Household gas alarm circuit

Household Gas Alarms More and more households use liquefied petroleum gas and coal gas as fuel. However, these gases are harmful and explosive, and there are many hidden dangers and accidents. If gas leakage cannot be detected and dealt with in time, it will bring catastrophic harm to the family and neighbors. . The gas alarm introduced in this article is suitable for leak alarms of natural gas, coal gas, liquefied petroleum gas, etc. 　　

Working Principle  　　The working principle of the device circuit is shown in Figure 1. It is composed of power supply circuit, sensor circuit, voltage controlled oscillator circuit and alarm circuit. The 220V mains power is stepped down to about 5.5V through the power transformer T1, and is directly used as the heating voltage of the gas-sensitive semiconductor sensor QM-N10 without rectification or filtering. The power supply of the control circuit is supplied by U full-bridge rectification and C1 filtering. The resistance value of the QM-N10 gas-sensitive semiconductor sensor in clean air is about tens of kΩ. When exposed to harmful gases, the conductivity increases and the resistance value drops sharply. The decrease is proportional to the gas concentration below 0.5%. NAND gates IC1A and IC1B form a gate control circuit, and IC1C and IC1D form a multivibrator. When the QM-N10 gas sensor is not sensitive to harmful gases, due to the extremely small conductivity, the ② pin of IC1A is at a low potential, and the ① pin of IC1A is at a high potential, so the ③ pin of IC1A is at a high potential, and after being inverted by IC1B, its ④ The pin is at low potential, the multivibrator cannot vibrate, and the transistor VT2 is in a cut-off state, so the alarm circuit does not sound. Once QM-N10 is sensitive to harmful gases, its conductivity increases and its resistance drops sharply. The voltage drop on resistors R2 and R3 causes pin ② of IC1A to be at a high potential. At this time, pin ③ of IC1A becomes low level. , becomes high level after being inverted by IC1B, the multivibrator starts to oscillate, and the transistor VT2 is turned on and off periodically, so the positive feedback oscillator composed of VT1, T2, C4, HTD, etc. works intermittently and issues an alarm. Voice. At the same time, the light-emitting diode LED1 flashes. This achieves the purpose of warning of harmful gas leakage. The component selection and production component list is shown in the table below. No. Name Model Quantity R1, R2 resistance 510Ω (needs adjustment) 2 R3 resistance 1K (needs adjustment) 1 R4 resistance 2M (adjustable delay time) 1 R5 resistance 3.6M 1 R6 resistance 470Ω 1 R7 resistance 2.2K 1 R8 resistance 10K 1 R9 Resistor 150K 1 R10 Resistor 270Ω 1 R11 Resistor 330Ω 1 C1, C2 Electrolytic capacitor 470u/16V 2 C3 Polyester capacitor 0.1u 1 C4 Polyester capacitor 0.047u 1 U Rectifier full bridge 1A/50V 1 VD switching diode IN4148 1 DW Zener diode 5.1V 1 LED1, LED2 1 red and 1 green light-emitting diode 2 VT1, VT2 transistor 9013 2 IC1 NAND gate CD4011 1 QM gas sensor QM-N10 (see the text after the table for performance) 1 HTD piezoelectric buzzer plus Acoustic cavity 1 T1 Power transformer 5.5V 1 T2 Pulse transformer homemade 1 QM-N10 is a high-sensitivity, high-stability harmful gas sensor. Its main technical parameters are as follows: Response time: ≤10s Recovery time: ≤60s Heating voltage: 5V ± 0.5V Heating power: ≤0.5W (heating wire cold resistance is 50Ω ± 2Ω) Anti-interference ability: When the butane alarm concentration is set to 0.3%, there will be no false alarm under the following conditions: The temperature is -10-+50℃ ; Humidity is ≤95%RH. T1 uses a 3-5W, 5.5V power transformer; T2 uses an 8:1 semiconductor radio output transformer, which can be used upside down. HTD uses piezoelectric ceramic sheets with Φ=27mm and requires the installation of a resonant sound cavity. Since the QM-N10 gas-sensitive semiconductor sensor has a warm-up period of about 10 minutes when it is turned on, unstable factors are likely to occur during this time, leading to false alarms. Therefore, there is a power-on delay circuit in the circuit design, which consists of It consists of R4 and C2. Adjust R4 so that the delay time is about 10 minutes. The resistance values ​​of resistors R2 and R3 determine the alarm starting point of the circuit. Place the gas sensor in a butane gas sample with a concentration of 0.3%, and adjust the resistance of R3 so that the circuit is at the alarm critical point. Finally, you can conduct a field test and adjust R3 to the required alarm concentration point to alarm.