Help Building a Water Meter Gauge using LM3914 and PreAmp

Needlerp said:

No, it's a CA3140EZ

Click to expand...

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

ericgibbs said:

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

Click to expand...

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.

Needlerp said:

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.

Click to expand...

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.