Designing touch anti-theft alarm circuit with 555 integrated chip

NE555 (Timer IC) was released by Signetics Corporation in 1971. It was the only very fast and commercial Timer IC at the time. It was very widely used in the next 30 years and extended many application circuits, despite recent CMOS technology versions. The Timer IC such as MOTOROLA's MC1455 has been widely used, but the original specification of the NE555 is still available on the market. Although the new IC has some improvements in its function, its foot strength has not changed, so Currently available for direct substitution.
NE555 is one of the 555 series timing ICs. The pin function and application of the 555 series IC are compatible, but the different models have different stability, power saving and oscillation frequency. It is not the same; the 555 is a very versatile and fairly versatile timing IC that requires only a few resistors and capacitors to generate pulse signals of various frequencies required by digital circuits. The features of the NE555 are: 1 A simple resistor and capacitor can be used to complete a specific oscillation delay. The delay range is very wide, ranging from a few microseconds to a few hours. 2. It has a large operating power range and can be used with TTL. Logic gates such as CMOS, that is, its output level and input trigger level, can be combined with the high and low states of these logic series. 3. The output current of the output terminal is large, which can directly promote various automatic control. Load 4.4. It has high timing accuracy, good temperature stability and low price.
The touch alarm can be widely used for anti-theft alarm, dangerous object touch alarm, etc. The alarm device can automatically alarm when triggered, and can automatically stop after a predetermined time.
The circuit is shown in the figure. IC1 is the time base integrated circuit NE555, which forms a monostable trigger with R1, C1, C2, C3. When no one touches the metal piece M1~Mn, the circuit is in steady state, that is, IC1 3 feet output low level, the alarm circuit does not work. Once someone touches any one of the metal pieces M1~Mn, a negative pulse (actually a clutter pulse) is input to the 2 pin of IC1 due to the human body induced potential, the one-shot circuit Triggered by flipping into the transient steady state, IC1's 3 pin jumps from the original low level to high level. The high level signal turns on the transistor V1 through the current limiting resistor R2, so V2 is also saturated and conducting, and the sound is integrated. The circuit IC2 is powered on. The audio signal output by IC2 is amplified by the complementary amplifier formed by the transistors V3 and V4, and then the electric speaker BL is driven to emit a loud alarm sound. Since the one-shot circuit is triggered to flip, the power supply starts to pass the RI. Charging C2, after about 1.1R1C2 time, the one-shot circuit automatically returns to the steady state, the 3-pin output becomes low level, the alarm stops alarming, and it is in the forecast state. M1~Mn can be used steel or aluminum sheet, middle drill a small hole that receives any metal parts that require protection. For example, a door lock , the metal casing of the instrument, etc. The time base integrated circuit ICI selects NE555, 5G1555, uA555, LM555, FX555, etc. V1, V2, V3 can use 9013, 9014, 3DG6, 3DG8, or 3DG12, etc., B>=100 .V4 uses PNP type triode, such as 3AX81, 3AX31, 9012, etc., B>=80. IC2 uses KD9561, which is a four-tone analog integrated circuit. IC2's peripheral components only have one oscillating resistor R3, and the value can be in the range of 180~510KO. Inside, the smaller the resistance value of R3, the faster the alarm rhythm, and vice versa. BL uses 0.8W, 8O electric speakers, and can also be used with piezoelectric ceramics. It must be equipped with a sound-assisting cavity. However, its volume is still small. More. The device can be powered by a 4.5~6V battery, or it can be supplied by a rectifier after step-down rectification.
If the circuit is changed to a positive pole connected to the power supply by a resistor on the 2 pin of IC1, a mercury switch is connected to the negative pole of the power supply, and the mercury switch acts as a trigger device, so that the circuit can be used as any vibration alarm, tilt alarm, Earthquake alarm, temperature rise alarm, etc.