Pressure sensor circuit advice

Discussion in 'The Projects Forum' started by CreativeKitty, Sep 27, 2013.

  1. CreativeKitty

    Thread Starter New Member

    Sep 27, 2013
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    0
    Hi,

    I am building a pressure sensing device and would like some advice on what type of circuit to build. The device will sound an audible alarm when a fixed pressure is detected.

    I have built a pressure sensor that has an initial resistance of about 40 ohms (+/- 10 ohms) which drops to 11 ohms (+/- 2 ohms) under pressure. I am working on a second pressure sensor which will have a very high resistance (mega ohms) initially, that will drop to almost no resistance under pressure.

    I have looked at transistors, super-alpha pairs, microcontrollers, and Schmitt triggers as a possible basis for my circuit. I also wondered about a simple switch circuit for the second sensor. I am not sure which would give the best results and be simplest.

    Any advice on which route to take would be greatly appreciated :)
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    For the first circuit, an adjustable constant current source driving the sensor will probably be your best bet. Follow the output with a gain amp, then to a comparator.

    You could also add an intermediate offset and gain adjustment, if your application requires it. Personally, I'd pump the raw analog from the gain amp directly into the A/D of a CPU, and apply offset and gain adjustments digitally. Alternatively, a good delta-sigma converter could reliably convert the low voltage output of the current source (no need for adjustibility in this case) driven sensor without having to deal with the drift and noise of the gain stage.

    For the second, if the resistance change is abrupt, a simple voltage divider configuration (using the sensor as one of the two R's) followed by a comparator should be acceptable.

    Do you have datasheets on the sensors?

    What kind of pressure accuracy due you need, and what is your tolerance for drift over time and temperature?
     
  3. CreativeKitty

    Thread Starter New Member

    Sep 27, 2013
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    Thanks for your reply. I shall have a look at your recommendations. I'm relatively new to electronics and have not used many of the things you mentioned, but I shall look them up. Thank you.

    I did forget to put in my initial post that the device needs to be portable. Sorry about that. I don't know if it makes a difference.

    I don't have datasheets as I have made the sensors myself.

    I'm hoping that I won't have much drift over the life of the sensors as they're only for short term use and will have been 'pre-conditioned' before they are installed.

    As they are going to be embedded in a material with a very low temperature conductivity and used within a small temperature range I anticipate the output of the sensor will be fairly stable over its life in the conditions I'm making it for.

    Thanks, Kitty
     
  4. Nykolas

    Member

    Aug 27, 2013
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    What type of pressure sensor are you using? E
     
  5. CreativeKitty

    Thread Starter New Member

    Sep 27, 2013
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    It's made of conductive rubber between two copper electrodes.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You might need to tin the copper, otherwise it will oxidise/corrode over time and increase the resistance of the sandwich.
     
    #12 likes this.
  7. wayneh

    Expert

    Sep 9, 2010
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    The first one is simple. Put the sensor in series with other resistors to form a voltage divider, and compare (with a comparator, LM339 for instance) the voltage across the sensor to an adjustable reference voltage. Total resistance of the series string should be less than 1KΩ or so because you are looking for swing of less than 30Ω. The comparator output would control the gate of a MOSFET and the MOSFET would control - like a switch - the current through your alarm.

    Can the sensor tolerate a continuous current of 10mA or so? This might cause corrosion over time. If this is a problem you'll need a bit more complexity, an op-amp to amplify a smaller voltage (from a smaller current) across the sensor. As noted already, at that step you could build in offset and span control.

    The second sensor will absolutely require a high impedance op-amp. How many megohms are you expecting? You may want to prepare to use the same basic circuit for both sensors, the only difference being the bias resistors.

    You may want to read about using op amps in "instrumentation amplifier" circuits.
     
  8. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Not so simple. The low ohms is going to cause self-heating at moderate voltage levels (as well as unnecessary current consumption). This is why I suggested a current source (with an implied low current level), followed by an amp.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Yes, but a few milliwatts may be trivial.

    I agree with using the same approach for both sensors away, though. Op-amps are in the OP's future. ;)
     
  10. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    I won't disagree with you on that point. But, from my vast experience interfacing similar types of sensors over many years, drift and non-linearities caused by self-heating will be an issue -- depending, of course, on his desired accuracy. This is especially so since he's constructed his own sensor, and I am sure he has not taken thermal stability into account.
     
  11. #12

    Expert

    Nov 30, 2010
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    Without even looking anything up, I can name that tune in 3 milliamps.
    30 ohms to 13 ohms difference, assuming a V offset guarantee of 5 millivolts on an LMC type of op-amp, you would need 50 millivolts difference to get ten times the chip error.

    I = .05/17
    .00294A
    3ma X 40 ohms = (< 1/8 watt).

    Get serious and it can be done with even less current.
     
  12. wayneh

    Expert

    Sep 9, 2010
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    Yes, I don't think it matters all that much for #1. Especially since I'm picturing a big pad (which it may not be). But the OP needs a very nice circuit for #2 anyway, and I think it makes sense to just use one design for both.
     
  13. #12

    Expert

    Nov 30, 2010
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    This will do it fairly well. Still have to choose an output fet. It depends on the load, which we don't know.
     
    Last edited: Sep 27, 2013
  14. KaguraHime

    New Member

    Sep 25, 2013
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    0
    Novice here.

    In my diagram:
    LM339 is a comparator IC.
    Connect it to the Vcc and Gnd, of course. Wires not shown.
    Tune Pot 4 to the reference voltage you need.
    Pot 3 is your sensor.
    Choose a relay according to your Vcc.

    When Pot 3's voltage exceeds Pot 4's, the NPN transistor lets in current through the Vcc and the buzzer goes off.

    I've tried a similar project that works with the below values but haven't figured the ones that'd work for yours.

    My project:
    R1 = 1k
    R2 = 2k2
    R4 and R3 = 5k pots
    Relay = my Vcc, 12 Volts
     
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  15. #12

    Expert

    Nov 30, 2010
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    It won't work. Even if you were lucky enough to adjust a 5K pot to within 10 ohms of where you're supposed to be, the OP switch does not have 3 terminals and the transistor needs a limiting resistor to avoid burning the LED in the isolator.
     
  16. KaguraHime

    New Member

    Sep 25, 2013
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    Something's wrong, cuz I just built a circuit like this physically and it worked perfectly.

    Not sayin' you're wrong, I mean I'm still learnin', but if it's gotta go wrong like in 1 week's time cuz I missed out something I definitely wanna know what could go wrong before I send that to a boardhouse.

    >"<

    The values were used for MY project. It needs scaling to meet OP's needs.
     
  17. wayneh

    Expert

    Sep 9, 2010
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    You may have gotten lucky with the missing resistor for the LED, by using a low enough Vcc and by limiting the base current to T1 with both R1 and R2.

    It makes more sense to limit the base current with R1 and the LED current with R2 in its current path. R2 really has no function where it is now - you could have just used a larger value for R1.

    The bridge approach #12 has shown for the inputs is a standard solution for this sort of thing, so I second the recommendation. You could still use a comparator for the low impedance one, but without reading the datasheet I'm not sure about the high impedance circuit. A high-impedance op-amp seems like a good approach.
     
  18. #12

    Expert

    Nov 30, 2010
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    Look at the math for the current through the opto LED.
     
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  19. #12

    Expert

    Nov 30, 2010
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    Rest assured, I mathed the Dickens out of this one. The low impedance measuring circuit is actually calculated to the limits of the OP07 amplifier. According to the Mouser search engine, you can't buy a comparator that can do this job as well.

    The high impedance circuit is just a guess based on the OP's stated limits, multi-megs to less than one meg. Still, the OP07 has a 30 picoamp input current, so it stands a good chance of working in the multi-meg range.
     
  20. doug08

    Member

    Jan 30, 2011
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