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Topic: Variable Resistor Sensing  (Read 151 times) previous topic - next topic


Hi. I need help sensing a variable resistor, the problem is that i can only access one side of the resistor because it's screwed to the intake of an engine (it is used to sense the temperature of the engine), and i dont know how to get a read on it to use with my arduino mega.

PS: I need to use my arduino to control the cold start of the motor, on Honda A20A 2.0 engine. I already have everything working in theory, the only thing left is the temperature of the engine.

Thenks in advance


If you can get at the R and the gnd (and they are not the same!) you should be able to use it. You need to make sure the voltage does not exceed 5v.

Do you know what the signal is? Is it variable voltage? Is it pulsing? Is the voltage over 5v?



The signal can be anything, because i can access one side of the resistor(a cable that comes out of it), the variability is controlled by temperature and the out of the resistor is the ground of the engine. I have measured continuity between the cable that I can access and the ground of the motor, when the motor is cold there is not continuity but when the motor starts to warm up the continuity starts to go up until the motor has reached its optimal temp and there is full continuity.

PS: I think I could use the 5v output of the arduino connected to the resistor and the ground connected to an input pin, but there is so much electric noise in the gnd of the motor that there could be false readings


We would be interested in the voltage more than R as this is what can destroy the Arduino.



Rather than measure continuity, measure resistance, OHMS.
This will give you a better set of values for setting up the arduino input.

Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....


Why don't you glue a thermistor to the motor in the same place as the existing resistor. Determine what resistance the thermistor has at the desired temperature, is when the existing resistor conducts. Then simply read the thermistor with your mega, thermistors always give same resistance for specific temp.

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