8x8 LED Scanner, No uC - Did I do this right?

Thread Starter

HoraceTorys

Joined Apr 2, 2016
5

I'm a few months into teaching myself and my kids electronics with a breadboard, and I just designed my first circuit. It lights every LED on an 8x8 matrix in sequence, scanning top to bottom, left to right, and the speed is adjustable with a trim pot.

I know this would be easy with an Arduino or microcontroller, but I wanted to see if I could do it with a 555, two 4017s, and two 7406 hex inverters. I got it to work, and I imagine most here know the rush that comes from seeing your first personal project succeed.

However, since I just cobbled it together, I'd like to run it by a few of the old timers here to make sure I'm not putting undue stress on components, didn't forget anything, etc. When it's at full speed at 6V, the matrix looks completely lit up, but a dull pink. I ran it at 9V briefly, which looked great, but I'm not sure if I would be hurting the matrix that way.

As you've probably guessed, it uses one 4017 to source each row in sequence, and the other 4017 to sink a column through the 7406 inverters. When the first 4017 hits its 9th output, it sends a clock pulse to the second 4017 and resets itself. Attached are my (pseudo) circuit diagram and the spec sheet for the matrix. I know the diagram is a mess in the center, I colored the clock wires to make things a bit clearer.

Any feedback would be appreciated.

8x8LED_Scanner_Circuit.gif
 

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ScottWang

Joined Aug 23, 2012
6,860
Are you sure the circuit of 555 is right?
It seems missed the pin 7, the circuit only has the charge, but there is no discharge, so the output there is no any pulse output.

What is the purpose for this circuit, only scanning?
 

Thread Starter

HoraceTorys

Joined Apr 2, 2016
5
I used this Music Box circuit as a reference. That circuit works as shown, and so does my scanner thing, both with pin 7 floating on the 555.

555_4017_MusicBox.gif

The purpose of my circuit is just for fun, to teach myself the ICs and multiplexing...
 

bertus

Joined Apr 5, 2008
20,228
Hello,

The circuit shown in your opening post will not work as the 10K pot is connected to the power supply.
In the music box schematic the timing resistor is connected to pin 3 of the 555.

Bertus
 

ScottWang

Joined Aug 23, 2012
6,860
The circuit in #1 and #3 are different, the circuit in #3 has the discharge path through pin 3, so it can be generate the pulse, but the circuit in #1 there is no path to discharge, so it can't generate the pulse, if you try the first circuit again then you will know it.
 

ScottWang

Joined Aug 23, 2012
6,860
The circuit in #3 and #6 are ok to run, but in #1 was wrong, the reason that I already described in #5, you just rebuild the circuit in #1 and the output connecting a led and resistor as 2V/10mA/2mm led + 500 Ω or 2V/20mA/3mm led + 250Ω or /3V/20mA/3mm + 180Ω(200Ω).
 

hp1729

Joined Nov 23, 2015
2,304
Are you sure the circuit of 555 is right?
It seems missed the pin 7, the circuit only has the charge, but there is no discharge, so the output there is no any pulse output.

What is the purpose for this circuit, only scanning?
Since the circuit is actually working I suspect it is built correctly, just not drawn accurately.
 

Thread Starter

HoraceTorys

Joined Apr 2, 2016
5
Good catch, I did draw the 10K pot wrong. Revised diagram attached. Thank you all for your expertise.

Out of curiosity, if the 555 is not discharging through pin 7, where is it going through?

8x8LED_Scanner_1.1.gif
 

Thread Starter

HoraceTorys

Joined Apr 2, 2016
5
Both the wrong drawing and the correct one have the clock pulse leaving pin 3 to the 4017. Are you saying the discharge goes thru the trim pot on the correct drawing, joins the clock pulse wire, and then... ? I'm still not sure why the wrong drawing can't discharge, but the correct one can.
 
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