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Need help with input circuit for reset line for Deep Sleep for 8266

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tjodork

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1644082334583.png

the reset pin on the 8266 must be pulled low (<.8v) to bring it out of deep sleep.
I'm trying to make a battery operated water sensor. When there is water between the 2
leads the resistance is around 600K - 1.5M.
Although I'm an electrical engineer I cant say I understand transistors...
Is there a way with a transistor circuit to pull that reset line low WHEN
the resistance on the water leads goes from OPEN to 600K-1.5M ?
Or any other suggestions on a method to accomplish this, preferable where
it doesn't use power when no water is present : )
Thanks much for any help
tim

O
 
There are two problems that I can see:
You need a separate high-impedance input circuit with a greater-than-1M pullup resistor to sense the water probe, and that in turn switch the reset system.

Also, I believe the reset pin must be pulled low and released to high level again, to restart the MCU from sleep.

That would mean using a schmitt trigger stage from the water detector to produce a fast level transition, then a capacitor-resistor edge edge detector into the reset control.

You could do both with a couple of sections of such as a 4093 or 40106 (or 74HC14) schmitt input gates.

Note that if the water probe will be submersed long term, it will eventually cause some electrolysis and corrosion on such as copper or iron; you would need to use stainless steel probes/contacts to maintain conductivity.
 
dang, I forgot about the fact that it has to go back up!!! That does complicate it!
I did confirm you are correct.
I see that a schmitt trigger will clean up a signal..but not sure how to cause the pulse and once the pulse
is generated...would I have to do something to reset?.....could by a user pressing a button going to some circuit
good idea about stainless steel but hopefully water should NOT be typical
 
Look at the first two stages of this example:
1644101434419.png


The push button would be the electrodes. When the first input changes to low, the capacitor passes a high to the second input for some time, depending on the capacitor and resistor values. 0.1uF and 100K could be a good starting point.

The output of that second inverter gives you the low-going pulse for the reset input.

Adding a small capacitor across the electrodes would give a power-on reset effect and reduce noise sensitivity.
 
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