Almost all restaurants have a hand washing sink for their customers to use. Since it is used by many people, the faucet stops working properly after some time and starts leaking, which leads to wastage of water. This circuit prevents the problem because it automatically flows water from the faucet when a person goes to the sink.
The circuit is built around a DC barrel jack (J1), PIR sensor HC-SR501 (MS1), transistor BD139 (T1), a normally closed solenoid valve (SV1), and other components. The circuit can be powered by a 12V battery or an external 12V DC adapter.
Before understanding the working of the project, let us understand the working of the PIR motion sensor, which is the heart of this circuit. Fig. 2 shows the front side of the PIR motion sensor while Fig. 3 shows its back side.
The HC-SR501 is a compact passive infrared sensor used to detect any type of movement. It has a wide operating voltage of 4.5V to 20V and two operating modes-repeatable mode and non-repeatable mode.
In repeatable mode (H: repeat trigger in Fig. 3), the sensor output increases when motion is detected. It will remain high as long as the person is in range of the sensor. When man moves from his size, his output decreases—after a predetermined amount of time.
In non-repeatable mode (L: single trigger in Fig. 3), the output of the sensor is only high for a pre-set time when motion is detected. Any movement during this time does not affect the output, so water continues to flow during this time. It will only trigger once and cannot be triggered again. In this project, we use repeatable mode.
On the back side of the sensor there is a jumper and two variable resistors, as shown in Fig. 3. The jumper is used to select the operating mode. A variable resistor is used to change the sensitivity of the sensor and the other to change its time delay.
The working of the circuit is simple. Usually, the output of the sensor is small, and the valve is not strong. When a person comes in front of the PIR motion sensor, the output of the sensor will be high as long as the person is inside and the solenoid coil is energized. The valve opens and water begins to flow from the faucet.
D1 – 1N4007 rectifier diode
T1 – BD139 NPN transistor
Resistors (all 1/4-watt, ±5% carbon):
R1 – 220-ohm
J1 – DC barrel jack
J2 – 3-pin terminal connector
J3 – 2-pin terminal
MS1 – HC-SR501 PIR motion sensor
SV1 – 12V NC solenoid valve, Single-changeover relay
If the sensor does not detect any of its contents, its output will go short, and the solenoid coil will be de-energised. Therefore, water flows from the faucet only when a person is inside it and automatically stops when a person moves away.
Build a test
Simply put, the circuit can be assembled on a general purpose PCB. After the circuit is assembled, insert it into the appropriate box. Install the motion sensor in front of the hand washing sink so that when someone steps in front of it, the tap water starts flowing.
In the non-repeatable mode (L: single trigger in Fig. 3), the time for the water flow can be pre-set by the time-delay adjustment pot (on the right side of Fig. 3). The sensitivity can be set via the sensitivity-adjustment pot (on the left side of Fig. 3).
- Adjust the solenoid valve to the correct direction of water flow. The direction is marked on the valve housing.
- There is a minimum pressure required for proper valve operation. The water storage tank must be at a height of at least three meters above the valve, otherwise the use of a water pressure pump is necessary.