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Temperature sensor

Discussion in 'Sensors and Actuators' started by Century92, Dec 9, 2016.

  1. Century92

    Century92

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    Dec 9, 2016
    Hello!

    Was wondering if anyone knew of any cheapish devices that I could use to monitor a three foot by three foot surface from about a foot away? I'm thinking something like a thermistor but I'm not sure if I can use those if not direct heat. am I wrong?
     
  2. Century92

    Century92

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    Dec 9, 2016
    Sorry I'll be more specific. I want to monitor this surface and if it gets above say 50 degrees C then it'll run the gate on a transistor. hope that makes sense, sort of like a heat detector for a fire alarm but I just want to watch a 3 foot by 3 foot surface instead of a room
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    An IR based temperature sensor comes to mind, e.g. this one.
    You may be able to modify an inexpensive PIR motion sensor such that it detects the temperature instead of the motion.
     
  4. Bluejets

    Bluejets

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    Oct 5, 2014
    If you tell us a bit more about the application as your query raises a dozen or more questions about your application.
    Such as, is the 3' x 3' surface concrete or steel, is the 3' x 3' section enclosed or in a wind tunnel....what range of temperatures are you talking about here .......on and on..
    Do you see the drift?
     
  5. OBW0549

    OBW0549

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    Jul 5, 2016
    Adafruit has a contactless IR thermopile sensor that will do what you want for about $10. It interfaces via an I2C bus and Adafruit provides a library for using it with an Arduino.
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    The problem with monitoring a specific area (3 feet by 3 feet) from a specific distance away (1 foot) is field-of-view (FOV) of the sensor. The FOV must be large enough to capture the entire area, yet not too large lest the sensor also "sees" the surrounding area or the background behind the monitored area. Perhaps for the sake of minimizing unwanted contributions, the FOV could somehow be restricted to slightly less than the desired area.

    Which brings up the problem of how to match a small finite-area sensor to a much larger area. Typically this requires a lens, and if a thermal radiation sensor is required, a lens that passes infrared radiation and (depending on the sensor) blocks shorter wavelengths. There are a number of stand-off radiation sensors that can be used to remotely "measure" surface temperature. The Melexis thermopile that @Harald Kapp linked to is quite accurate, but perhaps a bit difficult to use. The Texas Instruments TMP006 that is part of the link @OBW0549 referenced is a similar device to the Melexis Technologies MLX90614 series of sensors, but with perhaps the advantage of a stronger user community. Either one should do the job if the FOV of the sensor can be matched to the target area. TI has a nice little application note that describes the problem.

    Years ago, when they first became available, I acquired a patented pyroelectric sensor from Harshaw Chemical here in Ohio to "play" with. The company I worked for in the 1970s was just getting involved with high-energy laser research and we spent a good deal of time investigating different types of infrared sensors, both cryogenically cooled and un-cooled sensors. The pyroelectric looked very promising because of it's simplicity and sensitivity. It later went on to become an integral part of passive infra-red (PIR) intrusion detectors, which use two pyroelectric sensors staring at slightly different FOVs by means of plastic Fresnel lenses placed in front of the two sensors. Motion within the two slightly different FOVs causes a differential output signal from the two sensors.

    I like Harald's idea of re-purposing a PIR as a stand-off temperature sensor, but there is a big problem with that. The pyroelectric sensor only "sees" changes in radiation. A static input radiation field produces zero output. To use a single pyroelectric sensor as a temperature sensor requires that it's FOV be alternated between a reference FOV and the measured FOV. This is typically done with a mechanical shutter, such as a rotating disk with alternating cut-outs and (sometimes) mirrors. Mirrors are often used to direct the sensor FOV to a temperature-controlled reference surface, but for this project a simple two-bladed shutter, painted black on the side facing the sensor, would be sufficient. That's what I used, back in the day, to detect people from across the room. No lens.

    Because the FOV was huge, the signal to noise ratio was terrible. The optical chopper ensured that the sensor saw the entire background infrared radiation, but the additional radiation from a human body had to be detected in the presence of the radiation from the room. As it turned out, the only way to do this was to use a lock-in amplifier and suppress the steady-state background. So, if the background was at 300 K (approximately room temperature), then a human being was only a few degrees warmer but their "signal" was only a small fraction of the FOV of the sensor. IIRC, the lock-in required almost of minute of integration time to "recognize" the appearance of someone standing in a doorway I had roughly aimed the sensor at. Compare this with how fast a modern PIR, using differential FOVs to detect motion, works. You have to walk realllly slooow to fool a PIR motion sensor. Or wear a suit that matches your radiation profile to the background, which is not an easy task.

    Still, I think it would be worthwhile to open up a PIR and block one of the sensors. Use a small motor with a two-bladed "chopper" wheel to provide an output signal from the un-blocked sensor. Depending on how "warm" the target area is compared to ambient temperature, you should easily be able to "see" a 3' x 3' surface from one foot away and "measure" it's temperature. Although I had to use a lock-in amplifier to "see" people, this should not be necessary to measure the temperature of a static area. The pyroelectric sensor produces a substantial current output (a few microamperes) when the radiation impinging on it, modulated by the optical chopper, is a few degrees higher (or lower) than the chopper blade temperature. You can probably use the existing high-impedance amplifier already connected to the two pyroelectric sensor elements. In any event, a simple current-to-voltage circuit using an op-amp with a FET input should suffice. Construction, however, require minimizing the leakage paths associated with the sensor connections and preventing microphonics that can be introduced by the megohm or so feedback resistor. "Dead bug" construction techniques and Teflon insulation (where necessary) recommended.

    People do make thermistor-based radiometers, usually in a temperature-controlled bridge configuration, IIRC. But you can purchase pyroelectric sensor based PIRs for just a few bux and they are much more sensitive. If you decide you need more sensitivity, you can add an optical interrupter to the chopper and use it's signal to synchronize a lock-in amplifier.
     
    Harald Kapp, OBW0549 and bushtech like this.
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    @hevans1944 : Hop, you're having too much free time :rolleyes:
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    That's why a modification is required, e.g. by blocking half of the sensor, so it will no longer 'see' a differential IR signal. The inexpensive PIR sensors I envision to be used don't use mirrors and such, they contain two sensors with different field of view to create the differential signal only.
     
  9. hevans1944

    hevans1944 Hop - AC8NS

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    Not yet! Got the 26-foot Penske truck loaded, but we still have stuff left over that we want to take to Florida. Gotta pack some more cardboard boxes and see if we can (somehow) fit them on top of what's already in the truck. So, I took a break last night to see what's going on here... hard to sleep when I am having so much fun.:cool:
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    Yes, that is why the input radiation field must be optically modulated (chopped) for comparison against a reference field. As long as the object whose temperature is being measured is not at the same temperature as the chopper blades, a signal will be observed at the modulation frequency.

    A bare pyroelectric sensor connected to a charge converter circuit will easily "see" your bare hand waved across the FOV of the sensor, but this signal is transitory, present only while the radiation from your hand is increasing the sensor temperature. Move your hand away and the temperature decreases, causing the signal polarity to reverse. Again this is a transitory event and the output quickly returns to zero. Useful to determine that the sensor is working. Add a chopper for steady-state temperature measurements.

    The pyroelectric sensor is modeled as a small capacitor (a few picofarads) with a permanent dielectric charge polarization. Very similar to an electret microphone. Change the temperature and the effective charge changes. You can measure the change in charge (but not the charge itself) by feeding the sensor into the inverting input of a high input impedance op-amp and providing a large-valued feedback resistor (upwards from one megohm) to make this small input current appear as an output voltage. The non-inverting input is "grounded" of course, being connected to the other pyroelectric sensor lead and circuit common. This is really simple and inexpensive remote temperature sensing. I was amazed at how easy it was to "see" infrared "signals" with minimal fuss. Of course, I did have a commercial chopper wheel available for other electo-optical experiments, but a small fan motor from a PC would do nicely. Might want to remove some of the blades to get a larger signal.
     
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