Production of touch-type automatic water faucet

Compared with ordinary faucets, the hand-touch automatic water faucet introduced here has the advantages of being easy to use, sensitive, hygienic and reliable. The disadvantage is that the water volume cannot be controlled. Since the switch is completely sealed, it can be wrapped in the winter in the north and become a non-freezing faucet below 0℃. It has universal promotion value.

1. Working Principle
The circuit of the hand-touch automatic water faucet is shown in Figure 1.

The power switch IC and a few peripheral components form a touch-type electronic switch. After the 220V AC is stepped down by the power transformer T, full-wave rectified by the crystal diodes VD1 and VD2, and filtered by the capacitor C, it outputs a DC voltage of about 12V to supply the IC control circuit.

Normally, the transistors VT1 and VT2 are both in the cut-off state, the control terminal of the power switch integrated circuit IC has no input voltage, its internal switch circuit is disconnected, and the electromagnetic relay K is not attracted. The entire circuit is in standby state.

When the "open" metal sheet is touched by the hand, the various clutter signals (mainly 50Hz AC) sensed by the human body from the surrounding space will turn on the transistor VT1, and the 5th pin of the power switch integrated circuit will obtain an input voltage of ≥1.6V. The internal switch circuit is controlled to turn on, so that the 2nd and 3rd pins output DC voltage. On the one hand, the voltage is fed back to the 5th pin of the IC through the resistor R. After the hand leaves the touch sheet, the internal switch circuit of the IC remains in the on state, realizing the "self-locking" of the circuit on state; on the other hand, the electromagnetic relay K is driven to attract, so that the electromagnetic valve controlled by the normally open contact KH is energized and opened, and the faucet automatically discharges water.

When you touch the "off" metal sheet with your hand, the weak electrical signal induced by the human body turns on the transistor VT2. The input voltage of the ⑤th pin of the power switch IC drops below the turn-on threshold voltage (1.6V), and the switch circuit inside the IC is disconnected. The electromagnetic relay K is released, the electromagnetic valve is powered off and closed, and the faucet stops discharging water. At this time, the circuit returns to the original standby working state.

2. Component Selection

The IC uses the TWH8778 power switch integrated circuit. It uses a TO-220 five-pin plastic package, and its appearance and pin arrangement are shown in Figure 2 (a). The TWH8778 is equipped with overvoltage, overheating, overcurrent and other protection circuits, and its internal circuit block diagram is shown in Figure 2 (b).

TWH8778 can be used as a high-speed switch below 28V/1A. It only needs to add a voltage of about 1.6V (maximum 6V) to the "control" end (pin 5) to quickly control the on and off of the load power supply at the output end (pins 2 and 3). The input current of the control electrode is only about 50μA.

TWH8778 can also be directly replaced by QT3353, a similar product with the same appearance and function. K uses JQX-4F medium-power electromagnetic relay, with a contact form of 4H. When in use, all four groups of normally open contacts are connected in parallel to increase the load capacity. The electromagnetic valve can choose DF-1 AC 220V two-position two-way gas-liquid electric control valve, and its diameter size is selected according to the diameter of the water supply pipe to be controlled.

3. Production and Use

Figure 3 shows the wiring diagram of the printed circuit board of the hand-touch automatic faucet. The actual size of the printed circuit board is about 65mmx50mm. The welded circuit board and the power transformer T are placed in a small insulating box of suitable size, and are connected to the solenoid valve and 220V AC power supply through double-strand wires.

The touch metal sheet can be made of brass sheet or stainless steel sheet, cut it into two circular sheets of φ25mm, and glued to an operation panel made of plastic or organic glass board with strong glue. Because the trigger sensitivity of this switch is high. If the lead between the touch metal sheet and the crystal triode VT1, VT2 is long, interference failure will occur. For this reason, the author repeatedly experimented and directly welded the crystal triode VT1, VT2 on the back of the touch metal sheet (as shown in the dotted box in the figure), and then connected it to the circuit board in the control box through three wires (length is not limited), which solved this problem simply and cleverly.