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How to build an under the hood temp. sensor on a budget

To: spridgets@autox.team.net
Subject: How to build an under the hood temp. sensor on a budget
Date: Thu, 20 Jun 2002 23:26:24 -0400
Several of you have asked how I made a CHEAP temperature
sensor for under the hood of my MG.  It was made using a simple
thermistor.  A thermistor is a device that changes the resistance
based up the temperature.  The unit I bought was 10K ohms (+|-) 1% at 25
degrees C.  You can buy thermistors that are already calibrated.  The
first thermistor I choose was a radio Shack thermistor, part number
271-110A, which is available for < $4.  This device's resistance
changes based on the temperature.  This thermistor range is from -50C
to +110C. Here are some of the values:

        Temperature(C)  Resistance (kohms)
        -50            320.2
        0              27.28
        50             4.151
        100            .9375
        110            .7579

If you connect one of these devices to Digital Multi Meter (DMM) you
can read the resistance and then compute the temperature.  To build
really cheap temperature sensor you need the following:

       1 - thermistor
       2 - lengths of wire to run from the engine compartment to the
       passenger compartment
       2 - 2 banana connectors that fit into you DMM
       1 - Digital Multi Meter (DMM)
       1 - 1 clicking type BIC pen

Step 1.  Measure two lengths of wire so they can run from the
carburetors to inside your passenger compartment.

Step 2.  Solder these wires to each of the thermistors leads.

Step 3.  Disassemble the BIC pen.  Keep on the lower barrel.

Step 4.  Insert the thermistor into the pen so that only the ceramic
head protrudes from the pen barrel.  This may require you to trim the
the pen opening wider.

Step 5.  Once you have the correct fit, use 3M (plug for a US company)
electrical tape to tape the wires to the pen barrel.  This will hold
the thermistor in place.

Step 6.  Cable tie the BIC pen in the area that you want to monitor
the temperature.  I tied mine to the hose that runs from the valve
cover to the carburetor using 3 cable ties. Since you are measuring the
air temperature do not touch the ceramic tip to anything.

Step 7.  Route the two wire through the firewall into the passenger

Step 8.  Attach the banana connectors to each wire.

Step 9.  Plug the banana connectors into the DMM and set the DMM scale
to 10K ohms.

Step 10.  Turn on the DMM and read the value.  Take that value and use
the lookup table that came with the thermistor to get the temperature

------ Results ----------

Here are some of the value that I got today.  The ambient temperature
was about 80F.  These numbers are in the correct order based upon my
trip home.

Kohms     C   F      Comment
4.0       50  122    Running in forth gear at 3K RPM on country roads 4 miles 
                     starting the trip
2.86      63  145    Traffic light waiting about 45 seconds
2.42      67  152    Stop sign waiting about 10 seconds
2.41      67  152    Traffic light waiting about 20 seconds
3.5       55  131    Running in forth gear at 3K RPM on country roads
                     on the second half of the trip (car got warmer
                     over time)
2.16      72  161    Traffic light waiting about 20 seconds
.889      103 217    Idling in front of the house for 2 minutes
.55       N/A N/A    1 minute after the engine is shutdown (scale only
                     goes down to .7579)

This method is easy and cheap.  The only problem right now is the
thermistor that I am using needs to have a higher temperature range.
They make thermistors that will do the job, I just need to find one on
a store shelf in my area.  I am interested in the number when the
temperature gets into the 90's.

Anyone have any comments on my methods?  Any major flaws or problems?


Bill Gilroy   w.gilroy@verizon.net

77 Midget
95 Toyota
01 Irish Terrier

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