Simple Overhead Tank Water Level Controller without MCU

In this project, we will learn how to create an automatic water pump system circuit that turns on and off based on the water level in the tank.

Basic idea
The presented project demonstrates a simple circuit built using digital logic gates, IC NE555, transistors and some additional components that detect the water level in the water tank and turn on/off the water pump when the tank is empty/full. We will also illustrate what to do if there is no water in the underground water tank. A small addition to the main circuit will solve the problem. Finally, we will present some circuits for different indications such as pump on/pump off, tank empty/full, etc.

Circuit Description
As shown in the figure, the circuit is built using a NAND gate, a NOR gate, a timer IC NE555 and some other components like transistors, resistors, relay, etc.

1. The two transistors Q1 and Q2, connected in switched mode, serve as water level sensors. Their bases are inserted into a water reservoir that maintains contact with the water and is known as a probe.
2. The +Ve supply is supplied to water from a tank, so if the probe is in contact with water it also receives a +Ve supply.
3. There are two sensors to detect empty and full two levels .
4. The final sensor output is taken from a transistor collector. Both outputs are provided as inputs to two gates: NAND gate (7400) and NOR gate (7402)
5. The output of the NAND gate is connected to the trigger input of IC NE555, and the output of the NOR gate is connected to the threshold input of IC NE555. IC NE555 is connected in bistable mode.
6. The output of the NE555 activates 1 C/O relay (switching) through another NPN transistor Q3.
7. The water pump receives 230 VAC power through the C (common) and NO (normally open) relay contact terminals

Circuit operation

1. When both probes are in contact with water, they receive a supply of +Ve. Since transistors receive +Ve input and conduct. They provide low outputs (logic 0).
2. Both outputs are provided as inputs to both ports. Since both inputs are 0, the NAND gate output is high (logic 1 – means +Ve), and the NOR gate output is also high (logic 1 – means +Ve).
3. Both high outputs are provided to trigger and limit the NE555 inputs, so its output remains low. The relay does not turn on, so the water pump does not receive supply.
4. When the water tank is empty, the probes do not come into contact with the water. Therefore, transistors do not conduct and provide high outputs (logic 1).
5. When NAND gate takes both inputs as logic 1, it gives low output (logic 0).
6. As this low output is provided to drive the input of the NE555, its output increases due to its bistable operation.
7. This will turn on the relay and immediately the water pump will receive the supply and turn on. So, when there is no water in the tank, the pump turns on automatically.
8. As water fills a tank, the first bottom probe comes into contact with the water, providing low output. But this does not change the state of IC NE555 and the water pump remains ON.
9. As the water reservoir fills to the maximum level, the upper probe also comes into contact with the water; again, both transistors provide low output.
10. This time, the NOR gate comes into play – setting both inputs low results in a high output.
11. As this high output is supplied to the threshold input of IC NE555, it immediately provides a low output.
12. This turns off the relay as well as the water pump. So, as the water tank fills up, the water pump stops automatically.

Adding intelligence
Typically, there are two water tanks – an upper tank and an underground (or lower) tank. The pump supplies water from the lower tank to the upper tank. Here, the water pump turns ON when there is no water in the tank. If there is no water source, the pump starts but runs without water, which can burn or damage it.

Therefore, we need to turn off the water pump when there is no water in the lower tank. This can be done by adding a simple circuit as shown below.

The same transistor switch circuit is used as a water level sensor. Insert two sensor probes into the lower water tank – one for +Ve supply and one to detect the lower tank water level. The transistor receives +Ve input until the probe connected to the base comes into contact with water, the transistor receives +Ve input. This output is combined with the pump to initiate the output of the previous circuit using the OR gate.

When the pump starts, the output is low and the sensor output is also low. In this case, only the trigger pin of IC NE555 receives low input from the OR gate and the pump is turned ON through the relay. But if there is no water in the lower tank, the transistor does not conduct and its output increases. The OR gate output increases immediately and there is no effect on the pump start signal. IC NE555 does not receive trigger input and does not turn on the pump through the relay. As a result, the water pump will not start when there is no water in the lower tank.

Indications
To add an ON and OFF pump indication, simply modify the NE555 IC block as shown in the figure by adding two LEDs.

1. RED LED to indicate pump off
2. GREEN LED to indicate pump on

The RED LED is connected between Vcc and the IC output (with a 220E current limiting resistor). When the output is low and the pump is OFF, the RED LED lights to indicate the pump is OFF.

The GREEN LED is connected between the IC output and ground. When the output is high and the pump is ON, the green LED is ON.

Below, we have indications of empty, full and lower empty tanks. There is a common type of circuit known as transistor-emitter biasing for all these three indications. The basic inputs of the transistors are connected to the respective TE, TF and LTE terminals of the above two circuits. When the tank (top or top) becomes empty, the TE output goes high, then the empty tank RED LED lights up to indicate that the top tank is empty. When the upper tank is full of water, production is low. The GREEN LED on the PNP transistor lights up to indicate that the upper tank is full.

LTE output increases when the lower tank is empty, then the empty RED LED glows to indicate that the lower tank is empty.