First off, disclaimer; I'm a computer scientist in the progress of my first baby steps in electronics.

I'm trying to hookup a 7segment with a 4511, 4518 and an Arduino. The 45-series chips and the 7segment are receiving 12v, while the Arduino is getting 5v.

I've confirmed that each part of the system works as intended in isolation, i.e. when clocking the 4518, it increments the binary output, which is correctly feeded into the 4511 and to the 7segment.

The Arduino does nothing, but a square wave currently, my questions are regarding the connection between the 4518 and the Arduino.

  1. Can I safely connect the Arduino GND and the 12V GND?
  2. Can I connect the 5v Arduino output pin directly to the 4518's input clock pin?
  3. If (answer(Q2) == false): How should I do the connection?
    • (Assuming answer(Q1) == true): Via a transistor?
    • (Assuming answer(Q1) == false): Via an optocoupler?

I havn't decided on the source of my 5V and 12V, but for the above questions, let's assume an ATX power supply (i.e. molex {5v,GND,GND,12v}).

  • 1
    \$\begingroup\$ Why would you not have the Arduino do the counting and decoding itself? \$\endgroup\$ Dec 23 '14 at 3:42
  • \$\begingroup\$ @Ignacio: Several reasons; 1. I'm going to be running multiple 7segments from a tiny Arduino, hence the pin reduction will come in handy. 2. As the 7segments require 12v, wouldn't I need like a transistor for each of the LEDs inside it? 3. As I'm learning I'll eventually switch the arduino to a crystal. \$\endgroup\$
    – Skeen
    Dec 23 '14 at 4:10
  • \$\begingroup\$ Which 7-segment display are you using? I find it very suspect to hear that they require 12V. \$\endgroup\$ Dec 23 '14 at 4:11
  • \$\begingroup\$ @Ignacio: They are 5cm ones from China, they barely light up at 5v. Let me find the data sheet, give me 5 minutes. \$\endgroup\$
    – Skeen
    Dec 23 '14 at 4:16
  • \$\begingroup\$ They are 23011as, as I'm reading the data sheet minimum voltage is 5v; tinyurl.com/ow68muc \$\endgroup\$
    – Skeen
    Dec 23 '14 at 4:19
  1. yes - you must connect all the grounds together.

  2. No - the Aduino's 5 volt High (probably) won't be recognized as a high by the 4518 running on 12 volts.

  3. Use an NPN transistor and a pull-up resistor:


simulate this circuit – Schematic created using CircuitLab Note that the transistor will invert the signal - a high output from the Arduino will be a low input to the 4518.

  • \$\begingroup\$ Can you explain, or point me to a video lecture, alike explaining the different transistor types? :) \$\endgroup\$
    – Skeen
    Dec 23 '14 at 3:57
  • \$\begingroup\$ I !!must!! connect the ground, or I may? - Can you elaborate on that? - Couldn't I keep the two grounds and powers separated, and signal using an optocoupler, and if so, what would be the pros/cons of that approach? \$\endgroup\$
    – Skeen
    Dec 23 '14 at 4:00
  • 1
    \$\begingroup\$ If you want to keep the two grounds separate, you will need an optocoupler to pass the signal between the two isolated circuits. Since you suggest using an ATX computer power supply, there is no reason to use an optocoupler, as all the outputs of the ATX supply will be referenced to a common ground. \$\endgroup\$ Dec 23 '14 at 4:10
  • \$\begingroup\$ Right, that makes sense, when would I want separate grounds? - If something is affecting the ground, high power components? \$\endgroup\$
    – Skeen
    Dec 23 '14 at 4:12
  • \$\begingroup\$ You would require separate grounds if you knew that the "ground" potentials of the two circuits were different (or couldn't be sure they were the same). Note that, in most electronics, "ground" is just a name we give to that point in a circuit that we wish to call "zero volts", and there need not be any relation between "ground" in circuit A and "ground" in circuit B. \$\endgroup\$ Dec 23 '14 at 4:19

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