Electronics > Beginners
How to make an analog circuit to proportionally scale a sensor output voltage?
GerryR:
The multipliers across the pressure range are specifically as follows:
Bar: 0.5 1.0 1.5 2.0 2.5
Multiplier: 1.029 1.145 1.183 1.202 1.213
I'm probably complicating this more than necessary, but from the chart, above, given by the OP, the sensor output requires a different multiplier (gain) at the different pressures in order for the new sensor to match the original sensor output. A fixed multiplier would seem to address the output difference at only one point. It would seem that a variable gain amp, or as a previous poster stated, a MPU with a look-up table, to set the output per the new input levels, would be what's needed??? What am I missing? (I told you I can be a little slow.) Thank you.
sokoloff:
--- Quote from: GerryR on July 05, 2019, 01:32:26 pm ---The multipliers across the pressure range are specifically as follows:
Bar: 0.5 1.0 1.5 2.0 2.5
Multiplier: 1.029 1.145 1.183 1.202 1.213
I'm probably complicating this more than necessary, but from the chart, above, given by the OP, the sensor output requires a different multiplier (gain) at the different pressures in order for the new sensor to match the original sensor output. A fixed multiplier would seem to address the output difference at only one point. It would seem that a variable gain amp, or as a previous poster stated, a MPU with a look-up table, to set the output per the new input levels, would be what's needed??? What am I missing? (I told you I can be a little slow.) Thank you.
--- End quote ---
Probably just missing this later update from OP:
--- Quote from: HwAoRrDk on July 04, 2019, 04:43:57 pm ---
--- Quote from: Dundarave on July 04, 2019, 05:34:53 am ---But since there is an inflection point at 0.5 Bar, it makes things a bit more complicated. However, I noticed that at 0.5 Bar, the output of the original looks to be 0.7ish volts...
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Yes, I screwed up the original graph which made this crucial attribute not obvious. The figure for 0 Bar on the original sensor actually comes out to -0.2V according to the transfer function, but seeing as the sensor can't actually output a negative voltage, I manually entered zero, inadvertently making the graph slightly misleading. :palm: Correct graph below.
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rs20:
--- Quote from: GerryR on July 05, 2019, 01:32:26 pm ---The multipliers across the pressure range are specifically as follows:
Bar: 0.5 1.0 1.5 2.0 2.5
Multiplier: 1.029 1.145 1.183 1.202 1.213
I'm probably complicating this more than necessary, but from the chart, above, given by the OP, the sensor output requires a different multiplier (gain) at the different pressures in order for the new sensor to match the original sensor output.
--- End quote ---
You are right in that you would have to mess with different gain values for different pressures IF all you had was gain. But mikerj's circuit has gain AND OFFSET. You're missing the offset part. With a fixed gain and a fixed offset, a nearly perfect conversion is achieved.
GerryR:
Got it!! :-+
max_torque:
--- Quote from: vk6zgo on July 05, 2019, 10:15:05 am ---
--- Quote from: HwAoRrDk on July 04, 2019, 03:15:21 am ---I have this device that takes an input from an air pressure sensor. The sensor gets a 5V reference, and the device's MCU reads the sensor's output voltage using its ADC. The sensor reads up to 2.5 Bar.
However, this particular sensor is expensive and hard to get, so I have been thinking about the possibility of substituting it with something more commonly available. My mind turned to automotive MAP sensors, which are ubiquitous and cheap. In particular, the common GM 3 Bar MAP sensor would be a great candidate. But, of course, it works over a different pressure range (2.5 versus 3), so therefore the output voltage will not match for any given pressure.
I found spec sheets for both my existing sensor and a Delphi-brand GM sensor, and made a graph of pressure vs. voltage using the provided transfer functions:
Existing sensor is in green, GM 3 Bar in dark red. As you can see, the voltage difference is proportional to pressure; for example, at 2.5 Bar the GM one is approx. 82% of the other, but 87% at 1 Bar.
The obvious solution might be to say "modify the MCU firmware with different sensor calibration data", but I can't because I didn't make the device and I can't re-program it. :)
So, my question: is there some way, using purely analog means (i.e. no MCU/ADC/DAC), that I can scale the voltage from the new sensor to match the output of the old one? Ideally, I would be able to add the circuitry in-line in between the sensor and the device.
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The OP isn't using it in an automotive application, see part of post I emphasised in red.
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er, the OP hasn't actually said what the application is have they? All the talk has been about "engine vacuum" and "boost pressure" etc, so i was thinking the application is automotive??
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