs - Search Thousands of New Jobs http://www.hotjobs.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Todd and everyone who replied Thanks for all your help. I really need to swot up on my analog stuff. I DID read Horowitz & Hill Art of Electronics (transistor section) before I posted the question, but I find it heavy going. I need to find a book which explains things in more basic terms - rather like the responses I have received from my posts to the PICLIST on this subject. So, if I understand your response Todd et al, that means I can drive the base of a PNP BJT with the collector of a NPN , the emitter of which goes to ground, and the base of which will connect to my PIC pin - rather like the circuit that Rick C posted on this thread, and Matt Pobursky pointed out? Appreciate your time Les PS So my circuit has +9v at the collector, +4.4V at the emitter...have I invented a logic controlled 4.6v heater? :o) > {Original Message removed}