Computing stuff tied to the physical world

CC-RT: Prototyping

In Hardware on Oct 19, 2011 at 00:01

This is part 2 of the Crafted Circuits – Reflow Timer series.

Now that the initial requirements of the Reflow Timer have been laid out, it’s time to design and build a first prototype of the whole setup. Normally, I’d do this with either a solder-less breadboard (shown on the left) or a soldered protoyping PCB (bottom view of an example shown on the right):

DSC 2691

But in this case, I’ll skip those since I already built up a system using a JeeNode, some plugs, and my favorite circuit hacking setup, which I call Projects On Foam:

DSC 2189

That’s a JeeNode mounted upside-down, with a few plugs: a Thermo Plug, a Blink Plug, and an LCD Plug with a 2×16 character LCD on top. In the bottom right is a 4x AA battery pack, stuck to the board with double-sided tape.

I’ve been using this setup for about a year now. There are some “features” listed which don’t actually exist, such as the calibration mode: the current sketch has fixed values, tweaked by trial and error for my specific grill. But apart from that, it works well: prepare grill, turn on, press start, wait for beep, open grill, turn off, done.

The reason to skim over this step in this series of posts, is that your setup will differ anyway, unless you intend to build exactly the same thing. But it’s still a crucial step to go through. This is where you get to test that your idea actually works, and where you create the setup needed to develop and test the software, i.e. sketch.

Having constructed this setup, I know it works. I also gained valuable experience with it – and I’m still not 100% satisfied (besides being a mish-mash of plugs stuck together on a foam board). The main issues are:

  • this is using an AD597 thermocouple chip, which needs at least 5V – hence the 4x AA
  • there is no provision for a mechanical or solid-state relay to switch the heater
  • I’d like to have more control over the backlight, i.e. dimming through PWM
  • it’d be nice to support an auto-power-down mode which draws virtually no current
  • the LCD plug could be omitted if we have enough I/O pins to drive the LCD directly
  • maybe the transistor driving the beeper can be omitted as well

Those last two items are all about reducing the number of components. Less components = lower cost = simpler build instructions = better chances of success = everybody happy :)

Tomorrow, I’ll go through the main choices / trade-offs.

  1. I’ve succeeded in dimming the backlight and brightness control with PWM, but that didn’t use a JeeLabs plug to control the display.

    You will need to add a few capacitors small capacitors to control the brightness of the screen. If you don’t, the microprocessors of the LCD can go haywire.

  2. I run piezo speakers direct from the ATmega ports all the time. Just need a series resistor (volume control) and maybe a zener to be nice and protect the clamping diode if you happen to knock the piezo and generate a nice high voltage spike.

    Although I’ll confess that often the later component is left out, and so far I haven’t killed anything.

    Just out of interest, how do you secure all your bits and bobs to the foam-core board?

Comments are closed.