Simple timer circuit diagram

When you turn on the radio, it will automatically turn off after a while. In many aspects of life and production technology, time control is required. Here we introduce a simple timer with a simple circuit and easy production. Figure 2-2-1 is the circuit principle, and Figure 2-2-2 is the printed circuit board.

Figure 2-2-1

Figure 2-2-2

　　Recognize

　　Test it

　　1. Measure the value of resistance (R), see Figure 2-2-3. Set the multimeter to Ω×1K, and the resistance is about 3.6KΩ.

Figure 2-2-3 Figure 2-2-4

　　2. Measure the variable resistor (W), see Figure 2-2-4. When measuring terminals 1 and 3, the resistance is about 2MΩ (1MΩ=1000KΩ). When measuring terminals 1 and 2 or terminals 2 and 3, the resistance increases or decreases between 0 and 2 MΩ when the rotor is rotated.

　　3. See Figure 2-2-5 for measuring diodes. The forward resistance is about tens of Ω, and the reverse resistance is hundreds of KΩ. The forward resistance is measured when the black test probe is connected to the positive electrode of the diode and the red test probe is connected to the negative electrode of the diode. The reverse resistance is measured when the test probes are reversed.

Figure 2-2-5 Figure 2-2-6

　　4. Measure electrolytic capacitance (C), see Figure 2-2-6. When the resistance value is first measured, it is very small, and then the pointer gradually deflects to the right and returns to the original position near the right end.

　　5. Measure transistors (T1 _, T2 _, T3 ₎ , see Figure 2-2-7. T1 _and T2 _are NPN type, T3 _is PNP type. You should select their respective ranges and not make a mistake. The three wires (EBC) of the transistor should be inserted into the corresponding three small holes and not make a mistake.

Figure 2-2-7

　　Give it a try

　　According to Figure 2-2-1, solder the components onto the circuit board in Figure 2-2-2.

　　1. Clean the leads of each component and apply tin.

　　2. Install resistor R and variable resistor W. Terminal 2 of W should not be confused with terminal 1 or terminal 3.

　　3. Install diode D and electrolytic capacitor C. The leads of these two components have positive and negative poles, so they must not be soldered incorrectly.

　　4. Install T1 _, T2 _, and T3 _. Note that T1 _and T2 _are both NPN type, while T3 _is PNP type. They cannot be used interchangeably. The three EBC leads of the transistor cannot be interchanged either. Be careful when soldering and do not make mistakes.

　　5. Check the soldered circuit board repeatedly, connect the 3V power supply after confirming that it is correct, and lead a wire to each of the A and B points of the circuit board. The two wires should be distinguished by different colors and cannot be used incorrectly. Find a small bulb used in a flashlight, connect it to the lead wires and you can test it. Press the button AN, the small bulb will light up, and after a while, the small bulb will automatically go out. Adjust W to change the length of time the bulb is lit.

　　6. Take a radio (with two batteries inside). Take out the original batteries in the radio, connect point B on the circuit board to the negative pole of the radio, and point A to the positive pole of the radio. Turn on the radio, press button AN, and the radio can listen to the radio broadcast. After a period of time, the radio automatically turns off. Adjust W to change the timing time.

　　Think about it

　　There is a device called a relay, and its circuit symbol is . It is mainly composed of a coil and a switch. When current flows through the coil, the switch inside it is closed or opened, which can control high-power electrical equipment. If a relay is used instead of a radio, what other occasions can this small project be used for?