In this guide, we are designing a Liquid Level Switch using 3 transistors (BC107), some passive components, a diode and a couple of relays. This is basically a level sensing circuit designed using minimal components. If you’re looking for an efficient level sensing circuit, you would be better off with our water level controller circuit.


Here is a simple level switch circuit that switches on one relay and switches off another relay when the fluid level exceeds the set limit.This circuit is a modification of the Simple Water Level Indicator previously posted.When the water level touches the probes positive supply is connected to the base of Q1 through fluid.This makes transistor Q1 ON.Base of Q2 is connected to collector of Q1 and base of Q3 is connected to collector of Q2.As a result Q1 will be on and Q2 will be OFF.So K2 will be OFF and K1 will be ON.Net result, we get two relays operating according to level ,one ON and Other OFF.The contacts of the relays OP1 and OP2 can be used according to your need to operate any device like motor,pumps,solenoid valves,light,alarm etc.When water level falls K2 will be ON and K1 will be OFF.

Liquid Level Circuit Diagram with Parts List.

Water Level Switch-Fluid Level Switch
Liquid Level Switch Circuit Schematic


  • Adjust R1 keeping probes touched in water to obtain the relay K1 ON.Then remove probes from water and see relay K1 goes OFF.During this time the opposite will be happening in K2.If you cannot attain the setting replace R1 with a 500K POT and repeat the procedure.That’s enough.
  • For the probes cut 2 1 square cm aluminum sheets and connect it to good quality insulated Aluminum wires. 2 cm spacing between probes is enough.Mount the two probes parallel at the desired level.Circuit is ready.
  • One problem of this circuit is the corrosion of probes since DC is used.Any way this setup will run problem free for at least one year. We are developing a corrosion free version of the same circuit.That will be here soon.
Note:- We have recently developed a fully functional Water Level Controller using Microcontroller. This water level controller monitors the level of the over head tank  and automatically switches on the water pump when ever the level goes below a preset limit. You may see the circuit, if you are interested.

We have more interesting circuits that you should take a look at:

1. IR Sensor Circuit

2. Photo Switch Circuit

3. Touch Switch Circuit

4. Fire Alarm Circuit

5. Digital Temperature Sensor



  1. Gopi nathan

    Hello sir,

    Hope ur fine. Last one month i m trying this circuit. but i did not get the output. one relay K2 is working(on/off).another relay K1 always on.either i should change any resistance value.i m awaiting for ur reply.

  2. is the common terminal for the relay is connected to the negative supply?

    • Seetharaman

      yes it is connected to common negative supply.

  3. can i use 12 v spdt relay without changing the supply or making it also 12v?tnx sir

  4. hi sir, can i use 12v spdt relay in place of 9v spdt relay because its not available in the market.???

  5. Edgardo Diolola

    Hi F.Rees,
    Yes, the circuit featured maybe old, but it helps to people like me who are among the newbies.
    I know yours are of industrial type.Can you feature in this site the circuit diagram of the level control product your company used, so that many will benefit from your advertising? I checked out what you wanted and saw the video also.It helps a lot if you can furnish the circuit diagram, functional one to fully support and help the newbies.From what country were you from? me I am from Philippines.
    Thanks in advance.