# Help needed on 7 seg LED driver - Newbie

#### rawlinm

Joined Feb 24, 2010
4
I'm new to the forum and returning to electronics after about 20 years.

I have two problems the first is that I want to drive a 3 digit LED display to count pulses in increments of 25. ie 025, 050, 075, etc. I think I've got my head around a basic counter and led driver circuit, but can't figure out how to get it to jump in 25's and to drive the 3 digits.

The second problem is that I need to drive a 4 digit LED display driven by a bank of 10 press switches (numbered 0 to 9), such that pressing the first switch sets the first digit and the second switch the second digit etc (pressing the 5th switch should reset the first digit) - I'm stumped by this one and need some pointers in the right direction.

Any help would be very appreciated.

#### bertus

Joined Apr 5, 2008
22,275
Hello,

For the 3 digit display, is the display multiplexed or are there seperate digits?

For the buttons, are there more buttons for one digit?
Explain the functions a bit more.

Greetings,
Bertus

#### rawlinm

Joined Feb 24, 2010
4
Hi Bertus,

It's just 3 separate 7 segment LEDs, either common cathode or common anode. The idea is that someone can use a rotary encoder type selector switch to select the number being displayed on the LEDs but this number has to increase by 025 units at a time.

For the second problem, the press buttons are non-latching, press to make switches. The idea is that someone can select the 4 digits being displayed on the 4 (7 segment) LEDs. There are 10 buttons (one for each possible value being displayed, 0-9) so by pressing four of the buttons in a sequence the values being displayed on the 4 LEDs will be selected.

Hope this helps.

#### bertus

Joined Apr 5, 2008
22,275
Hello,

For the 3 digit display:
make a 4 counter, decode the 00 25 50 and 75 from it.
A second counter 0 - 9 for the hunderds.

For the button question:
are there 4 X 10 buttons? 10 for each number?

Greetings,
Bertus

#### rawlinm

Joined Feb 24, 2010
4
Hi,

There are just 10 buttons, numbered 0 to 9. For example if you pressed button 1, then button 3, then button 5 and then button 2; 1,3,5,2 would be displayed on the 4 LEDs.

How would I code the 4 counter to give the correct output? Is there a logic IC that would do this or do I need to build it up using gates?

#### bertus

Joined Apr 5, 2008
22,275
Hello,

For the 3 display:
00 => 00 => display 10's a,b,c,d,e,f , display 1's a,b,c,d,e,f
01 => 25 => display 10's a,b,g,e,d display 1's a,f,g,c,d
10 => 50 => display 10's a,f,g,c,d display 1's a,b,c,d,e,f
11 => 75 => display 10's a,b,c display 1's a,f,g,c,d

You will have to make a decoder that activates the given elements.

Greetings,
Bertus

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