Discrete Binary to 7 Segment Display

Part of the purpose of this project was to act as a testing ground for future projects made out of discrete logic. With this in mind I was curious to find out the max effective speed of the decoder. To test this I attached my function gen to the 8th's position on the input of Bit 3 depending on how you describe it. This produced a path 5 gates long untill the path reached the buffer or F segment driver for the display. I attached my scope to the gate of this transistor and simply feed increasingly high speed signals into it.

I observed some odd 50KHz oscillation on the positive portion of the signal when I drove it at 10Khz. It could be some sort of harmonic effect but the frequency of the noise seemed independent of the input frequency.

At 50KHz the wave form gets a bit nasty and I deem this the max effective frequency. Currently I am using NAND gates pulled high with a 10K resistor. I'm going to experiment with lower values and see if the max effective speed changes.

Now on to V3. I never planed to make a 3rd version but I might have a opportunity though my college to make a more educational version. I showed V2 to the professor who teaches the digital logic classes, a class I took and was able to apply to this project. She is interested in a though hole version that ideally will show the individual logic state of each gate. I thought of several ways to accomplish this but I settled on the most reliable option.

I'm simply attaching an LED buffer to each gate. This double the part count but with a board this size, (I esitmate 15-30 inches) I want it to work the first time!

On the left you have the old gate design and the right is the gate plus a buffer. For any one new these are simply 2N3904 BJT transistors.

Getting boards is always a fun event. There's just some thing about holding a board that you designed. Being impatient I got started on it immediately.

My method of SMD work is sticking the parts down with solder paste (I have a syringe of it I use) and then using a hot air gun to melt it. It's tedious but so far I've always gotten good results with it. I saved the though hole parts for last and I was ready to see if it worked!

It didn't...... My worst fear was an issue with the core logic circuits and it seems that seemed to be what the issue was. But then I noticed that the decimal point would light up in some positions. I never connected the DP to any thing, left it floating. I went back and reviewed the foot print. Sure enough I had reversed the foot print. Remember kids always triple check your work.

This was fixable though and some tedious soldering later it LIVES!

I did go back and fix the error. All the files on gihub are correct.

When V1 failed it was nearly a year before tried again. I redid all the logic design portion as well making liberal use of logic Friday. The file is include in the Git hub. I then recreated the diagram in dip trace. However this time I had a new trick up my sleeve. I figured out how to use hierarchy blocks and was very generous in their usage. This made the schematic a lot cleaner witch for a project this large helps keep mistakes at a minimum.

This time I learned from my mistakes. I printed out the schematic and went though the first few numbers to make sure the out put was as expected.

With the knowledge the the core was sound I moved onto the actually PCB portion. I was hoping all the parts would be grouped close to their respective gates but I was massively disappointed. Turns out all the parts were directly on top of each other. As you can see in the picture below there are stacks of fets 2-5 fets deep.

I still took a shot at manually routing it but after getting frustrated I give in to the auto route. Initial results were pretty bad. I played around with it quite a bit but I managed to get it working reasonably well. The resulting PCB was ~5 square inches. I think it would be possible to get is smaller but that is getting close to the limit. The auto router did have to chew on it for a few minutes.

The final result

V1 was a fail for multiple reasons. For some reason I decided to try to do the project Manhattan style. With the amount of connections and wires I had problems with connections. In addition to that I used cheap Fets from ebay of dubious reliability. What ultimately killed it however was the discovery that my schematic had multiple fundamental flaws.

Reviewing the schematic

I think this problem arose when I tried to convert it to using a common anode display. I got confused during this process and never discovered my mistake in time.

V1 trying to display a 1