Resistance convertor

[GreenEnvy]

Hi,
I'm working on a project to convert a newer set of gauges from a motorcycle to work on an older one.
Everything is done so far except one last bit.
There is a temperature reading that is displayed on the gauges.
Basically how it works, is there is a temp sensor in the engine, that varies it's resistance depending on the temperature. This resistance is read by the gauges and converted into a temperature to display.

The problem is, between models they changed the resistance output by the sensor, so the readings are way out of whack. The difference does not seem to be linear either, so I can't just make a simple divider, and thats about as far as my electronics skills will take me.

Does anyone have any idea how to wire up a circuit to convert this?

Here are the values so you can see the problem

Old sensor:
50C/122F = 9560 ohms
80C/176F = 2780 ohms
120C/248F = 750 ohms

New sensor:
50C/122F = 209.8 ohms
80C/176F = 69.1 ohms
120C/248F = 21.2 ohms

Any ideas on how to wire something up to convert the older sensors values to line up to the newer ones?

Thanks.

 

[picbits]

You'll probably find that the clocks provide a current regulated output to the sensors which then make one side of a potential divider.

I've been toying with this idea and come up with possibly using a PIC microcontroller to do the maths and output an 8 bit DAC conversion. This is then fed into an Opamp driving a low side mosfet with the feedback making sure the voltage at the top of the mosfet is what the PIC is outputting on the DAC.

Not actually tried it yet but its a project (and a product) I'll have to work on at some point as I've had a few requests for this sort of application.

You could also consider using PWM as the output if the gauges are good old bi-metallic strip types.

 

[GreenEnvy]

I don't think the gauges do anything to the temp sensor. You can completely unhook the temp sensor from the gauges, and just hook an ohm meter across the temp sensor while it is sitting in a bot of hot water, and get the same readings off it.

The old style sensor is just a single wire coming off it, as the sensor itself is grounded to the bike.
The newer one has 3 wires, 2 of which go to the ECU, and are not needed for the gauges.

 

[picbits]

The old style will just be a NTC thermistor (temperature controlled resistor).

On many cars there is a voltage regulator on one side of the gauge, this provides a stable power supply (normally around 9-10 volts). The output of this is connected to one side of a gauge which is itself technically a resistor. This forms the top half of a potential divider.

The other side of the meter is then connected to the sensor which forms the lower half of the potential divider.

The gauge itself (unless its a newer electronic gauge) is quite often a bimetallic strip with a winding of wire round it - when current passes through the wire it heats up the strip which then moves the needle of the gauge.

When the sensor is at a lower resistance there is more current flowing through the gauge and the needle moves higher, when there is less resistance the opposite happens.

By using a mosfet or transistor in to ground the output (input) to the gauges, by applying a small voltage to the gate (or base) of the transistor/mosfet you can effectively control the gauge.

Now lets take a practial example with some easy numbers.

Assume the meter itself has a resistance of 50 ohms, the onboard regulator gives out 10v and your sender has a resistance of 50 ohms.

You then have a potential divider of 100ohms, the voltage from ground to the middle of the divider is 5 volts and you have a current of 100ma flowing through the gauge. If you can use a transitor or MOSFET with an opamp as a comparator instead of the sensor, feeding the opamp with 5v on a comparator pin will make the opamp try and keep the voltage on the other comparator pin at 5v emulating the sensor.

It all gets a bit tricky when you try and match the resistance vs temperature curve of the sensor - they are notoriously non-linear and if you plot the resistance vs temperature on a graph you'll see a nice curve.

Hence using a microcontroller to perform a lookup on a table (this might not be necessary if your curves between the two sensors match and you can just bias them correctly with resistors with a simple comparator circuit). The microcontroller will look at the input voltage from your sensor/resistor divider, work out an ouput value, convert this to a voltage with a resistor network which then feeds the comparator to drive the gauge circuit.

Sounds more complicated than it might be - from personal experience it could be an easy job to do or an absolutely nightmare job to do.

At the best you might get away with a few resistors, a regulator, a transistor and an opamp, at the worst you end up with a few days worth of messing about with programming a microcontroler, input protection circuitry and plenty of caffeine.

Where abouts are you located out of curiousity ?

Dom

 

[GreenEnvy]

I'm in the Niagara Falls region, in Ontario, Canada.

The temp reading on the bike is a LCD, showing the actual temp rather then bars or a needle. I don't see any coils on the PCB for the gauges at all.

 

[IRL3103]

3 wire sensors on cars require 12V, ground and output an analog voltage like a pot. Could it be something like that?

 

[picbits]

I'm in the Niagara Falls region, in Ontario, Canada.

The temp reading on the bike is a LCD, showing the actual temp rather then bars or a needle. I don't see any coils on the PCB for the gauges at all.

Shame - a bit too far from me to be swapping prototypes and such.

Most gauges work as above with the sender acting as part of a potential divider even if they aren't mechanical.

 

[GreenEnvy]

I think I'll look into seeing if the temp sensor off the newer bike will fit on the older bike, it seems much simpler then making a whole circuit

Thanks for the help.