7-Segment SN74HC595N and BC327 Transistor

Other Hardware LEDs and Multiplexing
1

Hello,

I am trying to run a large 7 segment display.

My setup is currently a SN74HC595N connected to BC327 transistors. And should supply 8V to the individual segments. I have a display with a common cathode.

Somehow I have planned something wrong and don't understand what my problem is. Below is my diagram, to keep it simple I have only drawn one LED.

Thanks for your help in advance

Unbenanntes Diagramm.drawio
Unbenanntes Diagramm.drawio1243×1431 91.7 KB

2

There are many things wrong with your approach.

  1. The shift register pin outputs only 3.3V. It will not be able to turn off the pnp transistor. You need close to 8V on the gate to do that. So the pnp will be on at all times, or would be, but...
  2. No common ground between the 8V supply and the rest of the circuit, so the pnp will never switch on.
  3. No current limiting for the led segment. If the forward voltage of the segment is 8V, you need a supply of ~10V and a current limiting resistor
3

So I would need to connect the GND of my ESP with the GND of my 8V? What other solution is there so that I can control my 7-Segment display with a circuit like this?

4

You mean you don't want to connect the 8V ground to the Arduino ground for some reason? If that truly cannot be done, you could use opto-isolators.

5

No I can connect them. I thought from your first comment that my transistor would never do what I want

6

Ah, ok.

By itself, it won't. You could use an additional npn or MOSFET transistor, like this diagram:
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7

The gpio pin in this diagram would be my SN74HC595N output ?

8

Wait... There is an error in that diagram I posted. A current limiting resistor is needed between the base of the pnp and the collector of the npn.

9

Yes.

10

Then thank you for your help, I will implement this later

11

Ok, good luck. Return to this topic if you get problems, don't start a new topic.

With large 7-seg displays where the segments have high forward voltages, it's much easier if you can get common anode rather than common cathode.

12

This will make life a lot simpler

13

Good suggestion.

I may have dreamt this, but is there something like that which also contains a shift register? A little like TPIC6C595 but for switching high-side?

14

I don't think so.

15

Why would common anode be better?

16

Because switching the segments on/off on the "low side" (between the segment's cathodes and ground) is easier. For example that TPIC6B595 chip I mentioned can switch high voltages and currents such as your segments, and it is also a shift register just like 74HC595. So it would replace the 74HC595 and all the necessary transistors, making your circuit much simpler.

17

This?

18

Maybe. It probably has quite a large voltage drop, though. Uses BJT rather than MOSFETs for the output stages.

19

Assuming you are not multiplexing, there may be another option. Instead of connecting the two power supply grounds together, you could connect the two V+'s together. Then the 3.3V outputs of the 595 could switch the PNPs directly. The 8V supply terminals would in effect become +3.3V and -4.7V. The -4.7V would be connected ONLY to the common cathodes, not to anything else in the circuit.

But this depends on how you have the ESP32's power supply set up. Are you deriving it from the 8V supply, or somewhere else? Are you powering directly at the 3.3V pin, or supplying a higher voltage at Vin? That's not shown in your schematic. Can you post a power diagram?

How many digits will you have?

20

Currently I am using a USB port of my computer so 5v. I want to control 4 digit with this setup to create a countdown. But later I will use a step down converter to supply the ESP from the 8Vs