Simple and practical 1-hour timer switch

The timing switch is mainly composed of a NAND gate I and a transistor switch circuit. The logical function of the NAND gate circuit is that when all the input terminals are high level "1", the output is low level "0"; as long as one of the input terminals is low level "0", the output is high level "1". This logical relationship can be simplified as: "see 0, output 1, all 1 is 0". Mastering this logical relationship, you can analyze the circuit related to the NAND gate. In the figure, the NAND gate has two input terminals, namely pins 5 and 6, and pin 4 with a small circle is the output terminal. The "&" in the box represents the NAND gate. In the figure, VD2, VD3, and C2, C3 form a capacitor step-down half-wave rectifier voltage stabilization circuit. Normally, since SB and K2 are both in the open state, the entire circuit does not consume electrical energy. Socket X also has no 220V AC output.

The plug of the household appliance to be controlled is inserted into the socket X. If the appliance is working, just press the button switch SB, and the socket X will supply power to the outside. This is because when SB is pressed, the rectifier circuit starts to work, and the two ends of C2 output about 12V stable DC power for the NAND gate I and the transistor circuit. Since one of the input terminals of the NAND gate, pin 6, is grounded through the conversion contact, according to the logical relationship of "see 0 and output 1", it can be known that the output terminal pin 4 outputs a high level, so VT is turned on, the relay K is energized and attracted, and the conversion contact K1 is turned upside down, and the power supply charges the capacitor C1 through RP and R1.

Since charging takes a certain amount of time, pin 6 remains at a low level, and VT continues to remain in the on state. The other contact K2 of the relay is closed and self-locked. After SB is released, the timing switch is still powered on, so socket X supplies power to the outside to make the plugged-in appliance work. As charging continues, the voltage across C1 continues to rise. When it reaches 1/2VDD, the two input pins 5 and 6 are both high level. From the logical relationship of "all 1 is 0", it can be seen that the output pin 4 is a low level "0", so the transistor VT is cut off, K loses power and releases, K2 opens, and socket X stops supplying power to the outside. At the same time, K1 falls to the ground, and C1 discharges through K1 to prepare for the next timing. Since K2 is open, the entire timing switch no longer consumes electricity. The length of the timing time can be adjusted by the potentiometer RP. Using the data in the above figure, the timing time can be continuously adjusted within 1 hour.

Component selection :
NAND gate I uses a 2-input four-NAND gate CD4011 integrated circuit, which integrates 4 2-input NAND gates. This circuit only uses any one of the intact NAND gates. The input terminals of the other 3 unused NAND gates should all be connected to the positive or negative pole of the power supply. They cannot be left floating, otherwise they are easily disturbed by the external electric field and the logic level is confused. VT uses 9013, 9014 and other silicon NPN transistors, β ≥ 300. VD1 can use 1N4148 silicon switching diode, VD2 can use 1N4001 rectifier diode, VD3 is 12V, 1/2W voltage regulator diode, such as 2CW19, etc. RP can use WH15 potentiometer, and it must be equipped with corresponding buttons. R1-R3 are all RTX-1/8W carbon film resistors. C1 and C2 use CD11-16V electrolytic capacitors, and C3 is preferably CBB-400V polyphenylene capacitor. K adopts JRX-13F, DC12V small electromagnetic relay, which has two sets of conversion contacts to meet the needs of this machine.

Production and use :
The printed circuit board of the timer switch is shown in the figure below. The size of the printed circuit board is 60mm×40mm. Since this machine does not use a power transformer, the printed circuit board and electronic components are energized after power is turned on, so safety should be paid attention to during debugging. Use the experimental method to mark the set time on the knob dial of the potentiometer RP. When using it in the future, just turn the potentiometer to the predetermined time and then press SB. Socket X will supply power to the outside at a scheduled time as required. If you need to change the time of the electric big timing, you can increase or decrease the value of the potentiometer RP or the capacitor C1.