Electronics > Beginners
understanding zener and high side switching
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Ian.M:
Hmm, that sounds like a vacuum flush toilet on a vehicle, (maybe a large fast motorboat), and you want blocked bowl detection *BEFORE* it overflows.   

A capacitive liquid level switch with a PTFE or other water repelling face on the sensor, installed through the bowl wall,   located so the flush water rinses gross contamination off it is probably the best option, at least from a reliability and minimal maintenance point of view.  You should be able to buy a suitable sensor with relay or logic output off the shelf.
If the bowl is non-conductive, a sensor on the outside of it with enough sensing plate area may be able to work through the bowl wall. 


I had considered a pressure sensor on a port on the waste pipe, monitoring the pressure profile during the flush and comparing it to one recorded from a normal flush but IMHO that could be a maintenance nightmare even if its protected by a rubber diaphragm, and also there would be a lot of software development and testing required.

Your original idea of  a conductivity sensor with stainless probes is probably workable, but its going to be difficult  to locate the probes low enough in the bowl to have enough room left to handle the flush water volume of one normal flush + slop from vehicle motion, assuming the worst case situation when the blockage leaves the bowl level just below the sensor.

hsn93:

--- Quote from: Zero999 on December 18, 2018, 03:15:26 pm ---
--- Quote from: hsn93 on December 18, 2018, 02:10:30 pm ---hello Hero999, thank you.. i think its good idea to measure with two pins, once im sure there is water i can pull both pins to ground eliminating the potentional between electrodes. having water to the sensor is abnormal and when it happens it will need intervention from service. thats why i dont need to measure the water all the time and can do it once every 5 minute and then pull two pins to ground.
the thing is that im always afraid to expose mcu pins to outside board thats why i was putting the 24V with a high resistance on series.
but i guess its safe? i will just put 3v3 zener on both pins to ground and should be ok (i assume  :-+).

BTW, that link from analog devices doesn't work but i will search for it from the keywords. thanks.

--- End quote ---
Another resistor can be added in series with the input to protect it against high voltages.

By the way, there will be a time delay between seeing the voltage on the capacitor and the output going high, as it will take time for the capacitor to charge, so the code needs to account for this.


I don't know why the link didn't work. It works for me. Could it be the firewall? It's not the forum corrupting the link, as I've tested it again. Try Googling for "Liquid Level Sensing Using Capacitive-to-Digital Converters"
https://www.google.com/search?client=firefox-b&q=Liquid+Level+Sensing+Using+Capacitive-to-Digital+Converters

--- End quote ---

hi, to protect MCU we place 1M resistor. what happen if someone plugged 24V to the input? (limit current to 24uA is that all to protect mcu pin?)

im thinking to have voltage follower here op amps have better input impedance than mcu? if its not smart idea, why its not?
how about 3v3 zener after R2?

the link worked. i think it was analog website problem.


coppercone2:

--- Quote from: Ian.M on December 18, 2018, 01:16:32 pm ---complicated post

--- End quote ---

for very clean water, with resistivity of 100k-18M, can you get away with the small bias current from a opamp connected directly or should you still use a coupling transformer?

when you say galvanic isolation, what DC leakage is permitted?
Zero999:

--- Quote from: hsn93 on December 27, 2018, 03:01:35 pm ---
--- Quote from: Zero999 on December 18, 2018, 03:15:26 pm ---
--- Quote from: hsn93 on December 18, 2018, 02:10:30 pm ---hello Hero999, thank you.. i think its good idea to measure with two pins, once im sure there is water i can pull both pins to ground eliminating the potentional between electrodes. having water to the sensor is abnormal and when it happens it will need intervention from service. thats why i dont need to measure the water all the time and can do it once every 5 minute and then pull two pins to ground.
the thing is that im always afraid to expose mcu pins to outside board thats why i was putting the 24V with a high resistance on series.
but i guess its safe? i will just put 3v3 zener on both pins to ground and should be ok (i assume  :-+).

BTW, that link from analog devices doesn't work but i will search for it from the keywords. thanks.

--- End quote ---
Another resistor can be added in series with the input to protect it against high voltages.

By the way, there will be a time delay between seeing the voltage on the capacitor and the output going high, as it will take time for the capacitor to charge, so the code needs to account for this.


I don't know why the link didn't work. It works for me. Could it be the firewall? It's not the forum corrupting the link, as I've tested it again. Try Googling for "Liquid Level Sensing Using Capacitive-to-Digital Converters"
https://www.google.com/search?client=firefox-b&q=Liquid+Level+Sensing+Using+Capacitive-to-Digital+Converters

--- End quote ---

hi, to protect MCU we place 1M resistor. what happen if someone plugged 24V to the input? (limit current to 24uA is that all to protect mcu pin?)

im thinking to have voltage follower here op amps have better input impedance than mcu? if its not smart idea, why its not?
how about 3v3 zener after R2?

the link worked. i think it was analog website problem.

--- End quote ---
24µA is not going to damage the MCU input pin.

I doubt a 3V3 zener would make any difference.

MCUs already have a very high input impedance, but an op-amp is not a bad idea. It depends on what you want to do. Will you be using an ADC input? Does the MCU have Schmitt trigger inputs?

A Schmitt trigger input is a good idea to, as some MCU inputs behave unpredictably when given invalid logic levels.

Using an ADC input would make it more flexible as the threshold can be programmed, but there should be a low AC impedance path to 0V, so the sample and hold capacitor can be charged/discharged, otherwise the reading can be affected given such a high source impedance: 10nF between the input and 0V would alleviate the problem. Refer to the papers linked below:
https://www.st.com/content/ccc/resource/technical/document/application_note/9d/56/66/74/4e/97/48/93/CD00004444.pdf/files/CD00004444.pdf/jcr:content/translations/en.CD00004444.pdf
http://www.ti.com/lit/an/spna061/spna061.pdf
Ian.M:

--- Quote from: coppercone2 on December 27, 2018, 04:26:59 pm ---
--- Quote from: Ian.M on December 18, 2018, 01:16:32 pm ---To prevent electrolytic corrosion ... <snip complicated post>

--- End quote ---

for very clean water, with resistivity of 100k-18M, can you get away with the small bias current from a opamp connected directly or should you still use a coupling transformer?

when you say galvanic isolation, what DC leakage is permitted?

--- End quote ---
It all comes down to how much metal you can tolerate loosing over the max. expected life of the sensor.   

Assuming the electrodes don't normally corrode on their own with no current flowing:
Multiply the DC leakage, bias or sensing current by the life in seconds to get total charge.  Divide by the Faraday constant and further divide by the oxidation state of the metal's commonest stable soluble ion, to get the number of moles of metal atoms lost.   Multiply by the atomic mass to get grammes of metal lost from the anode.   You can then calculate the volume of metal lost from its density, and if you make some assumptions e.g. that the worst pitting wont be deeper than ten times the average loss of surface (not valid for stainless steel under anerobic conditions or for metals like aluminum that normally form an insulating highly adherent oxide film),  you can estimate the thickness that will be lost from the surface over the electrode life.  It wont apply unless the electrode geometry is such that the distance between the electrodes across their length and width is constant, and the electrodes are fully immersed.  If they are only partially immersed or the edges are significantly closer together than the rest of their surface, the immersed part or the closer part of the anode will be preferentially eroded.
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