The circuit contains two parts, which are a transmitter part and a receiver part. The transmitter part contains two 555 timer ICs, the transmitter part performs two functions here, first it will detect the water level in the tank and second it will send a laser signal to the receiver circuit.
The receiver receives the a laser light signal and switch ON the relay due to which the water motor connected with the relay will also become switch ON. When the receiver received another laser signal from the transmitter it will switch OFF the relay and hence the water motor will also switch OFF.
The operating voltage of the transmitter is 5V DC therefore use a 4.5V to 5V laser pointer with the circuit. If you want to use 3V laser pointer then use a 3V voltage regulator like LD1117V30 or any other 3V or 3.3V regulator after the BD139 transistor. The receiver circuit can be operated with any voltage from 5V to 15V DC and the relay in the receiver's section should be used according to the operating voltage. The current handling power of the relay should also be selected according to the load it is controlling, therefore if your water motor current consumption is under 10 ampere then a 10 ampere relay will work fine but if your motor current consumption is higher than 10 ampere then use a higher ampere relay.
The circuit requires tuning for the first time. First install both the circuits i.e. transmitter and receiver in two different enclosures. In the transmitter enclosure make a small hole from which laser beam will be transmitted, a small hole should also be made in the receiver enclosure for the LDR to receive the laser beam, also attach any opaque pipe at the outside of the LDR hole so the LDR only detects the laser beam and not any other light for example day light or any other bulb light. After completing both parts i.e. transmitter and receiver set both of them in the line of site at the desired distance. The distance depends on the output power of the laser pointer. Long distance can be achieved easily with a higher output laser pointer.
Here is a project of an automatic wireless water pump controller circuit based on laser. The circuit is fully automatic and once it is setup, it will serve for years. It can wirelessly switch ON a water pump when the water tank is empty and switch it off wirelessly when the tank is full. The circuit will also
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