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Old 08-30-2006, 09:21 AM   #1
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Default Electronics guru's - How to get a 5v reference signal?

General electronics question......

I need a 5v source to power my MAP sensor so I can log boost with my Wideband Commander. This is in my '68 Firebird. How can I convert 12-14 volts to a constant 5 volts? Is there something fairly simple I can rig up, or something I can buy to do this?

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Craig
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Old 08-30-2006, 10:29 AM   #2
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What you need is a 5V volt regulator. What type you get depends on the amperage that it draws. If it is less than 1 amp (1000 mA/milliAmps) then you can get a 5V cigarette style adapter from Radio Shack (cringe). The better solution would be to build a circuit around a 7805 regulator. A discussion is at http://www.minidisc.org/5v_regulator.html and a circuit is at http://www.seetron.com/an_vpwr1.htm. Remember, this will only supply one amp. I can build one of these for you if you like.

If it draws more than one amp, I would have to look around for a heavier duty circuit. I wonder if you can strip a little 5A Astron supply and just turn down the regulator...
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Old 08-30-2006, 10:34 AM   #3
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Thanks, I'll check out those links. I can't imagine that a MAP sensor draws more than 1 amp, but I don't know. All it is is a little rheostat? I think that has a 5v input and it varies the output from 0v-5v based on manifold pressure.
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Old 08-30-2006, 10:44 AM   #4
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78L05 from Digi-Key or 7805 from Radio Shack (a higher
power capable package, if you heat-sink it). See the
pinout at www.national.com (part search for datasheet).
These are super easy to use, VIN to ignition 12V, VOUT
to the MAP sensor supply pin, common all the grounds
and away you go. Maybe add a bypass capacitor to the
output just for quietness' sake, a 0.1u ceramic. I'd go
and bare-wire it, epoxy the regulator right to the MAP
sensor body and put the whole thing in a project box
with a tube for the manifold pressure and 12V, GND and
output. The National datasheets should have simple
application schematics for you too.

You need to find the documentation for whichever MAP
sensor you use, to determine the endpoint voltages,
they vary slightly between versions. Here is one 'sheet
off Delphi's web pages...
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Electronics guru's - How to get a 5v reference signal?-turbo_map_sensor_delphipdf.png  
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Old 08-30-2006, 10:54 AM   #5
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I'm using a GM p/n 12223861 3-bar MAP sensor I believe from an '89 Turbo T/A.

Here's an idea.....can I rig up one of my cell phone car chargers? It appears to have a 5v output and of course has a 12v input. Got a couple old one's laying around.
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Old 08-30-2006, 12:02 PM   #6
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It's possible, do not know how sensitive the MAP sensor
is to the 5V reference signal or how clean & tight the
charger output voltage is. Many sensors are ratiometric
and if you gave it (say) 5.5 instead of 5.0 volts you
might see a corresponding shift in output. Worth a try,
put the meter to it though and see if you are getting
the 100kPa value out (or whatever your local atmospheric
pressure is, today) in open air.
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Old 08-30-2006, 05:08 PM   #7
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Thanks. I thought I could just put my multimeter on it and see if the output varies at all and maybe rev the engine to make sure as the input 12v+ varies that the output doesn't follow it as well.
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Old 08-30-2006, 05:11 PM   #8
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Why not just tap off of the TPS's regulated 5V from the PCM? Excuse me if this got posted twice.
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Old 08-30-2006, 05:14 PM   #9
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Quote:
Originally Posted by dynocar
Why not just tap off of the TPS's regulated 5V from the PCM? Excuse me if this got posted twice.

What TPS? What PCM?

This is on a 1968 Firebird with a carb......not EFI.
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Old 08-30-2006, 07:30 PM   #10
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I used the Radio Shack 5V regulator to get 5V to a Autometer Fuel Pressure sending unit I am going to use for logging with EFI. It was cheap (under $3) and easy (just solder three wires to it, 12V+, Ground, and 5V output). Measured it with a meter when I was done and I had 4.97V at the sensor.
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Old 08-31-2006, 10:50 AM   #11
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Quote:
Originally Posted by 1fastWS6
What TPS? What PCM?

This is on a 1968 Firebird with a carb......not EFI.
Oops, sorry about that, did'nt look close enough at the year.
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Old 08-31-2006, 10:50 AM
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