How to reverse a pressure transducer output voltage

I am trying to set up an air compressor on my truck and run it through a Ford APCM (auxiliary power train control module). It has a 5v output. The APCM has a custom throttle control input on it. It will run the truck at 1200 rpm at .5v input and 2500 rpm at 4.5v

I have a 150psi 0-5v pressure transducer for the air tank pressure. The pressure transducer outputs .5v at 0 psi and 4.5v at 150 psi.

I would like the truck to run at 1200 rpm (.5v APCM input) when the air pressure is at 150 psi (4.5v transducer out put) and run at 2500 rpm (4.5v APCM input) when the system is at 0 psi (.5v transducer output).

I have messed around with and op amp circuit and I have got it close on both ends but can't get my desired voltages at both ends. Is an op amp correct for this or is there a better way to do what I am trying to do?

• You know that's hard to read with poor capitalization and no paragraph breaks? – JRE Mar 11 '17 at 19:37
• Leave out all the truck stuff and explain what input range of voltages needs to map to what output range. – Andy aka Mar 11 '17 at 19:47
• An OpAmp would do that, fairly easily. Or a good old transistor. – dannyf Mar 11 '17 at 22:51

Use a diff-amp circuit to subtract the sensor output from your high idle reference.

simulate this circuit – Schematic created using CircuitLab

I also added a buffer to the sensor input to give you a high impedance input.

It would be prudent to add some noise filtering in there though and probably voltage limiting to the output.

• Output: 4.5 to 0.5
• Input:0.5 to 4.5

Normally there is hysteresis to prevent the pump revving up with a small pressure drop.

You need an inverting comparator with a threshold of 4.5V and output swing of 0.5 to 4.5V using a 5V supply. If you want to make it proportionally reduce speed then a linear negative feedback gain is used. If you simply want to switch speeds with some deadzone or hysteresis after full pressure is reached then the reference voltage is reduced by the amount of positive feedback, e.g. 10%.

I chose a Schmitt Trigger inverter which has negative feedback but a comparator will give more precise thresholds with positive feedback ratio, variable threshold and a certain % hysteresis.

Simulation

result

This is just a quick and dirty solution, not an optimized one.

Rev A Linear control

And yet another way using BJT type Op Amps that cannot swing down to 0.5V

simulate this circuit – Schematic created using CircuitLab

yet another crude way

• I would like to keep it proportionally reduce/gain speed as air is used. Most of this stuff is over my head as i turn wrenches for a living. – Austin Sandford Mar 11 '17 at 20:51
• I have set up as per the schematic. i am using an adjustable power supply set to 5v. I am using a 5k and 10k pot to set my 2.5v and adjust my .5v to 4.5v. I am able to get the high end i am requiring but can only get down to 1.27v on the low side. what could i be doing wrong. I am in need of the .5v more so than the 4.5v end of things – Austin Sandford Mar 12 '17 at 20:47
• either need -ve Vee supply or Rail to Rail output Op Amp – Sunnyskyguy EE75 Mar 12 '17 at 21:03
• or if running from 12V then insert Darlington NPN between output and any R to ground with feedback from R. This will drop output 1.4V – Sunnyskyguy EE75 Mar 12 '17 at 21:19
• how would i create the -ve supply? or what rail to rail output op amp would you recommend? – Austin Sandford Mar 12 '17 at 21:20