**3.5 Gas-detection devices**

The Wheatstone bridge previously discussed was calculated when the sensor's resistance had a value of Rs ¼ 570 Ω (in C3H8) and Rs ¼ 582 Ω (in CO). These resistance values were calculated using the Eq. 1: *<sup>ρ</sup>* <sup>¼</sup> *RA <sup>t</sup>* ! *<sup>R</sup>* <sup>¼</sup> *<sup>ρ</sup><sup>t</sup> <sup>A</sup>* and the experimental measurements were shown in **Figure 8a** and **c**. Now, according to **Figures 1, 3**, and **8**, it is possible to establish three operating parameters for our devices: an operating temperature of 200°C, an operating gas concentration of 5 ppm, and the initial sensor's resistance *Rs*, mentioned above. On the other hand, another three operating parameters can be established for the electronic circuits: a supply voltage of 120 V AC, an operating voltage *Vcc* ¼ �12*V*,and an exit signal (or alarm signal) VAlarm ≈11*:*3 V. **Figure 9** shows our devices: **Figure 9a** depicts the C3H8 detector device, and **Figure 9b** shows the CO detector device.

Already manufactured the gas sensor and the electronic device according to the description of the previous sections, to apply the gas detection device (C3H8, CO), the

**Figure 9.** *Low-cost toxic gas detectors: a) for C3H8, b) for CO.*

#### *Toxic Gas Detectors Based on a MnSb2O6 Oxide Chemical Sensor DOI: http://dx.doi.org/10.5772/intechopen.107398*

sensor must be installed in the atmosphere to be monitored whose temperature must be 200°C, while its terminals must be connected to a pair of terminals of the electronic card developed for the detection of toxic gas (C3H8 or CO). The electronic card is manufactured using a copper PCB with the size of 10 cm by 10 cm, its design was made with the program Proteus®, it operates at room temperature and its supply voltage is through a plug (see **Figure 5b**), which is connected to a 110 V alternating current voltage source. Finally, using a multimeter, the alarm signal voltage VAlarm produced by the electronic card is measured: If the alarm voltage is ≈11*:*3*V*, the device is detecting the presence of gas in the monitored atmosphere. But if *VAlarm* ≈ 0*V* then the device is not detecting the presence of gas.

**Figure 10a** and **b** depict the operating principle of the C3H8 and CO detectors. It is as follows: If the concentration of the test gas is equal or greater than 5 ppm, the sensor's resistance diminishes, thus unbalancing the Wheatstone bridge, and causing the devices' exit voltage to be equal to that of the alarm signal *VAlarm* ≈11*:*3*V* (alarm state: "On"). However, if the gas concentration is below 5 ppm, Eqs. (5) and (6) are not satisfied. Consequently, the alarm signal is not active (alarm state: "Off"), that is, the devices have an exit voltage of approximately zero V. It is worth mentioning that, for our devices, the operating threshold value is selectable with the variable resistance *Rc*. For the C3H8 detector, if *Rc* >570 Ω (see **Figure 1a** and **9a**), the device will detect concentrations lower than 5 ppm. On the other hand, for the CO detector, if *Rc* >582 Ω (see **Figure 1b** and **9b**), the device will detect concentrations lower than 5 ppm. If *Rc* < 570 Ω or *Rc* <582 Ω, respectively, the devices will be able to detect concentrations higher than 5 ppm.

### **4. Discussion**

According to our results, our C3H8 and CO-detector devices possess good features, which include low cost, high sensitivity, rapid response, good behavior, ability to select the operating concentration through a variable resistance, an operating temperature of 200°C, dimensions of 10 cm x 10 cm, a supply voltage of 120 V AC, an exit voltage of *VAlarm* ≈ 11*:*3*V*, ease of construction, ease of repair, and ease of use.

We previously proposed a CO2 gas detector based on an analog circuit and on the dynamic response to the impedance of the oxide CoSb2O6 [22]. Such a device detected a gas concentration of 100 ppm with an operating temperature of 250°C. On the other hand, we also studied theoretically the dynamic electrical response of the oxide ZnAl2O4 and proposed a propane-gas detector [23]. That device detected gas concentrations of 1000 ppm with an operating temperature of 250°C. For both detectors, the design of the analog electronic circuits possessed high complexity since the analysis of the signal was conducted on complex planes and based on the sensors' dynamic response. In this work, the MnSb2O6 oxide was applied also in the detection of C3H8 and CO. However, its characterization and signal adaptation were done using DC currents, thus simplifying its analysis and implementation. Therefore, in comparison, our new proposal facilitates the construction of the device, lowering the operating temperature (from 250 to 200° C), the test-gas concentration threshold (from 100 ppm to 5 ppm for C3H8 and from 1000 ppm to 5 ppm for CO), and the device dimensions (from 15 cm x 15 cm to 10 cm x 10 cm). The overall components were also optimized.

Devices for the detection of C3H8 and CO find practical applications as explosion and intoxication-prevention measures, respectively. Our future work will be aimed at designing and developing gas detectors based on digital technology and quasidistributed systems for the detection.

### **5. Conclusions**

The reproducibility of the MnSb2O6 oxide was excellent. It was electrically characterized through static direct-current (DC) tests, obtaining its resistance-gas concentration behavior. Based on these results, the electronic prototypes for two toxicgas-detection devices were designed: one of them for C3H8-detection, and the other one for CO-detection. Both prototypes have an operating temperature of 200°C and an operating concentration of 5 ppm. They can produce an alarm signal of approximately 11.3 V. Their supply and operating voltages are 120 V AC and 12 V, respectively. They possess fast response, ease of construction, ease of operation, very low cost, and ease of repair. The detectors can find rapid application in processes involving combustion like, for example, boilers, smelting furnaces, and exothermic generators.

### **Acknowledgements**

The authors thank Mexico's National Council of Science and Technology (CONACyT) and the University of Guadalajara for their support. We also thank María de la Luz Olvera Amador and Jaime Santoyo Salazar for their technical assistance. This research was carried out following the research-line "Nanostructured Semiconductor Oxides" of the academic group UDG-CA-895 "Nanostructured Semiconductors" of CUCEI, University of Guadalajara.
