How to use ULN2003apg

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Joined:Tue Jul 26, 2016 8:17 am
How to use ULN2003apg

Post by Bartowski » Fri Jul 06, 2018 11:34 am

I want to control a 30x1 array of pins through a raspberry pi B+. On a breadboard, I set up a series of shift registers (74HC595) to an array of 2V leds, which I can control as intended. After the leds, I'd like to increase the current from 4mA to 25mA. I installed a series of darlington transistor arrays (ULN2003APG) to do this. I had read that such transistor arrays require 1.4V at the base in order to activate. Since I have connected to the Pi's 5V output, I used 460 ohm resistors (the closest to 400 ohms I had). However, the test leds that I'd placed downstream of the transistor arrays won't turn on. Thinking that I had overestimated the base voltage needed, I cut down on the resistance - eventually scrapping both the leds and resistors altogether. I still received no output from the test leds. What's wrong here? Any help would be much appreciated!

On the left are the five transistor arrays. They are oriented downwards (with the indentation at the bottom). As situated, the common is top left, the ground is top right. The common is connected to the Pi's 5V through a 100 ohm resistor (I will need 2.5V and 25mA post-transistors). The ground connects back to the Pi's negative terminal on it's 5V port.

On the left are shift registers, which are wired to the Pi. They are working appropriately. Not shown, but previously configured, was a wire leading from the transistor's output through a 125 ohm resistor to the test led bulb.

Joined:Fri Sep 20, 2013 7:25 am

Re: How to use ULN2003apg

Post by Brissieboy » Fri Jul 06, 2018 11:50 pm

The ULN2003A has an integral base resistor so you should connect direct to MCU output.
They require around 2V to turn on so they should be OK driven from the 5V output. Data sheet.
Not sure why you have the common pin connected - it should be left open when driving non-inductive loads.
Your post sounds like you intended to add photos but none is present so I can't see how you have connected the setup.

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