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Displaying Gibberish

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sgtfarmer

New Member
Hello!

My 7-segment display only shows gibberish when I set any or all of the 4 inputs of my 7447 driver to low.

The 7-segment display has a common annode(I think?) (one +5 terminal)

I do NOT have a counter and am concerned this is the problem.

I want to use a manual 4-position rotary switch to display 1,2,3, or 4.

I get a '0' to display when I set LT to low (as expected).

Here is the diagram I used, though I do NOT have a counter, and pins 3, 4, & 5 are given a +5 supply. (though no difference when they are left blank)

8951-Display.jpg


Please help! Seems simple, but I don't know how else to research a solution.


Best,
Farmer
 

ericgibbs

Well-Known Member
Most Helpful Member
Hello!

My 7-segment display only shows gibberish when I set any or all of the 4 inputs of my 7447 driver to low.

The 7-segment display has a common annode(I think?) (one +5 terminal)

I do NOT have a counter and am concerned this is the problem.

I want to use a manual 4-position rotary switch to display 1,2,3, or 4.

I get a '0' to display when I set LT to low (as expected).

Here is the diagram I used, though I do NOT have a counter, and pins 3, 4, & 5 are given a +5 supply. (though no difference when they are left blank)
Farmer

hi,
When you set LT low, does the DP light up.?
 

kpatz

New Member
The DP isn't connected to the 7447 so it won't light up on LT low. The other 7 segments should light though. If the DP does light, then something is wired wrong to the display.

With LT and the two blanking pins set high, and the 4 inputs low, you should see a "0". Set input Q0 (1) high, you should see a "1". Set Q1(2) high and Q0 low you should see a "2". What do you actually see?

You could have a bad segment in your display or a bad 7447, or more likely, something isn't wired right. If you mix up the outputs and connect them to the wrong segments, you'll get gibberish. Also, if you set the inputs to a pattern that doesn't correspond to 0-9 (e.g. hex A through F, or 1010-1111) you'll get gibberish patterns as I don't think the 7447 blanks on invalid inputs.
 
Last edited:

ericgibbs

Well-Known Member
Most Helpful Member
The DP isn't connected to the 7447 so it won't light up on LT low. The other 7 segments should light though. If the DP does light, then something is wired wrong to the display.

With LT and the two blanking pins set high, and the 4 inputs low, you should see a "0". Set input Q0 (1) high, you should see a "1". Set Q1(2) high and Q0 low you should see a "2". What do you actually see?

You could have a bad segment in your display or a bad 7447, or more likely, something isn't wired right. If you mix up the outputs and connect them to the wrong segments, you'll get gibberish. Also, if you set the inputs to a pattern that doesn't correspond to 0-9 (e.g. hex A through F, or 1010-1111) you'll get gibberish patterns as I don't think the 7447 blanks on invalid inputs.

hi,
I know the DP isnt connected to the 7447, IF the LED has been wired the correct way around, it shouldnt light!..:p

Note 'g' and DP at opposite corners of the LED, and he gets only '0' not '8' when LT enabled.
 
Last edited:

AllVol

New Member
You'll need one switch on each of the 7447 inputs in order to give a binary number that can be read and displayed on the 7-segment.
 

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  • binary switches.GIF
    binary switches.GIF
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Last edited:

sgtfarmer

New Member
Solved!

Unlike other projects I've done, this driver required low input on all four pins to get started. By removing the ground to Q0, I got the '1', removing Q1; 2, and expected binary combinations to follow.

I take it that this is what they call 'floating high'?

Can anyone recommend a driver and display combination that requires - or + input to activate? Rather than the removal of?

Thanks for everyone's input.

8979-7447.jpg
 

kpatz

New Member
That is typical of TTL devices; they float high and have to be brought to ground to be low.

How is your rotary switch wired? You'll want to wire it so that it grounds out the pins you want to be low, and float the pins you want high. A diode matrix is the easiest way to do this, or you can do it with logic gates.

Why not use a 4511 (CMOS) decoder? It'll draw less power and you can use pull-down resistors to make the pins low, and apply Vdd to the pins to make them high.
 

sgtfarmer

New Member
How is your rotary switch wired?

Why not use a 4511 (CMOS) decoder? It'll draw less power and you can use pull-down resistors to make the pins low, and apply Vdd to the pins to make them high.

Wise words.

Figured out a diode matrix yesterday, so it's not a big deal.
(I did learn the difference between source and sink in this project!)

I have some 4511 ICs, but wanted to make use of my common anode displays first.

Want to try a push button counter style project next with a 555 chip and 4026 counter/driver.
 

ericgibbs

Well-Known Member
Most Helpful Member
Wise words.

Figured out a diode matrix yesterday, so it's not a big deal.
(I did learn the difference between source and sink in this project!)

I have some 4511 ICs, but wanted to make use of my common anode displays first.

Want to try a push button counter style project next with a 555 chip and 4026 counter/driver.

hi,
Also the CD40110B is a great device, counter and decoder.:)
 

Attachments

  • cd40110b.pdf
    433.5 KB · Views: 163
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