In brief: Electronics for a gas sensor for Hydrogen gas (H2). I designed it for my Anodizing Lab.
It uses the Figaro TGS 822 sensor which is very sensitive for a.o. hydrogen gas.
Considerations / specifications.
I wanted to have a device which gives me an analog indication about the concentration of H2 in the area of my anodizing setup, and an audible alarm when the concentration becomes near critical = risc of explosion. (dutch: knalgas, eng: boomgas?)
I found the Figaro TGS 822 (Conrad 183466) a suitable device as it is extremely sensitive for a range of combustible gasses. Yes, alcohol too, so do not drink in your lab.
Datasheets (pdf): TGS 822 short form TGS822 use
The whole unit, including DC power-plug supply. Note the VU meter, the LED indicating that power is on, the Gas-Sensor amd the buzzer.
The circuit board.
Internal view.
Schematic diagram. Some explanations below.
I used a 7805-TO220 and a 9 Volt power-plug-unit because I was not sure about the accuracy of 5 Volt power-plug units.
The 7805 needs a small heat sink. The bare TO220 case became quite hot, the Figaro draws some 130 mA for the heater.
You may adjust R1 for the brightness of the LED you use. Mine was a pretty old low efficiency type.
R2 is the most important resistor, because it determines the sensitivity of the whole thing.
I used a left-over VU-meter from some audio application I never realized. Adjust R3 for about half-scale of the meter at 2 Volts across R2.
You can replace the microprocessor and its asociated circuits by any circuit which generates an audible alarm when the voltage across R2 exceeds 2.5 Volts, or you can leave it away when you do not need an audible alarm.
In my case the micro required 3.3 Volts, derived from the +5Volt by the two diodes D1 and D2. This micro does not need any decoupling capacitors.
The inputs PTA0 and PTA1 are analog inputs, processed by the internal A/D convereter. When the voltage at PTA0 exceeds the voltage at PTA1 the audible alarm goes beep - beep - beep - etc. The code for this program is written here.