The HW-685 4-20mA Current Sensing Module finds its place in a situation you want to measure small currents from 4 to 20mA. It is fairly cheap, about 1-2$.
I think the trend began when Arduinos became popular. With an Arduino platform you could plug a so called shield on top of the Arduino. With the shields you can add additional functionality like port expander, connection modules like WiFi or RS232.
Last modified January 1, 0001