Water Level Sensor Circuit

Thread Starter

JulesP

Joined Dec 7, 2018
383
As I said at the beginning the motor starts when the fluid level drops below the upper stud but doesn’t switch off when the fluid level is restored. My post is to find out how to make it switch off again. I’m guessing that once the 330uf cap is charged up it is hanging onto that charge/voltage and keeping the base of the first tranny on. Could that be the problem?
 

AlbertHall

Joined Jun 4, 2014
12,345
The time constant might be around three seconds - does it switch off after say 10 seconds?
If not, does it switch off with the pot adjusted to the bottom of the track?
 

AlbertHall

Joined Jun 4, 2014
12,345
Another check worth doing is with the screws disconnected, measure the voltage on the wiper of the pot and note the voltages at which the pump switches on and off (the pump doesn't need to be connected as long as you can tell whether relay pulls in or not).
 

Thread Starter

JulesP

Joined Dec 7, 2018
383
O
Another check worth doing is with the screws disconnected, measure the voltage on the wiper of the pot and note the voltages at which the pump switches on and off (the pump doesn't need to be connected as long as you can tell whether relay pulls in or not).
Ok I will do that tomorrow and also check the switch off time. 10s is far too long as a lot of water can be pumped into the system in that time. I’m looking for more like 1-2s so presumably need to adjust the RC constant in connection with the 330uf cap.
 

sghioto

Joined Dec 31, 2017
5,379
That circuit looks over complicated to me. Using the original circuit I would rewire it as in the schematic below.
C3 and R5 provide a 2 second delay before the pump shuts off when fluid level reaches the upper terminal.
Steve GEEE  Water level sensor.png
 

AnalogKid

Joined Aug 1, 2013
10,986
That circuit looks over complicated to me. Using the original circuit I would rewire it as in the schematic below.
Yours is a completely different circuit. The original circuit is a Schmitt trigger comparator. Your circuit has no hysteresis, and will not work well in this application.

My guess about the problem is that there is too much hysteresis - from an input voltage point of view, the two trip points are too far apart. Here is a basic analysis with transition level equations:
https://en.wikipedia.org/wiki/Schmitt_trigger#Transistor_Schmitt_triggers
https://www.electronicshub.org/schmitt-trigger-basics/

The "Supertrol" was very popular project in the March, 1967, Popular Electronics, and used the same approach (though not the exact same circuit). Note that the LM339 had not yet been invented.

ak
 
Last edited:

Picbuster

Joined Dec 2, 2013
1,047
Hi all,

I have built a water level feed control for an electrolyser using the attached circuit but it doesn't quite work as I would like it to.

The sensor consists of two Allen screws in the side of the electrolyser body (test rig) acting as the sensor and when the water level falls below the top screw the motor starts up as expected. However, when the water level rises back to the top screw the motor doesn't switch off.

There is a 10k pot in the circuit that doesn't seem to make any difference to how the device works and what I'm asking is how can I make the motor stop immediately once the water (0.05M KOH) rises to the top screw.

I'm thinking that perhaps the 330uF cap is holding the base of the first transistor on for too long? So maybe a smaller value like 50uF or a 1k bleed resistor across it?

Thoughts appreciated.

Jules
Measuring water levels using conductance is difficult it is not constant depending on temperature and pollution ( calcium, acid, lime and more)
Dc currents make it more difficult ( it will create a voltage source as result of pollution) a few micro/mili volts could destroy the results.

You want to switch on level.
use two poles of metal isolated with the top clean. Avoiding a water bridge between.
next step is to build a generator 10 Khz aprox and connect this via a capacitor to one pole.
Use a op amp with an input filter for 10Khz . Also coupled via a Capacitor.

advantage:
alternating signal no galvanic problems.
Measurement is reliable.


I used this many times and also added a pic to allow a threshold (eliminate water movements effects like waves)

Picbuster
 

Thread Starter

JulesP

Joined Dec 7, 2018
383
Measuring water levels using conductance is difficult it is not constant depending on temperature and pollution ( calcium, acid, lime and more)
Dc currents make it more difficult ( it will create a voltage source as result of pollution) a few micro/mili volts could destroy the results.

You want to switch on level.
use two poles of metal isolated with the top clean. Avoiding a water bridge between.
next step is to build a generator 10 Khz aprox and connect this via a capacitor to one pole.
Use a op amp with an input filter for 10Khz . Also coupled via a Capacitor.

advantage:
alternating signal no galvanic problems.
Measurement is reliable.


I used this many times and also added a pic to allow a threshold (eliminate water movements effects like waves)

Picbuster
I’m inclined towards thinking this but an even simpler method would be a neo magnet on a float in a tube with a reed switch on the outside to trigger a simple circuit. Still for completeness I will get the conductance method working.
 

ericgibbs

Joined Jan 29, 2010
18,766
hi Jules,
I think your attempt at finding the problem is a good learning exercise.
Have you completed the tests that Albert proposed.?
E
 

Thread Starter

JulesP

Joined Dec 7, 2018
383
hi Jules,
I think your attempt at finding the problem is a good learning exercise.
Have you completed the tests that Albert proposed.?
E
Just finished.

With sensor disconnected the centre tap of pot goes from 0-7.3V when motor on and pot taken from full L to R.

With Sensor connected, motor comes on when centre tap = 4.2V but does not switch off again, even after 30s whatever pot position.
 

Thread Starter

JulesP

Joined Dec 7, 2018
383
But you can't turn the motor off by rotating the pot? Even after waiting a few seconds?
Yes, when the fluid level has been restored the motor does not switch off even after 30s whatever the pot position. This was my reason for starting the thread.
 

Thread Starter

JulesP

Joined Dec 7, 2018
383
hi,
This simplified version, shows the result of turning the Pot from 0% > 100% > 0%. Looks OK.
No Water level Sensing.
E
So a simulation is saying it doesn’t work? It switches on the motor fine when the level goes below the top stud so it must work in part.
 

AlbertHall

Joined Jun 4, 2014
12,345
Yes, when the fluid level has been restored the motor does not switch off even after 30s whatever the pot position. This was my reason for starting the thread.
OK, something in the circuit is not as in the schematic.
Disconnect the sensor and the motor (no point in it running for this).
Measure the voltage at the wiper of the pot or the base of TR1, whichever is easier, and adjust the pot for the lowest voltage (should be close to zero).
Now measure these voltages:
TR1 collector and emitter.
TR2 base, collector and emitter.
 
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