This is part
2 3 of the Crafted Circuits – Reflow Timer series.
There are many design choices in the Reflow Timer. The goal is to keep it as simple and cheap as possible, while still being usable and practical, and hopefully also convenient in day-to-day use.
Display and controls – there are several low-cost options: separate LEDs, 7-segment displays, a character LCD, or a graphics LCD. The LEDs would not allow displaying the current temperature, which seems like a very useful bit of info. To display a few numbers, a small character-based LCD is cheaper and more flexible than 7-segment displays (which need a lot of I/O lines). The only real choice IMO, is between a character-based and the graphics LCD. I’ve decided to go for a 2×16 display because A) fancy graphics can be done on a PC using the built-in wireless connection, and B) a character LCD is cheaper and sufficient to display a few values, status items, and menu choices. And if I really want a GLCD option, I could also use wireless in combination with the JeePU sketch.
For the controls, there’s really only one button which matters: START / STOP. The power switch might be avoided if a good auto-power implementation can be created in software. For configuration, at least one more button will be needed – with short and long button presses, it should be possible (although perhaps tedious) to go through a simple setup process. A third button might make it simpler, but could also slightly complicate day-to-day operation. So two or three buttons it is.
Temperature sensor – this is the heart of the system. There are essentially two ways to go: using an NTC resistor or using a thermocouple. The NTC option is considerably cheaper and can be read out directly with an analog input pin, but it has as drawback that it’s less accurate. In the worst case, accuracy might be so low that a calibration step will be needed.
Thermocouples don’t suffer from the accuracy issue. A K-type thermocouple has a known voltage differential per degree Celsius. The drawback is that these sensors work with extremely low voltages which require either a special-purpose chip or a very sensitive ADC converter. Since thermocouple voltages are based on temperature differences, you also need some form of tracking against the “cold junction” side of the thermocouple. Thermocouple-based sensing is quite tricky.
But the main reason to use them anyway, is mechanical: although there are glass-bead NTC’s which can withstand 300°C and more, these sensors come with short wires of only a few centimeters. So you need to somehow extend those wires to run from the heater to the Reflow Timer. And that’s where it gets tricky: how do you attach wires to that sensor, in an environment which will heat up well beyond the melting point of solder? And what sort of wire insulation do you use? Well… as it turns out, all the solutions I found are either very clumsy or fairly expensive. There’s basically no easy way to get a glass-bead NTC hooked up to the reflow timer in a robust manner (those wires out of the glass bead are also very thin and brittle). So thermocouple it is.
Thermocouple chip – for thermocouples, we’ll need some sort of chip. There seem to be three types:
- dedicated analog, i.e. the AD597
- dedicated digital, i.e. the MAX6675 or MAX31855
- do-it-yourself, i.e. a sensitive ADC plus cold-junction compensator
The AD597 is used the the Thermo Plug and in my current reflow controller setup. It works well, with a voltage of 10 mV/°C coming out as analog signal. So with 250°C, we get 2.50V – this is a perfect match for an ATmega running at 3.3V. The only small downside, is that it needs an operating voltage which is at least 2V higher than the highest expected reading. If we need to go up to say 275°C (above what most ovens can do), then we’ll need a 4.75 V supply voltage for the AD597.
The MAX6675 doesn’t have this problem because the readout is digital, and works fine with supply voltages between 3.0 and 5.5V. But it’s a very pricey chip (over €14 incl VAT). Keeping these in stock will be expensive!
The MAX31855 is also a digital chip, and about half the price of the MAX6675. The main difference seems to be that it can only operate with a supply from 3.0 to 3.6V, which in our case is no problem at all (we need to run at 3.3V anyway for the RFM12B). I’ve no experience with it, but this looks like a great option for the Reflow Timer.
There is a slight issue with each of these chips, in that they do not exist in through-hole versions but only in a “surface mounted device” (SMD) style. The package is “8-SOIC”, i.e. a smaller-than-DIP 8-pin plastic chip:
For people who don’t feel confident with soldering it might pose a challenge. There are no sockets for SMD, you really have to solder the chip itself. Then again, if you’re going to create a reflow setup for building SMD-based boards anyway, you might as well get used to soldering these size chips. Trust me, SOIC is actually quite easy.
(note: there is an all-DIP solution with the LT1025, but it needs an extra op-amp, so I’ve not checked further)
If we can use the MAX31855, then everything can be powered with 3.3V. This means that either 3x AA or 1x LiPo will work fine, in combination with a 3.3V regulator. I’ll stick with the MCP1702 regulator, even though it’s not the most common type, because of its low standby current – this will help reduce power in auto power-down mode.
But how much current do we need? To put it differently: how long will these batteries last? Let’s find out.
The prototype I have appears to use about 35 mA while in operation. Let’s take a safety margin and make that 50 mA in case we also need to drive an opto-coupler for the SSR option. And let’s say we use 2000 mAh AA cells, then we’ll get 40 hours of operation out of one set of batteries. Let’s assume that one reflow cycle takes 10 minutes, plus another 5 minutes for auto power-off, then we can use one set of batteries for 160 reflow cycles. Plenty!
We could even power the Reflow Timer with an AA Power Board, and still get about 50 cycles – but that would increase the cost and require some very small SMD components.
Let’s just go for the 3x AA setup, with either a DC or USB jack as possible alternative.
AC mains switching
For switching the heater, there are several options. The one I’m using now is a remote-controlled FS20 switch from Conrad (or ELV). It can be controlled directly by the RFM12B wireless module. An alternative would be the KAKU (a.k.a. Klik Aan Klik Uit or Home Easy) remote switch, which operates at 433 MHz and kan also be controlled directly from the RFM12B. The advantage of this setup is that you never need to get involved with AC mains – just place the remote switch between mains socket and heater (grill, oven, etc) and you’re done.
Another option is to use a Solid State Relay (SSR), which needs 5..10 mA of current through its built-in opto-coupler. I built this unit a while back to let me experiment with that. The benefit of such a configuration is that all the high-voltage AC mains stuff is tucked away and out of reach, and that the control signal is opto-isolated and can be attached to the Reflow Timer without any risk. Note that with SSR, the RFM12B module becomes optional.
Yet another option would be to use a mechanical relay, but I’d advise against that. Some heaters draw quite a bit of current (up to 10A) and will require a hefty relay, which in turn will require a hefty driver. Also, very few power relays can operate at 5V, let alone 3.3V – which means that a 3x AA powered approach would not work.
So, RF-controlled switch it is, with an extra header or connector to drive the LED in an SSR as option.
That’s about it for the main Reflow Timer circuit design choices, methinks.