Electronic – arduino – Why is designed active low

arduinobest practicedesignschematics

I buy a 2 channel relay module for Arduino.
I surprised that why this relay module is designed active low?
enter image description here

i means that when we connect In1 to low V (GND) relay turn on. Is there any reason for using this way instead of turn on relay with high V?

sorry for bad English.

Best Answer

One reason is historic.

Early bipolar logic families such as TTL, DTL or RTL used only NPN bipolar transistors, they could easily drive an output signal to be pulled to within a few millivolts of ground but not easily drive a high signal. So many integrated circuits tended to use an active low signal output. That also allowed WIRED-OR functionality where a virtual gate could be created from open collector signals.

The use of active low-signals predates integrated circuit logic and is often used in cars where a switch may just ground a signal to assert it - for example the brake switch or dome light switch often just connects the signal to ground.

In systems where ground is available everywhere (the chassis of a car for example) it saves a wire as you just need one wire to the load rather than bringing power to the switch and also taking the signal away to the load.