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  • Author Author: AE7HD
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555 timer Remote Capacitance Sensor - fluid level, body detector, touch sensor

AE7HD
If you want to measure capacitance for a sensor some distance away, you can't do it at the end of long wires. Just temperature changes and nearby objects will make larger changes that can swamp what you are trying to measure.

This was originally for my first car in the late '70s. I put sensors on the windshield washer tank, the radiator hose, even inside the gas tank. I had already designed and built my own precision capacitance meter using two 555 timers. This triggered each sensor separately, then sorted which one was active with a decade counter and an LM3914 linear dot/display driver.
Two wire twisted pair is a lot cheaper than 3 wire. And when you aren't flush with cash, that can make a difference. This one is designed to send the signal back to an Arduino, which is then programmed to measure just the On time.

I've used this to measure liquid levels in water tanks, on roofs, even inkjet tanks. The sensor itself might be metal tape on the outside of a plastic or glass tank, or insulated wire in the liquid. Make sure the insulation is compatible with the liquid.

One 555 timer goes onto the remote sensor. The signal and power travel on the same line similar to how I2C and several other methods work, although in this case there are no addresses.
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  • in reply to Gururaj

    The wires are full insulated? Are they held rigidly? The cable between the sensor and 555 may affect readings as it flexes, depending on the cable.

    Inserting and removing the wires will give you different readings than filling and emptying the tank, as everything nearby including you affects the reading. 

    Why did you use wires from two different twisted pair? It defeats the purpose of using twisted pair.

    Do you have an oscilloscope to observe the output waveform? Perhaps you have some outside source of noise.

    Do not ground the circuit at the tank. That can cause ground loop problems.

    With wire that long, I'd try lowering R3 to 100 ohms. 100 ohms is the impedance of Cat 5 and Cat 6 twisted pair.

  • in reply to Gururaj

    An update. I tested 4ft Capacitive Sensor (CL) with a cable of 2ft between 555 out and Arduino. Got the acceptable results. When CL is connected as in Schematic, but not yet inserted into Water I get 0.11 nF.

    When I  start inserting it slowly into water, I get reading as 0.13nF, 0.15nF and so on till 0.32nF. When I remove it from water it comes back to 0.12 or 0.13nF

    With these result I wanted to test it in actual environment. The Sump (this is where Arduino is mounted) is around 75 ft apart from Over Head Tank (OHT). OHT level is what needs to be measured with 555 and CL  I have LAN Cable with 4 twisted pairs running between Sump and OHT. I used one wire from two different pair (Green for -Ve and Orange from +Ve/Pulse)  to connect 555 and Arudino.

    With this setup, the results were fluctuating. Initially it showed 0.11 nF before I inserted the CL into the tank. But when I started inserting CL into the tank, it used to raise a bit and fall back. Like value sometimes went to 0.2nF and fell back to 0.16nF. Even when I completely inserted CL into the tank (no movements at all) it used to show 0.19 nF, 0.16nF, 0.18nF, 0.13nF 

    I'll try to use the single pair and test it out. IMO, the result may not be too much of difference. Any suggestion would be very help at this stage.

    AE7HD : Should we reduce the value of R3 since the length of the wire is 75ft? Will T-Off as low as 693 nanosecond (at 0.1nF Capacitor) be recognised by Arduino with this length of the wire? Please suggest if I need to do any change.

  • in reply to Gururaj

    For testing purposes, you can build a capacitance meter using two 555 timers. With a DMM as the readout on 200.0mV scale, it reads down as low as 0.1pF resolution. I have a meter with the lowest scale being 600.00mV, I find it is stable down to 0.01pF aka 10fF.
    hackaday.io/.../183405-dual-tlc555-capacitance-meter-01pf-resolution

  • in reply to AE7HD

    Thanks for the replies jc2048 and . I'll order TLC555. Mean while I have prepared a 4ft Capacitive sensor for water in our house's Over head Tank. I'll test that with Arduino and NE555..Keep you posted on my results..

  • in reply to Gururaj

    jc2048 beat me to it. Use the TLC555 which is the CMOS version of the 555 timer. You will want to use shottky diodes.