Originally Posted by Needlerp

I realised that it wasn't working. It was either reporting 0v or 3.5v, but nothing in between.

I might give up and go for the simpler option after all. How do I wire the LEDs so that they are on by default, and go off as the voltage rises through the LM3914 as per Roff's suggestion?

When the tank is full, the voltage is lowest. If you positioned the vertical LED bar normally (low voltage at the bottom), the bar would be fully lit when empty (because the voltage is high), and as the tank filled, the LEDs would go off one at a time, starting at the top. If you turned the bar upside down, the bar would still be fully lit when empty, but as the tank filled, they would go off one at a time, starting at the bottom. That sounds a little weird (my bad).
Perhaps you should use dot mode (with the bar still inverted). I think it would look normal, your LM3914 would run cooler, and you would save power.

Originally Posted by Needlerp

I think I may be nearly there. I've now got the MaxSensor to output 0-.68v when on its own, and an op-amp circuit ased on Eric's information to take 0-.68v from a POT and output 3.4v to 0v.

However, when I swap the POT over for the output from the Maxsensor, although according to my multimeter they are giving the same outputs, the OpAmp will then only output 0v to 0.7v, and both the input and output seem to be doing a lot of oscillating on the meter before settling on a voltage.

Does anybody have any suggestions?

hi,
Dont quite follow you explanation regarding pot swopping over.?

Any chance you could post a simple diagram showing how the FULL circuit is connected.

Eric,

To see whether the circuit works, I used a pot to divide a 5v supply so that I could simulate a Vin range of 0-0.68v. Doing this, the OpAmp circuit works perfectly, and is set up as per the attached.

However, when I remove the Op Amp and replace it with my actual meter, which I separately check is also showing the above voltage range on a multi-meter, I get a completely different response from the Op-Amp. It's as though the Op-amp is clipping my input at anything below about 0.5v, even though I know it works when providing that low voltage via the POT mock-up.

Thanks

hi,
Thats not the same circuit as the circuit I posted.???

I will look it over, but have you changed the inputs and component values.

Originally Posted by Needlerp

I'm powering the OpAmp at 9v single supply, so went back to your original values before you updated them to 5v for me.

OK,
And is the opa a 741.?
I am going thru your cct on the LTspice sim.

Originally Posted by Needlerp

No, it's a CA3140EZ

OK,
Can you describe what is connected to the 1K resistor on the INV input.
That generates the 0 to 0.68V.?

Originally Posted by Needlerp

It's a MaxBotix MaxSonar EZ4, which provides an analog output of the sonar reading of 0.98mv per inch of depth. which for my purposes returns 0-0.68v as readings. When I power it up and check the output using a multimeter it is indeed returning 0-0.68v, but as soon as I connect it up to the rest of the circuit it seems to stop working properly.

I have just downloaded the d/s for the MaxBot, get back to you later today.

hi,
From the MaxSonar data and your values of 0V thru +0.68V [ about 68 inches range], using a CA3140 powered from a 9V supply, the CA3140 opa outputs +3.8V thru 0Vish. I am assuming that the 9V is a regulated stable value.

I have added the LM324 opa buffer thats part of MaxSonar circuit in order to show the the source of the 0V thru 0.68V.

As per this attached image, the circuit works OK over the full range, the only minor limitation in using a single supply is the 0V is about 10mV.!

You must have a problem in your layout.