#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..
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 is available form aliexpress sellers for around SGD$2.90 as I write this.
A well-made single-sided PCB:
|C1||22nF (223) ceramic||1|
|C2,3,4,5,7,8,9||100nF (104) ceramic||7|
|D6,D8||5mm LED red||2|
|D7||5mm LED yellow||1|
|D3||5mm LED green||1|
|14 pin DIP socket||1|
|J3||2 pin terminal block||1|
|J4||3 pin terminal block||1|
|J1,J2||2.54mm pin header x2||2|
How It Works
There are two inputs:
- J1 - “low state” input switch
- J2 - “high state” input switch
|State||J1||J2||D6 Red||D7 Yellow||D3 Green||VIN||Relay State|
|low -> medium||closed||open||OFF||ON||OFF||~ VCC/2||ON|
|medium -> high||closed||closed||OFF||OFF||ON||Low||OFF|
|high -> medium||closed||open||OFF||ON||OFF||~ VCC/2||OFF|
|medium -> low||open||open||ON||OFF||OFF||High||ON|
- 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:
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: