How to switch a darlington array from a shift register?

General
1

So I’m building a HUD speedo for my car and I’m driving the high side of an LED 7seg display with a darlington transistor array (UDN2801). Can I just connect the output pins from a shift register straight to the inputs of the transistor array, or do I need diodes or resistors?

(Note: I could multiplex directly but I need maximum brightness to compete with sunshine)

2

You should always use a resistor when connecting a LED’s.

The easiest way for a 7seg display would be to connect it on the common pin. Though this will cause the LEDs to be slightly dimmer for each LED that is turned on.
The best way for maximum brightness across all LED’s would be to connect it to the between the Darlington array and the LED it’s driving.

This post should help you understand more and calculate a resistor to suit your application.

3

I’ve got currently (but haven’t powered on yet) shift reg -> darlington -> resistor -> LED -> common ground.

I was just wondering if I needed any passive components between the shift register and the darlington. Any idea?

4

Apologies, misread the question.
Looking at the data sheet it’s slightly confusing.
I would think that this would be ok, but the UDN2801 is the only chip without a internal resistor on the gate.

Edit:
The data sheet states “the ULN2801A is designed for general purpose applications with a current limit resistor”

So yes insert another resistor between shift register and Darlington array, or get one of the other ones with TTL logic with built in resistor. Though I don’t think maybe helpful to you as you most likely have all the bits already.

5

Thanks for that. Looks like I’ve got a bit more soldering ahead of me.

6

one sec, “with a current limit resistor” if my brain worked late at night, this could be interpreted as that no resistor is needed. This would made more sense. Look over the datasheet again and make sure I haven’t assumed anything…

7

Alternatively, do we need to specify an area ala ‘this is the room with too
much power’ for people to be aware that dangerous things may happen within?
I don’t think that sits well with our public liability insurance.