I'm a newbie of arduino and I'm playing with air quality sensors.
I want to purchase this MQ-135 sensorhttp://www.ebay.com/itm/170969528013
There are 4 pins (S,Vcc,TTL, G)
There is a LM393 chip on pbc
Description says that there are 2 outputs:
analog output, and TTL-level output
I do not fully understand what is the usage of TTL-level output:
TTL output valid signal is low (low output signal light, which can be accessed microcontroller IO port)
what will be read from an arduino digital pin connected to TTL?
let me know where I can found a sketch that explains usage of TTL pin