Short circuit, disconnection, vibration alarm

Short circuit, disconnection, vibration alarm

The alarm is divided into two parts: vibration monitoring and alarm host. The detection part is composed of piezoelectric ceramic Y1 and parallel resistor R3, and is connected to the host in the room through a shielded cable. The host consists of a power supply circuit (Figure 1), a differential amplifier and rectification detection circuit (Figure 2), a delay and alarm circuit (Figure 3), etc.

In Figure 1, the power supply uses a typical integrated voltage regulator, which is a transformer that steps down the voltage, a diode that rectifies, a capacitor that filters, and a voltage regulator integrated circuit that stabilizes the voltage. LM7812 provides a 12V stable voltage to the circuit, and a green LED is used as a power indicator.

In Figure 2, R5, R6, R3, R4, R7, and R8 are equal to each other, forming a bias circuit, and together with transistors Q1 and Q2 with the same parameters, they form a single-stage dual-output first-stage differential amplifier. R13, R14, R15, and R16. R10, w2, and Q3 and Q4 form a second-stage differential amplifier circuit with a dual-input and dual-output symmetrical structure. The rectifier bridge D3 detects the output of the two-stage differential amplifier. And drives the photocoupler U1. The connection of C5 and C6 is to further improve the amplification effect of the two-stage differential amplifier on the audio vibration signal, so as to improve the vibration detection sensitivity.

In Figure 3, R2, D5, C29, Q5, J1, etc. form a delay circuit. This delay circuit can delay the opening of the subsequent alarm circuit for tens of seconds. It is used to prevent false alarms caused by circuit instability at the beginning of startup. The red light-emitting tubes D1 and D2 serve as the power supply of the music alarm integrated block and also as an alarm indicator. The one-way thyristor is the final execution switch of the alarm circuit, which is controlled to open by the voltage across C7 and R20.

In the preset state, the alarm is turned on with a delay. If the alarm is not triggered, that is, there is no vibration, disconnection, short circuit, etc., the output of the two-stage differential amplifier of the symmetrical structure is zero. The rectifier bridge D3 has no rectifier output, and the photocoupler U1 is in a high-resistance state due to no working current. The thyristor Q6 is cut off. The alarm circuit does not operate.

If someone tries to pry open the lock of the storage room and causes vibration, the vibration signal is converted into an audio signal by the piezoelectric sheet close to the door, and transmitted to the first differential amplifier composed of Q1 and Q2 through the cable between AB. After the first stage of amplification, it is sent to the second stage of differential amplifier composed of Q3 and Q4 through R15 and R16. After the second stage of amplification, the AC signal caused by the vibration is converted into a DC signal by the rectifier bridge D3 and the capacitor C1, the photocoupler U1 is turned on, the thyristor is turned on, and the alarm sounds.

Similarly, if someone tries to cut or short-circuit the cable between AB, the bias of the first differential amplifier is asymmetric, and the output of the collectors of Q1 and Q2 causes the output of the second differential amplifier circuit Q3 and Q4 to be asymmetric, and the subsequent circuits are activated, triggering the alarm.

Selection and debugging: In order to ensure the stability and reliability of the alarm circuit, enhance the adaptability of the circuit to different environments and temperatures, and reduce false alarms, when selecting the components, the resistors should be selected as five-ring metal film resistors with good stability and accurate values. Q1, Q2, Q3, and Q4 should be selected as high-quality authentic transistors. If conditions permit, a professional transistor characteristic tester must be used to select transistors with consistent characteristics for pairing. At least a digital multimeter with a magnification measurement function should be used to select two groups of transistors of the same model with the same DC magnification. Y1 and R3 in the storage room must be reliably shielded with shielded wires from the host between the rooms. Since this circuit works in the audio range as a whole, there are no special requirements for the printed circuit board during production. I use a universal board to connect it. As long as you pay attention to power decoupling, you can succeed. When debugging, first disconnect Y1, R15, and R16, carefully adjust the potentiometer W1, and measure it with a digital multimeter to make the collector potentials of Ql and Q2 equal. Then solder R15 and R16 and adjust w2 to make the collector potentials of Q3 and Q4 equal. Finally, the test was conducted. The piezoelectric piece should give a normal alarm if the cable between AB is short-circuited, disconnected or vibrated. A universal chassis was purchased from the electronic department of the market. After punching holes, the assembly was successful.