MaxBotix Sonar Sensor - Help please

[Needlerp]

Hi,

I'm trying to wire up a MaxBotix MaxSonar EZ4 to an inverting op-amp using the analog output, to turn the 0-0.68v range I'm expecting from the MaxBotix (for my distances) into 3.4-0v.

I can get the opamp to work perfectly when using a test voltage, but as soon as I connect the MaxSonar it stops working propertly, and testing the voltages across the Analog output of the maxsonar I no longer get the full range I did when disconnected.

Am I missing something obvious? Do I need to add a resistor/capacitor somewhere in the wiring?

Many thanks

[RyanCahoon]

Can you post the schematic of how you have the sensor, op amp, and related components hooked up? It'll help us help you.

--Ryan

[Needlerp]

Thanks. Attached is what I think I'm working to. Sorry it's not very elegant!

The opamp part of the circuit works fine when I put a test voltage across the inverted input, but as soon as actually hook it up to the MaxSonar is when it stops working.

[DonRotolo]

Two problems:

First, the input impedance of this circuit is far too low, which is what is dragging the sensor output down. Increase the values of the 1k and 10k/15k resistors by a factor of 10 or even better 100.

Second, this circuit appears to have a gain of -10, while you seem to want a gain of something like -(3.6/5=0.72), so the 1k and 10k resistors need a different ratio. (I could be wrong here - it's been a long time since I did this, but check into it please)

Here is a spec sheet for the LM358, study it (especially the examples at the end) and use Google to look up op-amp circuits (you want an inverting gain amplifier, even if the gain is -0.72).

Good luck!

[Needlerp]

Thanks, I'll give that a go. Will I also need to adjust the resistances of the part of the circuit driving the Bargraph, to stop a similar problem happening there?

I need a gain of -5, which seems to be OK based on 5k across my 10k pot and 1k in serial with Vin.

Thanks for your help.

[DonRotolo]

Probably not. But, Google the part number (LM3914), find the data sheet or a sample circuit, and go from there.

As an electrical engineer, when designing a circuit like this it would be rare for me to design it from scratch. The first step is always the manufacturers data sheet and applications notes, even in the days before the Internet. Then on to other ideas, reducing my job to adjusting the components to what's available/cost effective and still works.

[Maxbotix]

Needlerp,

The analog voltage output from the MaxSonar product line is best suited for input into a high impedance amplifier or an AD converter. An op-amp, when connected as a non-inverting amplifier will have a high impedance input for the sensor to drive.

Alternatively, if you prefer an inverting amplifier, use 100K in the first stage (i.e. the resistor leading from our sensor to the op-amp) and 500K in the feedback from (the op-amp output to the input) should do the trick just fine.

If you have any further questions, please let me know (or post here and I will check back)

Carl Myhre, Systems Engineer, Carl @ Maxbotix."com" (remove spaces and quotes)