Hazardous Gas Alarm

Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling

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About 1 years have passed since the first publication of the description of such a device in the Radio magazine [10]. During this time, several similar devices have been described: from the simplest to those assembled on a microcontroller [2]. All of them, having both certain advantages and disadvantages, are based on a typical gas sensor connection scheme [3] recommended by the manufacturer.

Over the past years, both new, more advanced and sensitive to the presence of various gaseous substances, sensors have appeared, as well as other elements that are very convenient for building signaling devices - sound signaling devices with a built-in generator, electromagnetic relays in microcircuit cases.

The proposed device for detecting the leakage of combustible and explosive gases is built on the basis of a well-proven scheme from [1], taking into account the recommendations of [3], and on a modern element base. The signaling device can be used to detect gas leaks in everyday life, in cars with gas-balloon equipment and on gas pipelines.

The differences of the proposed device from the previously published ones are in the improved temperature compensation of the sensor and the possibility of connecting an external actuator. If suitable sensors are available (and their range is quite wide today), the device can be made sensitive, for example, to alcohol vapors and other gases, the presence of which in the air is undesirable or dangerous.

Technical specifications

Sensor type .................TGS813
Types of gases detected .........Methane, propane, petroleum gases
Sensitivity in air, ppt .......................... 500
Consumed current, mA, no more ....................... 200
Operating temperature range, °С ................-10...+40

Fig. 1

The diagram of the signaling device is shown in fig. 1. A voltage of 2 V is applied to pins 5 and 1 of gas sensor B5 to heat its sensing element to operating temperature. In the presence of gas, the conductivity of the sensing element (it is connected between terminals 1, 3 and terminals 4, 6) increases. In proportion to the concentration of gas in the air, the voltage on the load resistance of the sensor - resistor R1 increases. It goes to the non-inverting input (pin 3) of the DA1 voltage comparator, which compares it with the reference voltage applied to the inverting input (pin 4). The exemplary voltage forms a divider from the thermistor RK1 and resistors R2, R3. It is equal to approximately half of the sensor supply voltage. Thermistor RK1 makes the reference voltage dependent on temperature, thereby compensating for the temperature dependence of the sensor sensitivity.

While there is no gas, the voltage at the inverting input of the comparator is greater than at the non-inverting one, as a result of which its output 9 is set to a low logic level. When a certain concentration of gas in the air appears and reaches a certain concentration, the voltage at the non-inverting input exceeds the exemplary one and the voltage level at the output of the comparator, as well as at inputs 3 and 12 of the elements of the DD1 microcircuit, becomes high.

The second inputs of these elements are connected to the R5C4VD1 circuit, which provides a delay in the start of the signaling device by about 2 minutes. This time is necessary for the sensitive element of the sensor to warm up and come into working condition. After the capacitor C4 is charged, this circuit does not affect the operation of the device. When the supply voltage is turned off, the capacitor C4 will quickly discharge through the diode VD1, and the next time it is turned on, the delay will be repeated.

If a sufficiently long time has passed since the signaling device was turned on, the presence of a high level at all inputs of the logic elements of the DD1 microcircuit will lead to the opening of the output field effect transistors built into these elements. As a result, the HL2 LED will be turned on, the signal supplied by the HA1 emitter (with a built-in generator) will sound, and the K1 relay will work. Closed contacts K1.1 of this relay, if necessary, can actuate an external actuator, such as a siren.

The power supply unit of the signaling device consists of a socket X1 and an integral stabilizer for 5 V DA2 with capacitors C1-C3, C5. LED HL1 indicates the presence of power. Socket X1 was supplied with voltage from a network adapter for a Panasonic cordless telephone (8 V, 500 mA).

Fig. 2

The signaling device was assembled on the one shown in Fig. 2 single-sided printed circuit board made of foil fiberglass 1,5 mm thick. The board is placed in a ready-made KR-4 case 90x60x32 mm in size, in the cover of which holes 5 mm in diameter are drilled opposite the LEDs. A large number of holes with a diameter of 1 mm were drilled near sensor B1,5 for free access of air to it in the wall of the cover.

Thermistor RK1 - MMT-1 with a nominal resistance of 2,2 ... 3,3 kOhm. You can use the same 4,7 kΩ thermistor by connecting a conventional 11 kΩ resistor in parallel with it. Capacitor C4 - necessarily K53-14. AL307 series LEDs can be replaced by any other red (HL1) and green (HL2) glow colors. Relay K1 - TRR-1A-05D-00 By changing the configuration of the printed circuit board conductors, it can be replaced by domestic RES55A (version RS4.569.600-03) or RES64 (version RS4.569.724).

For ease of change, the sensor can be installed in the PLC-7 lamp panel for printed wiring. When setting up the signaling device, resistor R2 must be selected so that at a temperature of +20 ° C, the voltage at terminal 4 of the DA1 comparator is 2,5 V.

Instead of the TGS 813 sensor, you can use the MQ-6, HS133 and TGS2610 without changing the circuit (the latter has pins 1 and 4 - a heating element, pins 3 (+) and 2 (-) - a sensitive element). If the signaling device will be used as an alcohol tester, a TGS2620 sensor should be installed in it (the pinout is the same as that of the TGS2610), and the fixed resistor R1 should be replaced with a variable one in order to be able to adjust the response threshold.

All these sensors are designed for a supply voltage of 5 V. There are others that need to be powered by a large voltage. To use such a sensor in the described signaling device, it is necessary to replace the voltage stabilizer KR142EN5A with another one that has the desired output voltage. The buzzer and relay must also be replaced. The remaining nodes remain operational when powered by a voltage of 5 ... 18 V.

Literature:

1. Vinogradov Yu. Control of explosive gases. - Radio, 2000, No. 10, p. 37
2. Surov V. Air gas contamination alarm. - Radio, 2009, No. 9, p. 37
3. Gas sensors from FIGARO. - chipdip.ru/library/DOC000052164. pdf

It is permissible to use the described signaling device as the only or main means of gas contamination control only after it has been certified by an authorized body and subject to periodic verification.

Publication: radioradar.net

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