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:

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!