Some PCB design questions. Mosfet Switches, connectors, LED source drivers - Page 1
Forum section: electronics / beginners

author: 4a4ik
author_rank: Contributor
timestamp: January 02, 2016, 09:56:34 pm
content: Hello, right now I have several problems and would like to know your opinion about how best to handle them.I am building some device and can order components only from Farnell (EU), and they don't have everything.It's second version of PCB, I need to make it smaller (Not "really" small, I can use through hole components and DIP) there were also some other problems.So here are my tasks:1) I have 4 LEDs 2 color, common cathode 20mA. How to connect them to the MCU with fewer pins (if possible)As i found out most popular LED drivers are SINK drivers, but I have LEDs with common cathode:So I can't use them, are there LED source drivers available?I found TLC59213 "8-BIT PARALLEL IN/OUT DARLINGTON SOURCE DRIVER WITH LATCH"http://www.farnell.com/datasheets/1648676.pdfBut I will need to use shift register and resistors. I found some analogue switches with serial input, but they have big impedance and small continuous current. Instead of IC I can use several mosfets, but I think it's better (and easier to solder =]) to have 1 IC instead of a several discrete elements. (maybe I am wrong ?).2) I need to power on 4 devices independently, 12 V ~100 mA. I thought to use relays, but they are big, consume a lot of power and need additional diode for coil. I looked at solid state relays, but they have 1 channel, expensive and r>1, it would be great if I found 4 switches in 1 package. I tried to look for some analog switches, but as I already said they have small current. I tried to look for some IC with several mosfets in 1 package, I found FDS6898A "Dual N-Channel"http://www.farnell.com/datasheets/13253.pdfAnd my plan is to use 2 of them, they are cheaper than relays .What do people use for switching purposes? Are there IC's with 4/8/16 mosfets inside? There are discrete mosfets, they are cheap, high current, low resistance, why not to put them in a package? Or maybe I just couldn't find them?3) I have 24 lasers and I need to detect if the beam (of at least one of them) was interrupted. I use photoresistors, 10K resistors and shift registers. It works fine, but it's a little bit slow (it doesn't react to fast fingers movement), MCU have to check every bit and it takes relatively big part of PCB. In the future I think to use CPLD, I will have only 1 output to tell MCU if lasers were interrupted (still need to use resistors). But I haven't made a PCB with them so I don't want to gamble. I had other ideas with only analogue check, or using latches and digital comparators/XOR's, but I think it's easier to use shift registers.Maybe someone had simillar task?4) Which connectors you think it's better to use to connect 24 lasers and 24 sensors to the PCB ?Previously I used "Mini D Ribbon" connectors:It was terrible You can't change the length of a wire once you attach it to a ribbon cable, it's 2 big chunks of 48 wires 5m long, some lasers/sensors were smashed and to change them you need to solder new ones back to the ribbon cable. It would be great if I could change only 1 item without soldering on the spot.Now I think to use average 90° terminal blocks:I tried to find double-decker blocks, but they are quite expensive, to use less space I decided to make 2 PCBs and place one on top of another.But maybe it's better to use some sort of pitch connectors? (Is it theirs real name?):It will be easier to swap components, but you will need extra equipment, time and materials to make a connector. I (or any other person) will not be afraid to insert wires incorrectly.What do you think?

author: onesixright
author_rank: Frequent Contributor
timestamp: January 02, 2016, 10:51:08 pm
content: Just a heads-up, didn't read though all your components. You tried tme.eu as-well?

author: 4a4ik
author_rank: Contributor
timestamp: January 02, 2016, 11:03:02 pm
content: I can't order components from any other place except farnell, just because my boss said so. (it's not really a job, boss == rector and it's complicated =/ ). And I can make the device with components that are available, I just want to know maybe someone has interesting ideas how to make it better. And if there are components that I can use (even if I can't get them right now) please tell me about them, so I could use them in future.

author: AndreasF
author_rank: Frequent Contributor
timestamp: January 03, 2016, 10:11:27 am
content: Some thoughts:re 2) I would go with the dual MOSFET device you found or with 4 discrete MOSFETs. 12V at 100mA is not that high current, so you should be able to find pretty small (e.g. SMD) MOSFETs that can easily handle this.re 3) The easiest way to detect ifanyof the 24 lines tripped is probably via awired OR. A CPLD seems like overkill (especially considering the overhead that comes with it). You'll need some diodes, but there are several available with multiple diodes in a single package (e.g. common cathode for wired-OR).re 4) I would go with the individual connector you showed at the bottom. Typically the plug can only be inserted one way, so if the cable is made correctly, there is no danger in inserting the wires incorrectly. It also allows you to very quickly troubleshoot problems, as you can simply swap cables in an out.

author: 4a4ik
author_rank: Contributor
timestamp: January 03, 2016, 12:23:45 pm
content: Thank you for your answer.3) wow, I didn't know about wired logic, but I didn't explain the problem correctly. I have 24 lasers, but any one of them could be turned on or off. So if 4 lasers are enabled, 4 "sesnsors" will tell that laser is working and 20 others that laser was interrupted.4) I think I will use them, don't know why I even bothered with terminal blocks.

author: LukeW
author_rank: Frequent Contributor
timestamp: January 03, 2016, 05:59:58 pm
content: Are you just doing one-offs, or volume manufacturing?Connector choices can be significant in terms of DFM.How much labor and cost has to go into crimping tooling and stuff for making wiring looms?Fidding around with wires by hand and terminating and crimping things and making looms can be very labor intensive.Spending more money on the PCB connectors can be OK if it makes the overall system easy to build, especially if it reduces the time and labor needed for the integration and wiring of the rest of the system.

author: 4a4ik
author_rank: Contributor
timestamp: January 03, 2016, 06:18:54 pm
content: It will be only one device. I didn't put much thought into it before, but with "pitch connectors" it's easier to fix/change several parts. There will be no Engineer around to change components if needed, so it's better if wires could be inserted only one way.What people use in their projects if they need to connect a mesh of wires to the device? I tried searching for some commercial LED systems, but with no luck, I don't know how to search for something with many wires.