Computing stuff tied to the physical world

CC-RT: Initial requirements

In AVR, Hardware, Software on Oct 14, 2011 at 00:01

Let’s get going with the CC-RT series and try to define the Reflow Timer in a bit more detail. In fact, let me collect a wish list of things I’d like to see in there:

The Reflow Timer should…

  • support a wide range of ovens, grills, toasters, and skillets
  • be self-contained and safe to build and operate
  • include some buttons and some sort of indicator or display
  • be created with through-hole parts as much as possible
  • (re-) use the same technologies as other JeeLabs products
  • be built on a custom-designed printed circuit board
  • use a convenient and robust mechanical construction
  • be very low-cost and simple to build

To start with that last point: the aim is to stay under € 100 as end-user price, including a simple toaster and whatever else is needed to control it. That’s a fairly limiting goal, BTW.

I’m sticking to “the same technologies” to make my life easy, both in terms of design and to simplify inventory issues later, once the Reflow Timer is in the shop. That translates to: an Arduino-like design with an ATmega328, and (for reasons to be explained next) an RFM12B wireless module.

Safety is a major concern, since controlling a heater tied to 220 V definitely has its risks. My solution to controlling an oven of up to 2000 W is the same as what I’ve been doing so far: use a commercially available and tested power switch, controlled via an RF signal. KAKU or FS20 come to mind, since there is already code to send out the proper signals through an RFM12B module. Range will not be an issue, since presumably everything will be within a meter or so from each other.

With wireless control, we avoid all contact with the mains power line. I’ll take it one step further and make the unit battery-operated as well. There are two reasons for this: if we’re going to uses a thermocouple, then leakage currents and transients can play nasty games with sensors. These issues are gone if there is no galvanic connection to anything else. The second reason is that having the AC mains cable of a power supply running near a very hot object is not a great idea. Besides, I don’t like clutter.

Having said this, I do not want to rule out a couple of alternatives, just in case someone prefers those: controlling the heater via a relay (mechanical or solid-state), and powering the unit from a DC wall wart. So these should be included as options if it’s not too much trouble.

To guard against heat & fire problems, a standard heater will be used with a built-in thermostat. The idea being that you set the built-in thermostat to its maximum value, and then switch the entire unit on and off via the remote switch. Even in the worst scenario where the switch fails to turn off, the thermostat will prevent the heater from exceeding its tested and guaranteed power & heat levels. One consequence of this is that the entire reflow process needs to unfold quickly enough, so that the thermostat doesn’t kick in during normal use. But this is an issue anyway, since reflow profiles need to be quick to avoid damaging sensitive components on the target board.

On the software side, we’ll need some sort of configuration setup, to adjust temperature profiles to leaded / unleaded solder for example, but also to calibrate the unit for a specific heater, since there are big differences.

I don’t think a few LEDs will be enough to handle all these cases, so some sort of display will be required. Since we’ve got the RFM12B on board anyway, one option would be to use a remote setup, but that violates the self-contained requirement (besides, it’d be a lot less convenient). So what remains is a small LCD unit, either character-based or graphics-based. A graphic LCD would be nice because it could display a temperature graph – but I’m not sure it’ll fit in the budget, and to be honest, I think the novelty of it will wear off quickly.

On the input side, 2 or 3 push buttons are probably enough to adjust everything. In day-to-day operation, all you really need is start/stop.

So this is the basic idea for the Reflow Timer so far:

JC s Doodles page 18

Ok, what else. Ah, yes, an enclosure – the eternal Achilles’ heel of every electronics project. I don’t want anything fancy, just something that is robust, making it easy to pick up and operate the unit. I’ve also got a somewhat unusual requirement, which applies to everything in the JeeLabs shop: it has to fit inside a padded envelope.

Enclosures are not something you get to slap on at the end of a project. Well, you could, but then you lose the opportunity of fitting its PCB nicely and getting all the mounting holes in the best position. So let’s try and get that resolved as quickly as possible, right?

Unfortunately, it’s not that easy. We can’t decide on mechanical factors before figuring out exactly what has to be in the box. Every decision is inter-dependent with everything else.

Welcome to the world of agonizing trade-offs, eh, I mean… product design!

  1. Patrick McCabe has a nice project design here.

    his design differs from your in some important ways, but I like the enclosure and over all design. Not bad for a 17yr kid!

  2. Here’s an alternative: No display and no buttons, just a WiFi board, so it’s all controlled from a web browser.

    • Interesting idea. Might be a bit pricey. How would you setup the thing to pick the base station w/ password? Right now, for day-to-day use I just press a button on my unit and walk away until it beeps, so the display is optional. For fancier stuff, I use the built-in wireless with a JeeLink in the desktop PC.

  3. 100 euro’s will probably be quite a challenge.

    Is your intention to use “standard” plugs, and a JeeNode, or will you be building a custom board specifically for this purpose (with maybe one or two breakout ports so you can use certain plugs as “extras”)?

    As for the remote control, FS20 is quite expensive, but doesn’t need extra hardware to control (RF12B can do that), whereas a KAKU start set costs only 16 euro’s for two switches and a remote control (which could be used as “buttons” if you like), but which requires the extra OOK433 hardware to be able to control it (and therefore also an extra “port” on the ATMega)…

  4. I recently noticed an electrical cooking top for two pans at “kruidvat” for 20 euro’s. I did manage to desolder components off of two old circuits I had. it looks like this :

  5. Some sugestions:

    • storage of profiles in flash
    • option to send profiles from the computer via the radio
    • for simple designs I usally use surface mountable electrical junction boxes. They have good size for most projects, very cheap and safe for even 220v. As they can sourced locally you don’t need to fit them in an envelope.
  6. I wouldn’t recommend using KaKu or any similar system because they do not provide feedback. You never know for sure if a device is turned off or on. Also, they are quite sensitive to interference, so they could be triggered by someone locking his car.

    Because these ovens are a simple resistive load I would personally use a solid state relais separated from your uC with an optocoupler. This way you’ll also keep the mains galvanically isolated from your microcontroller but it is a lot cheaper and probably more reliable.

    • I highly doubt that my KAKU switch would turn on if someone locked his car, but you are correct that a simple optocoupler would be cheaper and more reliable.

      However, I don’t think that KAKU (or FS20) is very unreliable. Especially given the short distance that we are talking about here! Also, I’m very happy to NOT have to mess with 230volts, as will a lot of the people that buy JCW’s stuff. So as for safety, I’d still go with a KAKU / FS20 like solution.

  7. we do have feedback : the temp sensor. But you raise a good point : please include a warning system, i.e. a beeper, flashlight or something like that

  8. what about using a triac and opto to control. Cheap and safe,

    • Sure – that’s also called a “solid state relay”. I will definitely keep that option open, but also the RF switch for those who don’t want to mess with 220V mains.

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