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I have been interested in this liquid level sensor (Waveshare 9525) which has this schematic.

enter image description here

I need to understand the principal behind its working because I'd like to replicate it. I have two platinum wires which I would like to put to use for it.

The reason I am asking this question is because I thought you could not use DC for liquid level sensor as that would lead to ionic migration and eventual drop in voltage measured, but I guess this sensor is not measuring voltage between any electrodes?

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2 Answers 2

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If the sensor plates are out of water it acts as open circuit. Once the water reach the sensor it has some resistance. So water causes opening the transistor since its base become supplied. So the principle is not measuring the voltage, the sensor plates act more like a switch or amplifier.

You can read some analog value from output regarding the water level but it will not be much accurate. You need to calibrate it.

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  • \$\begingroup\$ So ionic migration between dc powered electrodes is not going to be the problem? When the liquid level rises it will definitively quantify that rise? And where do the sensor plates go in the schematic? Between Vcc and R1? Its not marked. \$\endgroup\$
    – Iskander
    Commented Dec 19, 2023 at 5:53
  • \$\begingroup\$ The sensor probe is WA_R2. \$\endgroup\$
    – Michal Podmanický
    Commented Dec 19, 2023 at 6:03
  • \$\begingroup\$ aha, ok, thanks. I'll test this then. Could you suggest a particular NPN transistor? The schematic does not specify a part, and I'm not educated in electronics. \$\endgroup\$
    – Iskander
    Commented Dec 19, 2023 at 6:06
  • \$\begingroup\$ I am not much a physics guy, It measures with very low currents through plates (uA) if it helps you regarding ionics. The 2n3904 or bc547b transistor should be ok. \$\endgroup\$
    – Michal Podmanický
    Commented Dec 19, 2023 at 6:11
  • \$\begingroup\$ Thanks, I'll test and update \$\endgroup\$
    – Iskander
    Commented Dec 19, 2023 at 6:17
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You are right. You should not use DC to sense the presence of water. In this circuit, the current going through the WA_R2 is in one direction and goes from the Vcc to the base of the transistor. This will lead to ionic migration. The sensor will be covered and eventually stop working.

When the current that goes through the probe is low, the ionic will happen slowly. In your case, if you are not draining any current from AOUT, and for example, using 5V for Vcc, then when the transistor is fully on, the current going through the WA_R3 is about 5mA. If the gain of the transistor, for example, is 100, then the current through the probe is 0.05mA or 50uA. This is not very low!

However, even if you manage it with lower currents, if you are using this for an application where the probe is always in the water and you can not remove the probe, or clean or change it, it's not a good idea to use this circuit.

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