Fabricating LM34 as MAT Sensor
To get a linear MAT response this ECM can use an LM34 as the
main temperature sensor. The typical thermistor is very non-linear and subject to some self heating affects. This device will output a
linear voltage at 10mv/degF. Hopefuly the copper pipe across the bores will do nicely to pickup the air temperature as the air flows past it. The
switch gives me an easy means to switch back and forth between the thermistor and the LM34 IC. The little circuit board and its components
is there to reduce noise on the +5 volt reference and to assure that the LM34 can drive a capacitance load. Don't know for sure just how
much capacitance the 12 feet of cabling will present but the snubber at the sensor won't hurt anything if there is none and will stablize the
circuit if there turns out to be too much.
First I decided to loose some of the height of the air tunnel. I built this tube a few years ago trying to get a more representative air
temperature reading. Taking another .75" off the top will reduce some air restriction. Hopefully the thermal heat sink on the LM34
sensor will do a better job than the thermistor.
At first I was going to use some copper sheeting on the inside of the plastic tube and solder the sensor to it as the heat
sink. Two things stopped
me, one I didn't have any sheet copper handy and secondly I was concerned that if I glued a sheet of copper to the inside of the tube all I would read
was the temperature of the plastic pipe. The copper tubing should do better. Very little contact area to the plastic pipe so it shouldn't
affect it much, and it is right in the middle of the air flow and shouldn't cause much restriction. Another plus is the way it is mounted there
is very little chance of anything falling into the motor.
The LM34 sweated into the end of the copper tubing.
The little proto board holding the supply filtering and output stabilization.
A home made Weatherpack connector mounting tab.
Some DOW 3145 RTV and the connector will be forever secured.
The toggle switch will allow me to select either the thermistor or the LM34 sensor externally.
Final assembly. Lots of stuff inside this air cleaner.....