#629 Water-Level Control Kit

Build and test a high-low switching circuit with relay output. It is intended for monitoring a water tank level and switching a pump.

Here’s a quick demo..

Notes

This is a simple 5V circuit in a cheap kit form that demonstrates a high-low control circuit. An LM324 is used as a comparator to evaluate two switch inputs (“low”, “high”) and trigger a relay accordingly. The relay can be used to control a compensating service (e.g. turn on a pump if low state triggered, and turn it off when high state triggered).

I don’t actually have a water tank to control; I picked this kit up just to study the circuit, but now I’m thinking there may be other applications I can put it usefully to work in.

The Kit

The kit is available form aliexpress sellers for around SGD$2.90 as I write this.

A well-made single-sided PCB:

Parts

How It Works

There are two inputs:

• J1 - “low state” input switch
• J2 - “high state” input switch

State table:

Note:

• J1=open, J2=closed is an invalid/undefined state
• switch S1 is used to invert the relay output

The interesting trick in this circuit is how it arranges for the hysteresis effect through the “medium” state:

• when “filling” (i.e. low->medium->high), the relay (“pump”) stays on in the medium state
• when “emptying” (i.e. high->medium->low), the relay (“pump”) stays off in the medium state

This is achieved with feedback from the final output opamp unit LM324d(pin14) to its non-inverting input (pin12) i.e.:

• with J1 closed, LM324c(pin8) is low
• with J2 closed, LM324b(pin7) is low
• then the final output LM324d(pin14) is pulled high
• but when J2 opened, LM324b(pin7) goes high, but the resulting voltage on LM324d inverting input is not high enough to LM324d feedback to its non-inverting input
• but when J1 also opened, LM324c(pin8) also goes high and is finally enough to flip LM324d

Note: LM324a, the first opamp unit, is configured as a square wave oscillator running at about 710Hz. This provides the AC signal switched via J1 and J2 to pull their respective LM324 opamp units high via the DC-blocking capacitors. See the signal trace, tapped at pin 1 of the LM324:

Construction

Here is my redraw of the schematic and a possible breadboard layout:

I don’t actually have a water tank to monitor, so I did a quick simulation and test on a breadboard:

Credits and References

• LM324 chip automatic drainage dual-function controller DIY learning welding spare parts automatic water level controller kit
• S8050 Datasheet - NPN BJT
• LM324N Datasheet