The circuit diagram given here is a simple level switch that switches relays between two states according to the fluid level. The main parts of this circuit are three transistors Q1, Q2, Q3 (BC 107) and two relays K1 and K2.
Transistor Q1 is turned ON when fluid level touches the probes since it is connected to positive supply through fluid. The three transistors are interconnected in such a way that the base of Q2 is connected to collector of Q1 and base of Q3 is connected to collector of Q2. Since the interconnection of transistors, when Q1 is ON Q2 will be OFF. The state of relays K1 and K2 are also according to the state of Q1 and Q2 respectively. The contacts of the relays OP1 and OP2 can be used according to application to operate any device such as motor, pumps, solenoids, valves, light, alarm etc.
- 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.
- One of the main problems in using this circuit is the corrosion of probes. Corrosion free probes will release soon.