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My goal is to have a 7 segment display display the number from 0 to 3 in sequence and to repeat the cycle.

I have a 555 timer feeding into a 7474 (dual d flip flop) whose Q1 and Q2 outputs are giving me: {00, 01, 10, 11}.

I hooked this up to a 7447 bcd decoder that is feeding a common anode 7 segment display.

Anyhow, the display is changing but it is producing some funny symbols.

The 7 segment display is common anode and the model is ECG 3052. Here is some information on it:

enter image description here

I hooked up the 7447 like this:

enter image description here

Now, there are 2 things to mention:

  1. I'm not using the C and D inputs on the 7447. To avoid floating inputs, do I used pull up/down resistors?

  2. There are 2 common anodes on the ECG 3052. Can I use one and ignore the other? Do I tie it off somewhere? What about the pins I'm not using (like the decimal point)?

If someone could help me understand how to connect the 7 segment display I'd be very appreciative.

Like I said, my goal is to have the display show 0 o 3 in sequence then repeat itself.

Kindly,

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1 Answer 1

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You can just ground C and D to give a logic '0', otherwise TTL will behave as if the inputs are at logic '1'.

/LT and /RBI should be tied high, but TTL inputs are not supposed to be tied to Vcc, instead you can tie them to Vcc through a 1K resistor. Do the same with all inputs- either tie them to ground or to Vcc through 1K - do not leave inputs floating. Though it will appear to work if the inputs are supposed to be high, the noise immunity will be compromised without a pull-up.

You can just use one of the anode connections, they are tied together internally.

You should have appropriate value resistors in series with each of the segment outputs, of course.

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  • \$\begingroup\$ I had C and D to ground but with big resistors. they're going straight to ground and it works perfectly. I only connected one anode. Is it a problem that the decimal point input is floating? What I mean is, while it works, is there a best practice? \$\endgroup\$ Commented Oct 27, 2016 at 1:51
  • \$\begingroup\$ Best practice with TTL is to define logic levels on all inputs, otherwise you can get unintended operation (as you saw) or simply wasted power (input stages working in their linear range, not fully on or off). TTL inputs tend to float high so ou can pull up with 10K but need something like 470 ohms or less for pull down. \$\endgroup\$
    – user16324
    Commented Oct 27, 2016 at 9:45
  • \$\begingroup\$ What @Brian said- see my edit above. \$\endgroup\$ Commented Oct 27, 2016 at 12:22

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