Minimum voltage required to drive the input of ULN2803A

Thread Starter

Claudio Ikeda

Joined Aug 19, 2017
5
Hi guys
I am wondering if 2,4 volts is the minimum required to switch on the output of this IC. Because I am trying to check the voltage output of a project with 1,8 volts. Trying to connect to the Arduino is not a good idea, because it requires a minimum of 3 volts to detect a HIGH state in its digital input. So, if the ULN2803 is not a good idea to monitoring 1,8 volts, which IC do you recommend that meets the need of a low voltage input? Thanks a lot!
 

AlbertHall

Joined Jun 4, 2014
11,524
If your input goes from 0V to 1.8V and you want to drive an Arduino pin then a simple transistor will do the job.
The circuit below, with +V connected to the 5V from the Arduino, R1 = 10k and R2 = 100k, and almost any small NPN transistor would do it.
 

AlbertHall

Joined Jun 4, 2014
11,524
I think you would be pushing your luck with the '2803 because it uses darlington transistors so you have two base emitter voltage drops to overcome.
There are some transistor arrays available - 4 transistors in one IC package.
Another possibility is to use a low gate threshold MOSFET then you won't need a resistor on the gate and if you turn on the pullups on the Arduino you won't need any resistors at all.
How fast does it need to switch?
 

Thread Starter

Claudio Ikeda

Joined Aug 19, 2017
5
Not too fast at all. Actually it will check for led state at about 500 mS. So, do you believe ULN2803 can sense a high state with 1,8 volts in its input?
 
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