# Minimum voltage required to drive the input of ULN2803A

#### 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
12,324
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.

#### Claudio Ikeda

Joined Aug 19, 2017
5
Thanks, Albert. Do you recommend any IC that meets the requirements? I need to input a voltage of 1,8 volts?

#### AlbertHall

Joined Jun 4, 2014
12,324
How many inputs/outputs do you need?

#### Claudio Ikeda

Joined Aug 19, 2017
5
ABout 16 inputs + 16 outputs. I was wondering if I could use the ULN2803...thank you.

#### AlbertHall

Joined Jun 4, 2014
12,324
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?

#### 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?

#### AlbertHall

Joined Jun 4, 2014
12,324
No, the ULN2803 is ruled out for a 1.8V input.

#### Claudio Ikeda

Joined Aug 19, 2017
5
Is there any other option to work with voltages under 1,8 volts in its input?

#### AlbertHall

Joined Jun 4, 2014
12,324
Nothing that I know of.