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Lab Bench III

You guessed it! It's a lab bench & my third. The kicker? It's unlike any other...

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A lab bench is a very personal thing. You spend a lot of time working at it and, by impacting what types of tools and processes you have space for, it fundamentally shapes how you work. As a student, I've been very nomadic for the past few years and have had to move all of my equipment with me. This has mandated that I use a flimsy "bench" which folds in two and has rickety homemade shelves which are easily disassembled. With steadier housing on the horizon, I've decided to retire my trusty portable setup for something a little better. The bench I'm building is designed to be sturdy more than portable. It will also have shelves built directly onto the surface which are specifically designed to accommodate my test equipment in a 'form fitting' way :) . It will be broad enough to comfortably fit large electronics projects while also holding an A5 notebook and 15" laptop. Plus, a plethora of electronic devices will be integrated into the bench makin

WARNING - HOLD YOUR HORSES: Before you get too excited, I'm not going to have a chance to work on the bench itself until this December.  Although it should go together pretty quickly once I get started because I plan to have most of the electronics started or completed before I build the actual bench itself. 

Here are the key parameters I want in this bench:

  • Sturdy: This thing needs to be STRONK
  • Big-ish: I want to have room for my stuff.
  • Good shelves:
    • Form fitting: Make room for things like my scope, AWG, and chunky PSU but don't waste space!
    • Lots of space for hand tools
    • Component rack: Store 15+ boxes housing various sorted components
    • Lights: I installed LEDs on my current bench - I want the same (or better - stay tuned...) here.
  • Integrated Electronics:
    • Lights: I already talked about this
    • 12V DC bus: I build a lot of smaller devices which run on 12 V. Keeping a ton of wall warts around is a pain. Plus most of the cheap ones seem to use really crappy SMPS which are quite noisy. Having one big beefy 12V supply lets me ditch these for a better one & cut down on some wires.
    • Equalizer: I'm often either listening to music or building audio electronics. Installing one of the equalizers I've already built makes sense.
    • Flood lights?: I drop tiny parts all the time. Putting some flood lights on the bottom to light up the floor when needed could help.
    • Thermometer/hygrometer + data logger: Okay I can't justify this one. But why not :)
  • Tool mounts: On my previous benches I 3D printed some structures to mount specific wires, tools, etc. I use them daily and plan to make more for my new bench.

A few other details:

  • The legs and shelves will be bolted on rather than screwed on so that the bench can be disassembled into large chunks and thrown in a trailer without causing serious strain to the most important structural areas.
  • Surge protectors! I will live by the golden rule: there can never be too many outlets.

  • EDL PCBs (Part 1)

    Grant Giesbrecht08/10/2019 at 01:41 0 comments

    I soldered an Arduino Nano onto a piece of protoboard to wire it up to all the peripherals.

    This board also has the real-time clock, MicroSD card module, the sensor, a push button (w/ debouncing circuitry) for cycling through error codes on the display, a switch to disconnect Arduino power, and a power LED for the Arduino.

    I've designed PCBs for the display breakout and display drivers too.
    I wrote an Arduino sketch which you can find on the project's GitHub repo. Because I don't have the display driver board yet it's still a work in progress, but it can read the sensor values, log them on the SD card, and report them to the serial monitor.

  • Electronics I - Environmentals Display & Logger

    Grant Giesbrecht07/31/2019 at 08:14 0 comments

    The environmentals display and logger (EDL) displays the temperature, humidity, pressure and time, then logs the data in a microSD card. I'm using an Arduino as the brains of the operation. I bought  a COTS SD card module, thermometer/hygrometer/barometer module, real time clock (RTC), and LCD display. Later I decided to switch the LCD display to 7 segments because I like the look better (especially against natural wood). The switch to 7 segment displays severely increased the complexity of the system because instead of the prepackaged solution of the LCD I needed to implement a system to control all 96 LEDs for the displays (96 = (7 segments + decimal point) * 4 characters per display * 3 displays ). I opted to use shift registers and charlieplexed 7 segment displays because it would let me control all 96 lights with only four wires. This is a pretty commonly used system, but here's a quick run through of the idea:

    • Load data in 12 8-bit shift registers. Each register stores bits to control one 7 segment display (plus decimal point).
    • Turn one character on in each 4-character display at a time (the display's only let you activate one character at a time to reduce the number of pins) - ie. turn on 3 characters total at a time. 
      • A ring counter (run by a ~500 Hz clock) turns exactly one character's shift register's outputs on for each display, then shifts to next, and so forth.

    These diagrams taken from my notebook show the general idea:

    I decided it was complex enough to warrant a custom PCB - I don't want to all 200+ connections by hand! First though I tested all of the major subsystems on a breadboard so I wouldn't miss any huge problems when I ship my design out to a boardhouse. First I built the clock. I opted for a super simple timer 555 circuit.

    It worked quite nicely. Next I went for the ring counter. You can use a couple discrete D-type flip flops to make a one-hot ring counter (ie. one with only output on a time - we need this so we don't turn on two characters at once and short the outputs of two 74HC595 shift registers to each other). The problem here is that you need to guarantee an initial condition in which exactly one flip flop is set high and all other are low. I wasn't able to find a suggestion for how to do this on the web so I came up with this solution:

    The idea is to use a 74HC244 line driver to set the initial conditions correctly whenever the Arduino brings the blackout line low (this signal is also connected to the flip flops and disable's their output, thus allowing the Arduino to turn off the displays, hence the name). An RC network is used to add a delay between the Arduino bringing blackout high and the initial condition register turning off. This guarantees a few clock cycles occur while the flip flops are enabled and the initial conditions are still held. This worked nicely also. Next I set up the shift registers and wrote an Arduino program to load data into them. I ran into some software bugs and mistakes in my wiring (one of which shorted the shift register outputs and fried a few chips - oops). I worked out all the kinks though and got the displays to show what I wanted. The only catch was that the amount of current drawn by the display from the ring counter was too much, contrary to what my back of the envelope calculations predicted. Adding a P-channel MOSFET to drive the common anodes of the display fixed this. Not shown here is a 74HC04 hex NOT gate used to invert the signal from the ring counter - the 74HC595s use active-low output enables, so do the PMOS transistors.

    Here's the beast set up on a breadboard displaying demo data from the Arduino. I programmed the Arduino to display a series of characters and patterns to verify everything was working perfectly. I also only hooked up two characters to save some breadboard complexity. 

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