# Need help designing a circuit for my car

1. Oct 31, 2008

### Toolmaster

Hi all! Thanks in advance if you're able to help me here...

It's been 20+ years since I've toyed around with circuits, and I'll admit I forgot most of what I used to know.

My application is as follows:

I have a gauge in my car, that reads pressure (turbocharged car application). I'd like to drive that gauge with a different sender. Stock form, the sender drives the gauge from 0v to 5v, on a scale of 0psi to 28ish psi (14.7psi is atmospheric pressure - or at least thereabouts) , and the signal is linear. I'd like to replace the sender with a different one, which will also provide 0v-5v output, but on a different scale.

To clarify - right now, when my car is boosting to 1 bar (14.7psi), I get 2.5v out of the existing sender. IF I place the new sender in there, I'll get a smaller voltage, because the new sender has a larger range of values. Figure the new sender will have a range of 0-44 psi, so if I do the math, I expect to see about 1.666v. I want to be able to basically change the scale of the output so that the output of 1.666v from the new sender will actually send 2.5v to the gauge. Basically, it's a calibration circuit, and the new sender is also linear.

Ideally, I'd like to have a good way of selecting my range of voltages for the new sender, so I can use almost any sender I can find. Also, I'd love to be able to dial in the new sender and gauge, so that i can set the gauge to read any desired boost pressure. Like if I wanted the marking of 1 bar on the gauge to be atmospheric pressure, adjust a pot thusly, or if I wanted the 1 bar on the gauge to be 1 bar of boost, adjust accordingly.

So the circuit I'm looking for will read the input voltage, convert it to a different voltage, which could be higher or lower, (in a linear fasion) and then output that to my gauge. The circuit shouldn't send more than 5v to the gauge, ever.

Hoping someone can give me a hand...

Thanks!

Jay

2. Oct 31, 2008

### dlgoff

Simple way. Use a resistor divider. You can scale the output any way you want. If the gauge has a really low input resistance, then you would need to add an amplifer (op-amp) to provide the drive from the divider circuit. Probably just a voltage follower.

3. Oct 31, 2008

### Toolmaster

I had thought about that... The only issue I see is that if I use a divider, then I can never truely max the gauge out. Dividing the voltage always would mean (assuming R1/tap to drive gauge/R2) that technically I can never see a full 5 volts on the gauge.

Do I recall correctly?

4. Oct 31, 2008

### Toolmaster

Oh - and besides... the new sender would have a lower voltage to start with, so I'd really need to step it up which a voltage divider wouldn't do for me...

Thanks!

5. Oct 31, 2008

### dlgoff

Well if you want to use the existing meter, which has a full scale at 5volts, you will still need to use a voltage divider with your new sensor.

edit: if your new sender/sensor has an output of 12 volts @ 44psi, then you will need to divide that down to 5 volts @ 44psi

6. Oct 31, 2008

### Toolmaster

The new sender uses the same 0v-5v as the exisiting one - I just need to shift the voltage lower or higher through this circuit to make the gauge correct.

J

7. Oct 31, 2008

### dlgoff

Well I thought you were going the other way. In that case you will need to amplify your signal. To keep things linear, you should use an operational amplifier. There is a Op-amp circuit collection listed in the External links at the bottom of the page.

8. Oct 31, 2008

### Proton Soup

he'd still need a way to crowbar the voltage. maybe an appropriately sized resistor and zener on the output. or crowbar the input for that matter.