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04-04-2008, 03:05 PM
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#31
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Low Bitrate
Join Date: Aug 2003
Posts: 66
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Try it on a piece of scrap board, I'm quite sure that leaving an input pin at Hi-Z (tri-state) will give you unpredictable results.
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04-04-2008, 04:14 PM
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#32
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Raw Wave
Join Date: Apr 2000
Location: Surrey, UK
Posts: 2,101
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Quote: Originally Posted by scsirob  Try it on a piece of scrap board, I'm quite sure that leaving an input pin at Hi-Z (tri-state) will give you unpredictable results.
 Hence my original suggestion of a pull down on the ignition line. I would be most surprised if the device has internal pulldowns, most don't.
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04-05-2008, 06:08 AM
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#33
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Newbie
Join Date: Sep 2006
Posts: 46
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Ok I tried it and I was totally wrong, it weirds out completely  Thanks for bringing that up scsirob, I think my confusion came from other times when I was testing on a logic board but of course then it would be grounded or 1 input, not floating.
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04-05-2008, 07:54 AM
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#34
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Low Bitrate
Join Date: Aug 2003
Posts: 66
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Glad to have helped out. Good luck with the rest of the design.
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04-20-2008, 04:35 PM
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#35
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Newbie
Join Date: Sep 2006
Posts: 46
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Ok, it is now built. Working just great, I would make a couple of revisions for the next one I build (probably do a couple for friends). The only change was swapping the 4.7 Zener which regulated VCC for a 7805. The AVR just wasn't enough to take the full load from the zener and it was getting too hot. The 7805 in this circuit doesn't even get warm to the touch. With the AVR in sleep mode I can't even get a reading on my ammeter.
Pictures soon, great success!
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06-01-2008, 03:47 PM
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#36
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Low Bitrate
Join Date: Aug 2003
Posts: 66
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Looking at the value for R11 it's no surprise the zener became hot... 30 Ohms is way too low...
Your circuit (AVR + LED1) will draw no more than roughly 20mA. If you want to have regulated 4.7V with battery power down to, say, 9V then you should dimension R11 to bleed off (9 - 4.7) = 4.3V at 20mA. That would be a 215 Ohm resistor. Take 220 Ohm as the nearest standard.
When your battery is full at 13.8V, the current through R11 will be about 40mA, dissipating about 350 mW. The zener will see about 35mA when the LED is off, so it will dissipate 165mW worst case. Things should work fine using a 4.7V, 1/4W zener and a 220 Ohm, 1/2W resistor for R11.
Looking forward to the pictures...
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06-01-2008, 05:23 PM
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#37
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FLAC
Join Date: Mar 2004
Location: Florence Yall, BFKY
Posts: 1,722
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Linear regulators are cheap. I would have stuck with it since I'm always adding on later. It's really just a transistor, diode, and resistor. Like this...
http://www.satcure-focus.com/tutor/page5.htm
__________________
XPort 1.26 -GPS port splitter, logger, and USB device resume fix
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06-02-2008, 07:12 AM
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#38
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Newbie
Join Date: Sep 2006
Posts: 46
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I just ended up swapping out the zener for a quick and dirty 7805. Doesn't heat up at all and is fine, been running in the car for a couple of weeks now.
Currently having a problem where it seems to get confused and skip states on cranking, not had time to figure it out yet, a few code checks are a fix at the moment (the breadboard has ISP headers so I can program with it in the car and use Debugwire for realtime diagnostics if I want). The laptop very rarely just switches off as well (as if the power button has been pressed, it hibernates) whilst the avr thinks it is still on. This has occasionally caused some issues.
Currently I suspect the car wiring and will be redoing that when I have time (exams and stuff at the moment!).
I promise I will get pics soon!
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06-02-2008, 08:56 PM
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#40
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FLAC
Join Date: Mar 2004
Location: Florence Yall, BFKY
Posts: 1,722
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Ooh, that is quite a bit more. Thermal protection is certainly important.
Shad0wca7, add a capacitor after the regulator. Like a 470uF 16V elec. Bet that would help.
__________________
XPort 1.26 -GPS port splitter, logger, and USB device resume fix
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06-08-2008, 10:32 AM
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#41
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Newbie
Join Date: Sep 2006
Posts: 46
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Not had time to fully test it and identify issues, all the gear (AVR Dragon & Multimeter) is in the car just waiting but I still have bastard exams!
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08-15-2008, 06:41 PM
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#42
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Newbie
Join Date: Jul 2008
Posts: 11
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Did you ever get a chance to go back and take a look at this? I just designed a very similar controller, using a PIC, that controls the OEM navigation dash piece motor for my car (RX-8) since I replaced the screen. I've got the battery going to a lm2936 (regulator that uses very low current, circuit draws <1mA in sleep) and the ignition wire going to a 7805. Works fine on the test bench with 12v regulated, but I'm fearing problems with cranking and the controller resetting when I actually try it out. Ideas anyone?
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08-16-2008, 10:30 AM
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#43
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Newbie
Join Date: Sep 2006
Posts: 46
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I did solve it, surprised I forgot to post on here!
Turned out that after about 4 hours debugging, the microcontroller was faulty. I must have accidently sent 12v through it at some stage during testing. Replaced it and it as been perfect since.
I would use more protection than just a regulator. You should include some form of transient voltage supression just incase.
All my circuitry before the regulator on my diagram is to protect the system from cranking etc, seems rock solid so far.
Last edited by Shad0wca7; 08-16-2008 at 10:35 AM.
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