The first schematic I designed from scratch (decimal to binary)

[Pbm2001]

Hi Everyone,

I just designed my first schematic from scratch and I'm afraid to make a PCB out of it until I'm sure it will work. I was wondering if anyone here could help me with my design and if anyone saw any flaws with it.

Sorry if the lines are drawn messily

Thanks in advance, Patrick.

*edit

the epprom is programmed as follows

A0:  800s
A1:   80s
A2:    8s
A3:  400s
A4:   40s
A5:    4s
A6:  200s
A7:   20s
A8:    2s
A9:  100s
A10:  10s
A11:   1s

and the output is an 8 bit binary number

I added an updated schematic addressing some points made in the comments

[ataradov]

1. Your power supply pins are not connected to any connectors.
2. If you are using EEPROM for decoding, why not just attach buttons to the address lines directly? What is the purpose of the shift registers?

The schematic is really messy, so it is hard to read what exactly is going on with all those diodes.

[james_s]

Hard to say offhand, but I would suggest doing a bit of cleanup to make it easier to follow. Rather than physically connecting all the grounds with wires, you can ground various sections on the schematic using the Ground symbol. Same with Vcc to each chip, you can use a Vcc symbol to imply they are all connected together and the software will know to connect the netlist. Also don't forget decoupling capacitors, typically you'll want 100nF between Vcc and ground close to each IC, on the schematic I often draw them all together off to the side but that's a matter of preference.

Have you tried breadboarding the circuit? It's not a bad idea to do that, at least with parts of it. It looks simple enough that you could build it up on a solderless breadboard in a couple of hours.

[james_s]

Also it looks like the Master Reset pins on the shift registers are floating, watch out for floating input pins, normally you'll want to tie those to either Vcc or Ground, depending on whether they're active low or active high. Floating inputs are begging for weird random behavior.

[Pbm2001]

@ataradov

1. Fixed

2. I'm trying to use the keypad to input 8 bit numbers so if I press 2 three times it will recognise the first 2 as 200, the second as 20 and the third as 2

sorry about the diodes, I just have each button connected to the powers of two which make it up e.g. 7 is connected to 1, 2 and 4 and all of them connected to the clock advance of the shift registers.

[ataradov]

That is still too messy to actually check the validity. Get a breadboard and prototype it that way. But since you already have a programmable device, why not just use a microcontroller? You will avoid all this mess and get the final result much faster and cheaper.

[james_s]

I agree there, I was trying to figure out why there are both shift registers and a microcontroller. You can do the whole thing with just a keypad connected to a microcontroller.

[ataradov]

There is no MCU there, it is just a parallel EEPROM.

And holy carp, it costs more than $3. For that price you can get a few MCUs