1. Principle of circuit operation

The circuit is composed of voltage regulator circuit, signal generator circuit, logic pen circuit and buzzer circuit.

The transformer used in the circuit is 2.5W, the power is small, and the output current is about 200mA. Therefore, the power supply is only suitable for teaching and training, and can only supply power to some small circuits. You can't charge your phone. If a large load is connected, voltage drop or voltage fluctuation will occur.

The voltage regulation range of the LM317 is 1.25-37V, and the regulation range set by this circuit is 1.25-12V because there is a CD4069 on the circuit and the maximum operating voltage cannot exceed 15V. Therefore, the input voltage cannot be arbitrarily increased.

1. Voltage regulator circuit: The transformer reduces the mains power to 12V AC power, and converts it into DC power through the bridge rectifier circuit composed of D3, D4, D5 and D6. C4 is the input filter capacitor that suppresses input voltage ripple. C1 is that output filtercapacitor used to cushion the impact load and ensure stable operation of the circuit. Because of the existence of the capacitor C1, the LM317 is easily damaged by capacitive discharge. When the capacitor C1 is discharged by the external protection diode D1, the LM317 can be protected by discharging the capacitor C1 through D1. C5 is that filtercapacitor at the regulating end and has the function of stabilize the output. D2 is a protection diode for preventing damage to the LM317 during discharge of the C5. The D7 blue LED is the voltage output operation indication, and R2 is the current limiting resistor for D7. R1 and RP1 form a resistive network with adjustable output. The output voltage can be adjusted by changing the resistance of RP1. The resistance of R1 is 120-240. The output voltage is calculated as Uo= (1 + RP1/R1) × 1. 25.
2. Signal generator circuit: U2A, U2B, R5 and C6 form a non-gate oscillation circuit.Assuming the U2B output is high, since the voltage across capacitor C6 cannot be abrupt, then the U2A input is also high. The U2A output is lowas well as the U2B input. The output at U2B is high. At this point, the output pulse of U2B is guaranteed to be high. With the charging of the C6 capacitor, the C6 is charged to the voltage of a+, b-, and the negative voltage at the terminal b of the capacitor decreases the input voltage of the U2A. When the voltage drops to the non-gate-off voltage, the U2A outputs a high level and the U2B outputs a low level. Since the U2A outputs a high level, the capacitor C6 is reverse charged. As the voltage at terminal b rise to the open level of U2A, U2A outputs a low level and U2B output a high level, so that a square wave is generated. The square wave signal is output backwards through U2F, while the square wave status is indicated through the D8 yellow LED. By adjusting the resistance value of R5, the charge and discharge time can be adjusted, thereby adjusting the oscillation frequency.
3. Logic pen circuit: DS05 is the logic level input. When the input is low, the U2E output is high, the U2D output is low, and since the U2C output is high, D9 is bright (green LED), indicating low. When the input is high, the U2E output is low, the U2D output is high, and since the U2C output is low, D10 is bright (red LED), indicating high. If the input is floating, it is vulnerable to external interference and is in an uncertain state.
4. Buzzer circuit: It is composed of transistor Q1 and buzzer. When high level is added to the transistor base (DS03), the buzzer produces sound. For example, the DS03 is connected to the power supply by a wire. If the wire is on, the transistor turns on and the buzzer sounds. If the wire is broken, the transistor turns off and the buzzer does not sound. This principle can be used to test the wire on and off. Similarly, if a low frequency square wave signal is added to the DS03 terminal, the transistor is intermittently turned on, and the buzzer emits intermittent drop-drop-drop sound.
5. Installation Instructions
6. Install the low component first, then install the high component, the resistor has no positive and negative electrode, the long pin of the electrolytic capacitor is the positive electrode, the black circle position of 1N4007 corresponds to the black mark position of the PCB, the long pin of the buzzer is the positive electrode, the long pin of the light emitting diode is the positive electrode, the integrated circuit CD4069 first installs the seat, the notch position corresponds to the PCBwith notch mark position.
7. The LM317 is first fixed to the radiator with a screw of 3mm, and then welded to the LM317.
8. When installing the voltmeter, remove the 3 wires, solder the 3 component pins, and solder the corresponding voltmeter position-+ Vin to the PCB.
9. There is no difference between positive and negative electrodes in the connection of transformer, but it should be noted that the red wire (primary) is connected with strong current, and the blue wire (secondary) is connected with ~ position of PCB. The transformer is fixed on the housing with 3mm screws, and the PCBis fixed with tapping screws.
10. Protect the ends of the power supply wires connected to the transformer from short circuit and electric shock. Sleeve the welded power supply wires with heat shrinkable tubes, and then wrap the ends with electric soldering iron.
11. Secure the housing for 4 weeks with 2 mm screws.
12. Install crocodile clip: first insert the leather sleeve into the lead wire according to the color, and then solder the lead wire to the clip.

III. Troubleshooting

In case of failure, first check whether the components are erroneously installed and whether the welding is good. Especially for novice solders, solder joints are prone to defects, mainly in the solder joints are not smooth, burrs, lead too long lead to short circuit to the pad next to the heel, solder joint stolen goods, these are the main reasons for the failure.

The voltage cannot be regulated or the regulation range is small: 1. Check that there are two small pads on the back of the voltmeter close to the 3 pins and that there is no short circuit. 2. Check resistor R1 for installation errors. 3. Use a multimeter to measure whether the resistance of RP1 is variable.

No signal output or logic pen inoperative: Check if direction of CD4069 is reversed. Check the CD4069 for voltages greater than 3V between 7 and 14 pins. If the circuit voltage exceeds 15V, the CD4069 may be damaged.

Buzzer not ringing: Check 9014 for erroneous installation and check that the buzzer is reversed.