1PCS MQ-138 MQ138 Formaldehyde HCHO Volatile Organic Compounds (VOC) Gas Sensor Module with Alarm Signal Light

As described, there are 9 unique sensors in this kit. You can tell which is which by looking at the metal casing of the sensor where they silk screen the "MQ- " part number.

Worked great for a DIY breathalyzer on a MSP430, was able to get relatively accurate results compared to a certified police breathalyzer. Follow the burn in procedure, it's in the data sheet.

bought this for a science fair project. It worked great at sensing ammonia but did not exhibit a response to hydrogen sulfide. It was easy to integrate with an Arduino Uno board and I used it to trigger an alarm circuit. Good buy even though the target analyte was mis-advertised.

rellitivly inaccurate if your looking to get a decent ppm reading. This is more useful simply to detect the presence of NH3 rather than get exact levels. The other drawback is that it has a very long warm up time ...several hours. That being said if you just want to build an NH3 detector this will work ok. Be aware a good NH3 measurement sensor will run upwards of $400

Sensors were contaminated or overly old. Missing normal components or used poor components. Would never read <50% threshold when on 0% sample. Unable to read any alchohol concentration due to overly high sensitivity. On sample with 0.1% abv the sensor pegged >4volts and was flatlined.