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I'm trying reverse-engineer a circuit on an espresso machine water boiler. I was looking at this post: Water Sensing Circuit

which seems to be essentially the same idea. Really the only differences are:

*I'm only working with one probe. The other "probe" is actually just the surface of the boiler, which is grounded. I've measured roughly 2Mohms across the probe and ground when water is touching it.

*I'm trying to run low voltage AC instead of DC, to avoid corrosion. The original circuit produces a square wave at 1kHz, 3.6V Peak to Peak. I figured I could easily produce a similar signal with a microcontroller and circuit similar to: Changing a Signal's DC Offset

Is there an AC equivalent to a circuit controlled by a PNP transistor?

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Add a blocking capacitor with a weak bias (high value resistor) to system GND on the output side. This will produce an AC waveform centered about GND.

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  • \$\begingroup\$ Indeed, then make sure the sensing circuit can tolerate negative voltages, and simply only pay attention to the output during the positive part of the cycle. Making the system safe around mains power and water may be the ultimately hard part. \$\endgroup\$ – Chris Stratton Aug 2 at 19:42
  • \$\begingroup\$ that's what I'm wondering. What sort of sensing circuit is there that can tolerate negative voltages? \$\endgroup\$ – Tate Mazer Aug 2 at 19:56
  • \$\begingroup\$ An AC coupled one, or one that uses a bipolar supply. \$\endgroup\$ – hacktastical Aug 2 at 20:13
  • \$\begingroup\$ @Chris Stratton - this should give you nightmares: youtu.be/jiErqUkw690 \$\endgroup\$ – hacktastical Aug 2 at 20:15
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when I wanted an AC water sensor I used a 555 with the probe connected in place of the capacitor. the 555 frequency changes in response to water level.

schematic

simulate this circuit – Schematic created using CircuitLab

but you could probably do a similar sort of thing just using microcontroller ports, pull down and time how long it takes the port to change, then pull up etc...

schematic

simulate this circuit

this sort of thing but done in software

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