# Circuit to invert a signal

Discussion in 'General Electronics Chat' started by Fly_Champ, Mar 19, 2008.

1. ### Fly_Champ Thread Starter New Member

Mar 19, 2008
3
0
Hello,

I am a mechanical engineer who has a (I would think relatively simple) problem.

I have a legacy gauge that reads a resistance from a sensor, and the range is 85ohm full and 2ohm empty.

I want to change the sensor because the old one is not reliable, but new sensors are not made with this signal configuration. Current sensor configurations have a output of 0ohm full and 90ohm empty.

Without throwing out the gage, can I make a circuit with a transistor or something to invert the signal and have the gauge work as designed?

Thanks

2. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
You could possibly use an operational amplifier circuit to do that.

You can get an introduction to op amps here:

Your sensor is acting as a variable current sink, which is causing a varying amount of current to flow through your gauge, giving an indication by needle deflection (or similar).

You could use an op-amp circuit as an inverting voltage-to-current converter to sink a similar amount of current as the old sensor did.

3. ### Fly_Champ Thread Starter New Member

Mar 19, 2008
3
0

After looking at the op-amp tutorial, I don't quite see how to design a circuit that would sink current.

It appears that I need to wire an amplifier to invert with a gain of 1.

Could I apply voltage to the new sensor and feed the signal into the op amp, and then have the output of the op amp wired directly to the gauge?

4. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
By George, I think you've got it!

Actually, the gain will likely need a bit of "tweaking", as even inverted, your old sensor doesn't quite match up with your new sensor.

Try this experiment (where resistances are given, set your new sensor to the resistance using your digital multimeter):
Connect a 2 Ohm resistor to the input of the gauge, and on the other end of the resistor, an ammeter to ground. Current will likely be on the order of a few milliamps, but it could be as high as 100mA - perhaps more, if it's a really old design - so start from higher amperage settings on your DMM, and decrease range. Note the gauge position. Record the results (resistor Ohms, current, gauge position)

Then try the same experiment with an 85 Ohm resistor. Note the gauge position. Record the results.

5. ### Fly_Champ Thread Starter New Member

Mar 19, 2008
3
0
This is what I've come up with so far.

I will try to measure the true sensor resistance tomorrow after I get a couple of resistors with the right values.

I'm not completely sure if this is the proper way to get a gain of 1 and sink current to emulate the legacy sensor unit.

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