Monday, 18 September 2017

81 LED Chaser circuit using 2 NE555's!

Hi everyone!

This last week I've been busy making a little LED chaser circuit. I found a schematic online that used one NE555 to drive two CD4017 decade counter chips that drive the LED's. I thought I could improve on that by adding a second NE555 and it worked beautifully.

I started out by building the LED display on a separate perforated circuitboard. I wanted to have the LEDs very close together to get a nice dense row of lights and the board I used was exactly the right size to fit a 9 by 9 LED matrix. I used 5mm LEDs because I have about a thousand of those in my junk box, salvaged from an old display unit. They had short leads but long enough to still use. I used a green perforated circuit board but to fit them on next to eachother I had to trim each LED on 4 sides with a Dremel tool because these LEDs have a broad rim at the bottom. Anyway, I managed to fit them all on the board in a 9x9 matrix. Then I soldered all the Cathodes together, row by row. Then the same for the Anodes, to give me an X and Y axis to work with.

After that I started soldering the actual circuit.
Now, the original schematic, that you can find on the internet, only uses one NE555 to drive the movement of the LED lights and so you can really only change the speed of the X-axis of the display, or the Y-axis according to the schematic below, but I soldered the display in such a way that the initial movement was horizontal. So I thought why not add an extra NE555 and make the Y-axis adjustable aswell so you can get much more variation in the patterns displayed. So that's what I did. I made a new schematic and here it is: (Btw, the collectors of the vertical row of transisitors are all connected to the + of the power rail, just like the top transistor. It's not shown in the schematic because that would make the drawing very messy.)
(Click on the images to see them in full scale and right click to download them.)

Btw, instead of using the BC547 transistors you could also use the 2N3904 but in that case you need to change the 220 Ohm resistors for 100 Ohm resistors, although I can't imagine it making much difference You could even try 2N2222 transistors in which case you can do away with the resistors all together. They are just used as switches afterall.)

After having soldered on the first NE555 squarewave generator, I tested the output signal with my oscilloscope and I found that there were bursts of pulses with a frequency of about 60kHz on the output squarewave. This is a common problem of the NE555 which does not occur with the CMOS version ICM7555.
Here's a screenshot of the Squarewave from the NE555 with the pulses on top:

I added a big electrolytic capacitor of 470 µF to the output of the voltage regulator and that solved the problem. I proceeded to solder in the rest of the components. The 10K resistors for the base of the transistors I stuck into the same hole as the base, to save space. I had a bunch of cheap resistors from China which had very thin leads so they just fitted into the hole together with the base of the BC547s. That way I only needed 4 holes per transistor resistor combination.
I tested the circuit a few times during assembly to make sure everything worked because once it was all put together it would be very difficult to trouble shoot this thing with all the wires going everywhere. Luckily it all worked as I had imagined, especially the second NE555. It worked just as I thought it would do. I had a problem though with the two 100K potmeters. The ones I used were old ones from a valve radio and they turned out not to be up to the job. I didn't have more 100K pots but luckily I did have two 50K stereo potmeters, so I soldered the wires on, in such a way that the double 50K was in series and formed one 100K potentiometer and that worked very well. It's important that the potmeter goes all the way down to zero Ohm to get the fast movement of the LEDs and the ones I used did that very well.
Btw, if you decide to build this and want the display to appear as I have it in my video, with the lights going from left to right working their way down, you'll need to experiment with how you solder the wires from the transistors to the display. The way it's drawn in the schematic the light would go from top to bottom instead of left to right down the rows. Beware of that.

I made a video about this circuit which shows how it works with a little animation sequence, which you can watch here:

I did some measurements of the pulses and they are pretty messy to look at but they work just fine to trigger the CD4017s. I was surprised at the fast rise-time of the output pulses from the 4017s. They rise in about 12 nano seconds! Here are some screenshots from the scope:

This is the X-axis pulse going to the LEDs:

This is the Y-axis pulse going to the LEDs:

Here's a closeup of the rising edge of the output pulses showing how fast they rise. You could build a Time Domain Reflectometer with pulses this fast:

So now that I had it all working, I decided to round off this project by building the whole thing into a nice case. I found an old sewing tin which had just the right size. I spray painted it black and with a Dremel tool I made holes for the display and the knobs. Then using hot glue I glued in the display board. I left the board with the actual electronics on it floating. I didn't glue it down. All the wires connecting it to the display were enough to keep it in place and I needed the lid to be removable to make it possible to exchange the battery. I used a 9 volt battery which I kept in place with a strip of copper, bent to fit around the battery and glued to the bottom of the case. I lined the inside of the case with gaffer tape to prevent accidental short circuits should the print touch the case. I had just received a batch of knobs from China that looked a lot like the knobs on a Mini-Moog synthesizer and I put those on the potmeters.
After it was all assembled it looked like this:

This is the inside of the case:

(The hot glue underneath was meant to protect the wiring when I was testing the circuit.)

Doesn't it look cool?? Of course it doesn't do anything useful, but it's so much fun to play around with and also to build. Actually, you could use it as a game: try and make a diagonal line appear that doesn't move across the screen. It's possible but requires a very delicate touch on the controls. Perfect to while away those busy office hours, lol! And you could use it as a prop for a movie. Say like an artificial scanner of some sort, for tracking down ghosts  ^___^

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