As you all probably know, I am not an electronics engineer or had any training. I just try and understand why the things I do work or don't work.

In an effort to take a signal and convert it into on and off states, I used a darlington array. I found that I could take a 12V signal feed into one channel on a darlington and turn on an led. I also wanted to activate a relay and have another led that was illuminated when the 12V signal was off. By applying a 1k resistor to the output of one channel and feeding that output to the input of another channel, the output of the other channel was inverted. It works brilliantly for switching a number of circuits from just one input.

My question is why does it work. I have attached a quick schematic. if 12V is applied to Pin 1 then LED 1 is illuminated and LED 2 not. When 0V is applied to Pin 1, then LED 2 is illuminated and LED 1 not.

I am just interested to know why is works.

 

Its a Darlington output chip, so if you put a high on the input, the output goes low, and vice-versa, the second gate just inverts the first gate. QED

 

Note R3 is acting as a pull-up for IN2, but IN1 is floating when the switch is opened. This Darlington driver might not care because it uses BJTs that need current to operate, but most CMOS logic would act erratically because it would not know what to do when the switch is open, and pickup static charges.

 

Its a Darlington output chip, so if you put a high on the input, the output goes low, and vice-versa, the second gate just inverts the first gate. QED

Thanks Dave, good to know I am not doing something that is bad practice.

 

Its a Darlington output chip, so if you put a high on the input, the output goes low, and vice-versa, the second gate just inverts the first gate. QED

UDN2981 is a buffer, not an inverter:

 

good to know I am not doing something that is bad practice

Schematic isn't well drawn. Most would use the gates arranged to convey function vs using the IC footprint. If that's what your schematic editor does, you're stuck with it.

 

I am just interested to know why is works.

If I didn't make any mistakes transcribing your drawing, it doesn't do anything useful:

I substituted CD4050 for the buffer because I didn't want to bother creating one.

The buffers can't sink current so the LEDs can never turn on.

 

Schematic isn't well drawn. Most would use the gates arranged to convey function vs using the IC footprint. If that's what your schematic editor does, you're stuck with it.

As I said it was a quick drawing to show my point and not a full schematic.