Low-Quality Capacitors Turned Into High-Quality Temperature Sensors

When life hands you a bunch of crummy capacitors, what do you do? Make a whole bunch of temperature sensors, apparently.

The less-than-stellar caps in question came to [pyromaniac303] by way of one of those all-in-one assortment kits we so love to buy. Stocked with capacitors of many values, kits like these are great to have around, especially when they’ve got high-quality components in them. But not all ceramic caps are created equal, and [pyromaniac303] was determined not to let the lesser-quality units go to waste. A quick look at the data sheets revealed that the caps with the Y5V dielectric had a suitably egregious temperature coefficient to serve as a useful sensor. A fleck of perf-board holds a cap and a series resistor; the capacitor is charged by an Arduino output pin through the resistor, and the time it takes for the input pin connected to the other side of the cap to go high is measured. Charge time is proportional to temperature, and a few calibration runs showed that the response is pretty linear. Unfortunately the temperature coefficient peaks at 10°C and drops sharply below that point, making the sensor useful only on one side of the peak. Still, it’s an interesting way to put otherwise unloved parts to use, and a handy tip to keep in mind.

Temperature sensing isn’t the only trick capacitors can do. We’ve seen them turned into touch sensors before, and used to turn a 3D-printer into a 3D-scanner.

18 thoughts on “Low-Quality Capacitors Turned Into High-Quality Temperature Sensors

    1. greg, you posted exactly what i was going to post!

      i had this brilliant idea to make a cheap/simple capacitive humidity sensor but i started reading up on it and stability, stability, stability.

  1. In college we built a temperature gauge using a 1N1004 diode. The forward voltage drop is a function of temperature and with an opamp to boost the tiny changes in voltage we had a pretty accurate sensor.

    1. The nice thing about diodes is the effect is pretty much linear (unlike thermistors), and not too tiny (about -2 mV per degree C unlike thermocouples at about 40 uV per degree).

      1. I built a soldering iron holder based on a diode in about 1980 for a place I worked. I never liked the oxide and gunk that crusts on a tip after it has sat hot for a while so I put a diode down near where the tip sat when it was in it’s holder and built a gizmo with a triac and a trimpot that let me set the “idle” temp just low enough that my tip did not get smiggy, yet hot enough that when I pulled the iron out it got back up to temp fast. The nice thing about the temp sensor and why I did not use a microswitch was that it did have some thermal lag so it would not cut the power if you just shoved the iron in the holder for a short while. So if you holstered it while you turned your panavise around it would still be hot when you got it back.

        On the caps besides aging and being microphonic, a lot of them are also subject to humidity changes. None of which are good for long term temp sensing. Best use would be jumper wires from the leads, or perhaps bypass caps where the value is not critical.

        1. That is absolutely brilliant! Kind of obsolete now with the newfangled temperature controlled soldering stations. The old iron I had used a very big all-metal holder that would sink the heat out of the iron while holstered to keep it cooler until you pulled it out, also worked fairly well.

  2. “Charge time is proportional to temperature, and a few calibration runs showed that the response is pretty linear.” All functions are effectively linear if you narrow the range enough. :-) Good old Isaac Newton taught us that. :-)

      1. “Just google Weierstrass function.”
        Just googled it and read about it in Wikipedia. ( thanks :-) ) It’s a cool mathematical concept that definitely frustrates the linearity statement I made but it seems to only exist in the realm of mathematics. One would be hard pressed to find any physical measurements that act like that. The only exception I can think of is hardly physical like the price of a heavily traded commodity with about the same amount of detail visible at all scales, a property also found in fractals. I suppose we could come up with an example if we measured some attribute of something that naturally takes on a fractal structure but those are about as common as chicken’s teeth. :-) (unless one looks at snowflakes which don’t seem to be entirely fractal but have some of their attributes) I’d say the linearity stands for all practical purposes, but it is good to know about the Weierstrass function if things start getting weird. :-)

  3. This just shows one more context where our amazing ability to measure time precisely becomes useful. Any affect that can perturb an oscillator at all can be measured easily. It’s a wonderful world we live in.

    Timing the discharging of capacitors, reminds me of the old Apple][ days of reading PDL(0) … PDL(3) each with a range 0-255 all based on the discharge of a capacitor. You could write your own code and get even greater precision and measure light intensity with a CDS cell if you weren’t in a hurry. :-)

  4. I have a rather old LCD clock thermometer module, I think from the early 1990ies. This uses a ceramic capacitor as temperature sensor, about 5nF. But there is no marking on the device and I have no idea, which type of dielectric they used, which is reproducible enough for use as a sensor in a commercial product.

    1. Perhaps not a particularly accurate commercial product though, most people don’t calibrate their thermometers against each other, or even have more than one. As long as it vaguely matches the weather forecast plus a few degrees for being indoors where you can read it, people are going to be happy.

  5. OK, this is interesting as a laboratory curiosity, and it help illustrate the concept of temperature coefficient of capacitors.

    But as an actual useful temperature sensor, no. You can buy a thermistor for 12 cents from Tayda. It doesn’t even have to be calibrated for some applications.

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