An experimental model of an infrared radio alarm

The circuit schematic diagram of this infrared anti-theft alarm experimental model is shown in the figure below.

The circuit is mainly composed of a power supply circuit, an infrared transmitting circuit, an infrared receiving circuit, a logic processing circuit and an alarm circuit.

VT7, VT8 and related components form a multivibrator, and its oscillation frequency is determined by the values ​​of R14, R15, C8, and C9. The oscillation signal is input from the collector of VT7 to the base of VT6 through R17, and after being amplified by VT6, it drives the infrared transmitting tube to transmit signals to the outside world. When there is no object between the transmitting tube and the receiving tube, the transmitted infrared signal is received by D7, and after being amplified by VT1 and VT2, it is coupled from the collector of VT1 through C4 to the double voltage rectifier circuit to form a DC control voltage, so that VT5 is saturated and turned on, and the 8th and 9th pins of IC1 input low level. After "not" logic processing, the 10th pin outputs the level, and the 3rd pin of IC1 outputs the high level. This high level pulls up the level of the 13th pin of IC1 through R8, thereby maintaining the high level output of the 3rd pin, and the other way keeps VT3 cut off, and the alarm circuit does not work.

When there is an object between the transmitting tube and the receiving tube, the infrared signal is blocked, and D7 receives no signal. At this time, the signal voltage output by the voltage doubler rectifier circuit is very low, VT5 is cut off, and the state of the entire circuit changes. Pin 12 of IC1 becomes low level, and pin 3 outputs low level. This low level signal changes pin 13 of IC1 to low level through R8. At this time, even if the level of pin 12 becomes high level, as long as one input end is low level in the relationship with the non-gate, the output will be high level, so the output state is locked, and the output of pin 3 will be maintained at a low level; at the same time, VT3 is saturated and turned on, the alarm circuit is powered on, and the speaker will sound an alarm. Here, the alarm signal generation circuit uses a dedicated music integrated circuit. After power-on, it generates an alarm audio signal, which is driven by the speaker after VT4 power amplification. To release the alarm, two conditions must be met at the same time: one is that the infrared signal is received normally, and the other is that pin 13 of IC1 must be forced to input a high level signal. The reset button in the circuit is set to force a high level signal to pin 13.

1. Circuit Production

The manufacturer can complete the entire production by simply installing the components correctly according to the markings in the circuit schematic. Some of the components need to pay attention to the following points when installing:

1. When installing components, strictly follow the markings on the circuit board. For polarity components, be careful not to install them in reverse.

2. When installing the alarm sound music integrated circuit IC3, its oscillation resistor R22 is directly installed on IC3; then use the cut resistor pins to solder the pins out from IC3 and then connect them to the circuit board.

3. When installing the infrared transmitting tube and the receiving tube, the pins should be of sufficient length and bent after welding so that the two tubes are placed horizontally with the circuit board.

4. The power supply of this production can use a transformer with a power of not less than 2W and a voltage of 9-12V. In some places with DC regulated power supply, a 9-12V DC power supply can be used directly for power supply.

2. Function debugging

1. After all components are installed, connect the power supply, the power indicator light is on, and use a multimeter to measure the voltage across C1. Normally it should be around 5V. If it is abnormal, carefully check whether IC2 is installed upside down and whether the four rectifier diodes are installed upside down.

2. Debugging the alarm circuit: Use a wire to short-circuit the 8th and 9th pins of IC1 with the ground, and then turn on the power. The alarm circuit should not work at this time. If you short-circuit the C and E poles of VT3, you can hear the alarm sound from the speaker. If there is no sound, you should carefully check whether the music circuit is installed incorrectly and whether the speaker is wired correctly.

3. Debugging of the transmitter: After all components are installed, use a DC 5V power supply for debugging, and use a multimeter to measure the base voltage of VT7 or VT8 to ground. If negative voltage appears, it means that the circuit has started to oscillate and the transmitting circuit is working normally.

4. Debugging of infrared receiving circuit: Do not remove the short wires between pins 8 and 9 to the ground. Connect the transmitter power supply, align the infrared tubes, and keep a closer distance. Connect the power supply and measure the voltage across C7. Normally, when the infrared is not blocked, the voltage is greater than 0.6V; when the infrared is blocked, the voltage is less than 0.5V. If it meets this rule, it means that the infrared receiving circuit is working properly. Otherwise, you need to carefully check whether the components of this circuit are installed upside down, whether there is cold soldering or tinning during welding, etc.

5. Restore all circuits and then power on. If an alarm sounds, press the reset button to stop the alarm. Gradually increase the distance between the infrared emitting tubes and make sure they are aligned. For the convenience of debugging, the connecting wire provided here is 50cm. Then block the infrared rays. If the alarm sounds, move the object away and the alarm still sounds. Press the reset button to clear the alarm. If the function is normal, the entire debugging work is completed.