A low-cost combustible gas leak detector design (1)

Liquefied petroleum gas, gas and other gas as a fuel leak occurred and often result in poisoning and fire and other accidents.

Have some combustible gas leak alarm great variety, mainly used for industrial factories and mines, bulky, expensive and difficult to spread to the household. Although there are some simple alarm, but also the structure of complex, high price, promotion to household comparison free hard. This study used a simple digital and analog IC combining technology solves the problem of high cost equipment. Developed compact combustible gas leak alarm system, its simple structure, low cost (only main circuit component costs 3 Yuan, including shell and the power of all materials only shijiyuan) and alert sensitivity in sensor performance range of arbitrary adjustable. The trial results showed that the appliance is easy to use, very well, and simply improved to increase the control function, can facilitate the realization of automatic control device for opening fan, etc. The combustible gas leak alarm not only suitable for families, also suitable for industrial applications, and its sensitivity, alarm Reminder alarm functionality, reliability, and superior to the current product.

Comparison 1 system design optimization

Currently, combustible gas leak detector for technology divided into two categories, one category is single-chip at the core of intelligent alarm, including signal conditioning, A/D sampling and input and output circuits etc., relatively complex.

Another class of complete hardware implementation, the circuit is very complex. Both these techniques are more complex circuit resolve gas sensor of the initial alarm. While the initial report is due at the beginning of power on the sensor, the conductivity value has not yet been reached steady-state, leading to a false alarm, about 3 min, sensors up to steady-state values of electrical conductivity, alarm. In order to solve the problem of the initial alarm, you can use the following two options: 1) to adopt the second alarm program, which is a sensor's initial conductivity equivalent gas concentration as a first stage alarm, this phase concentration only alarm lights, Horn. The second stage alarm is when a combustible gas leak concentration reaches a critical risk concentration is then open sound alarm. Such programmes need a more complex two-stage comparison alarm circuit, remind the force is not strong and reliable, the sensitivity is adjustable alarm; 2) using comparison or feedback delay alarm options, but the circuit is very complex and costly. Therefore, the use of hardware design cost reduction is key. Here the use of digital and analog IC combining design, resolve the alert and the second alarm, and achieved very good results.

2 device selection and working principle

Gas sensors, performance varies.

Here choose MQ-KC-type sensor, it is a new type of resistance-type gas-type components, can be used for natural gas, town gas, LPG, leak detection and alarm. Has a high sensitivity, long-term stability, long life, low prices, low power consumption, you can easily use the battery, etc. MQ-KC-type sensor principle: the sensor connected to the specified load, in the initial stages of power, the sensor's electrical conductivity renders a higher value, about 3 min or so to reach steady state value. To place it in a certain concentration of combustible gas, its electrical conductivity will rise, in a certain range, the higher the concentration of combustible gas, higher conductivity sensor, if the sensor and the load-line, load that is caused by voltage changes, voltage reading this change, by comparison, magnification for alarm and control functions.

3 hardware circuit design

3.1 Basic alarm circuit

MQ-KC-type sensor rated supply voltage of 9 V to requested connection a load resistance.

Signal check out and compare the circuit design with discrete components, component count, high costs, and the result is not good. To do this, use a single 9 V power supply, a certain drive capability, integrated dual operational amplifier. Figure 1 as the Basic alarm circuits.

Figure l, R1 is the sensor required load resistance, resistance to 120 Ω, Vcc-9 V power supply voltage; A, b is A dual operational amplifier, LM358 to follow, the role of buffer, starting from quarantined to R1 the voltage VR1 basically all applied to the comparator B phase input.

Sensitivity adjustment for alarm RW potentiometers. Steady-state, adjust the RW makes to the comparator RP input voltage V-slightly higher than the steady-state when the voltage on the R1 VR1 this voltage is higher, the lower the alert sensitivity. Power and sensor reached steady-state, MQ-KC for more stable fixed resistance, when Vcc unchanged, VR1 Basic is a fixed value remains unchanged. When a combustible gas leak, the sensor access to gas, its electrical conductivity increased resistance drop, make rose, when the VR1 VR1 than V-, comparator output a voltage greater than 7 V, making the buzzer emits drip, drip of HA of alarm sound. If the voltage control a relay, can control function. In order to improve the antijam capability, respectively in R1 and b V-Terminal capacitor parallel a filter.

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