The first schematic I designed from scratch (decimal to binary) - Page 1
Forum section: electronics / beginners

author: Pbm2001
author_rank: Newbie
timestamp: April 03, 2018, 11:14:35 pm
content: 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 messilyThanks in advance, Patrick.*editthe epprom is programmed as followsA0: 800sA1: 80sA2: 8sA3: 400sA4: 40sA5: 4sA6: 200sA7: 20sA8: 2sA9: 100sA10: 10sA11: 1sand the output is an 8 bit binary numberI added an updated schematic addressing some points made in the comments

author: ataradov
author_rank: Super Contributor
timestamp: April 03, 2018, 11:30:01 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: April 03, 2018, 11:31:34 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: April 03, 2018, 11:33:33 pm
content: 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.

author: Pbm2001
author_rank: Newbie
timestamp: April 04, 2018, 12:20:21 am
content: @ataradov1. Fixed2. 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 2sorry 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.

author: ataradov
author_rank: Super Contributor
timestamp: April 04, 2018, 01:02:03 am
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: April 04, 2018, 01:30:48 am
content: 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.

author: ataradov
author_rank: Super Contributor
timestamp: April 04, 2018, 01:35:45 am
content: 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