Dual-timer-based system provides contactless water-dispensing solution

Article By : Praveen Kr. Agrawal and S.V. Nakhe

This Design Idea provides a simple retrofit solution to create a contactless water dispensing unit.

Contactless washbasin taps that can dispense water without the risk of communicating diseases can play an important role in the battle against the COVID-19 virus. At present, most of the world’s washbasin taps are hand operated, but this Design Idea provides a simple, cost-effective retrofit solution to implement a contactless water dispensing unit.

Contact-free tap controls can also help reduce the amount of water wasted during handwashing since most people tend to leave the water tap open during the entire 20 second cycle recommended by medical authorities.  This solution is reliable and easy to implement from commonly-available components with minimum modifications to an existing setup.

The schematic diagram of the electronic circuit is shown in Figure 1. The circuit has two independent sensors that generate a trigger signal when the optical path between their IR emitter and detector is interrupted. The first trigger signal will open the water line for five seconds, enough for the user to wet their hands. The second trigger signal keeps the water flowing for 20 seconds, allowing a user to thoroughly wash and rinse their hands. The timing can be easily adjusted to accommodate local requirements by adjusting the resistor capacitor combination (R3, C7 or R4, C8).

circuit diagram of a dual timer contactless water dispensing unitFigure 1 This dual timer-based contactless water dispensing unit uses two independent sensors that generate a trigger signal when the optical path between their IR emitter and detector is interrupted.

Both detector circuits use an infrared emitting diode (D1, D3 = OSRAM SFH 484, λp – 880 nm) as the transmitter. The bracket holding the IR emitters is aligned to ensure their narrow output beam (half angle = ± 8°) falls squarely onto their respective detectors (D2, D4 = Motorola MRD 821).

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The limiting resistors R6, R10 (560 Ω, 1/4 W) limit the IR transmitter’s forward current to ~20 mA, enough to ensure reliable operation for distances up to 150 mm between emitter and detector. The circuit operates on a single +12V DC supply, with detectors D2, D4 operated in reverse bias condition. The detectors’ outputs (i.e cathode pins) are connected to the inputs (pin 3 and 5) of a non-inverting hex buffer (U1, CD 4050). The outputs of U1 (pins 2 and 4) are fed to a dual Schmidt trigger (U2, CD 4098), which is configured to trigger on the rising edge (+ ve) and for one-shot (non-re-triggerable) operation.

The components R3, C7 (1 MΩ, 10 μF) and R4, C8 (4.7 MΩ, 10 μF) create time constants that cause U2 to generate signals with pulse widths of 5 sec and 20 sec from its output pins (6 and 10 respectively). U2’s output signals are fed to pairs of buffers in U1 (3/6, 4/6 and 5/6, 6/6), connected in parallel to provide the current needed to drive the transistor Q1 (TIP 122), which actuates the water valve solenoid. The drive signal reaches the transistor though either diode D6 or D7 (IN4148) with resistors R9 (3.3 k) depending on received input signal. Capacitors C1-C6 are used to reduce the effect of power supply noise. The indicators LED 1, LED 2, and LED 3 are used as status signal of power ON, 5 sec (wet mode), and 20 sec (wash mode). Q1’s collector is brought out to one pin of the circuit’s power connector (CN2) and the other pin is connected to +12 VDC.

The valve actuator’s solenoid coil (S) is connected between CN2 and a normally-closed ½” solenoid (12 VDC, 350 mA, available from Adrafuit Industries). This assembly is connected in series with washbasin taps, as shown in Figure 1, so that the solenoid valve now controls the water flow. The diode D5 (IN4002) is connected in parallel with CN2 to protect the transistor Q1 against the back-EMF voltage spikes that occur during solenoid switch-off.

The two optical sensors (D1-D2 and D3-D4) can be conveniently-located near a washbasin in the contactless water dispensing solution. When a user waves their hands near either sensor and interrupts the IR radiation between D1 and D2 , the circuit generates a five-second pulse at the output pin 6 of U2, which in turn drives transistor Q1, thereby energizing solenoid coil (S), which produces a five-second stream of water. Similarly, if an interruption is detected between D2 and D4, the solenoid coil will be energized by transistor Q2 to produce a 20 second flow of water, which can be used to complete a medically-approved hand washing cycle.

This article was originally published on EDN.

Praveen Kr. Agrawal and S.V. Nakhe work at the Raja Ramanna Centre for Advanced Technology in Indore, India.

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