Inside the Freshroast SR500

As I’ve mentioned it is my intention to try to improve on the control of the Freshroast SR500 by automating some of it with a microchip circuit.  I’ve got a lot of crazy ideas of things to do with this but most of it will take some time.

At the moment I’ve been unemployed for quite some time.  At my last employer I ran the IT department for a community bank.  That means I was in charge of the network, email, spam, spyware, virus protection, servers, desktops, printers, we had a Cisco Voice over IP phone system, and a variety of other things.  I got to deal with all the regulatory requirements from the Federal and State government writing policies, procedures, and supervising the IT staff as well doing budgets etc.  Anyway they outsourced the whole thing when the economy started getting bad probably because my salary and those that I would work with are not cheap and there were a lot of things that were changing and needing done.  Ultimately it was technically cheaper to just turn it over to a company that only does banks as an all inclusive package.  Since the economy has not improved much I’m still looking for employment which of course limits my funds to work on building control systems.  It does allow me a lot of time to work on projects such as learning to program microcontrollers.

Update July 2011 – Please check out my multi-part Teardown posting showing step by step taking apart a FreshRoast SR500 roaster after reviewing this post.

At this time I’ve written a significant amount of code that in simulators appears to operate in the proper voltage ranges and will turn on and off the heat and vary the speed of the fans.  Once I get the ability to start purchasing some of the devices and equipment necessary to build a prototype making circuit boards and programming the actual processors etc I will begin to blog about that as well.  For now I will write about what makes the SR500 work and my impression of what it does…

The Brain
Obviously the roaster has a fan, a heater, a roasting chamber, a chaff collector, a two number display, a switch, and fan speed knob.  What is not so obvious is what makes it work on the inside.  What you find inside are two circuit boards.  One has the knobs and switches connected to it.  The other board has all the power connections on it.  The control system for the knobs has what most people would refer to as “the brain”.  This is a 20 pin Atmel processor connected to a 12MHz oscillator running with about 2K of memory available to it.

Alternate view NTC Thermistor, ATmel AT89C2051-24PU

Next to the processor is a plug marked “NTC” where this is connected to a NTC Thermistor.  This is a type of resistor commonly used to test temperatures.  They typically have a narrow range of temperatures they operate in  but they are quite effective for the price.  The NTC is attached to some of the pins on the Atmel processor allowing it to sense temperature by measuring the electric resistance which changes with the temperature.

The NTC Sensor
Without knowing the specific NTC I cannot tell you how exact the temperature sensing is but while noticing that the switch has a high, medium, and low.   The temperatures listed by the manufacturer that it runs at are 490, 455, and 390.  These are 35 and 65 degrees apart from one step to the next.  This is a very large difference in temperature meaning it does not have to be very precise to tell the difference from one another.

The sensor is mounted in the path of the heater’s air.  You can see it through the grate at the top of the roaster.

NTC Thermistor mounting

As a result of the placement of the sensor this can measure how hot the air is coming off of the heater coils.  It does not get wired directly to power anywhere along the way.  Instead it connects directly to the Atmel processor which then feeds out to a MOC3043 Zero-Cross Opto-Isolators TRIAC Driver.

Left of High voltage board.

The MOC TRIAC is a special type of chip that isolates DC voltage from high voltage AC typically.  It often figures out when the AC voltage crosses zero volts.  In the US power runs at 110/120 volts and 60Hz.  Hz is a “times per second” rating.  In Europe the power tends to be somewhere in the 220+ volt area but at 50Hz… most parts in the roasting equipment are rated at either 400V or 250V so that it can be adapted to run using US or European (and other areas of the world) voltage without changing too many parts.  The Zero-Cross part of the chip helps to determine when the AC voltage goes from 0 volts up to +110/120/220/240 for example and then back down to 0 volts and then down to -110/-120/-220/-240 volts and back to 0.  That 0 to +volts  to 0 to -volts  pattern occurs 50 or 60 times per second or twice that when you just look at hitting the peak voltage from 0.  Effectively what happens when you try to “dim” the power (fan speed control, temperature control etc in some cases) it often is controlled with this sort of circuit.  In that sort of circuit (in otherwords) what happens is when you say run at 50% you let it go from 0 to +110 (0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110)  and then immediately slams it to 0 instead of rolling it down gradually (110, 100, 90, 80, 70, 60, 50, 40, 30, 20, 10, 0)  and then repeats it for the negative voltage.  Every time a MOC with Zero Cross connected to some TRIACs detects 0 point it gets reset and timing triggers occur to start over on the dimming at the appropriate points.

In the middle of the board are two devices that could be a TRIAC or a DC voltage regulator.  There are also numerous devices that could be TRIACS scattered over the board too.  The larger devices are attached to a large heat sink to keep them cool since they tend to generate a lot of heat.  Since I no longer have a multimeter to test some voltage I cannot determine whether the output of these devices are AC or DC voltage but there is obviously devices on here for creating DC voltage as well as those for switching and/or dimming AC voltage.

Fan, Bridge Rectifier KBPC606

This is one example of a place where DC voltage appears in the system.  Without being able to fully trace all of the wiring yet since this effort was only for a quick inspection and not to dismantle it I cannot see where this comes from and goes to but ultimately such a device (the black box) would normally take power coming in on two pins and through a series of diodes inside convert coming out the other two pins to DC voltage.  I am not sure where the fan knob (which connects to the two pins next to the MOC chip I believe)  is ultimately getting voltage from and whether it is AC or DC and if it is a lower voltage or higher one etc but it obviously controls some sort of dimming to this fan that apparently has AC voltage coming to the rectifier being converted to DC as it goes in.

Soooo… in the mean time I need to obtain a few things to get started….

1. PIC Microprocessor and/or development kit – I have decided I will be using a PIC micro-controller for this project.  At the moment I am working on a simulated PIC18 but will probably move to a PIC24 and perhaps if I can get a development kit circuit board will use the new PIC32 to allow for some “additional” ideas I have for the roaster and perhaps some “accessories” to make it even more useful than “just” roasting.  I had started with a Basic Stamp 2 series microprocessor and quickly overwhelmed it quite some time back and then began learning PIC microcontrollers.  At this time I can only use a PIC32 if I can get a physical chip and board to work with.  PIC24 options are very limited in the simulation and will likely be a stage before reaching the PIC32 physical hardware.  If I can get the right development kit I can swap the PIC24 and PIC32 interchangeably on the development board.
2. A lot of connectors similar to those used on the Fresh Roast boards pictured above.
3. A replacement Multimeter since I no longer have mine… I had a rather substantial professional quality one but after several moves it was lost.
4. A “rework” or “reflow” capable soldering system. These are typically used when prototyping SMD/SMT and high density chip based circuits.  There will be many miniature resistors and other devices that are actually soldered to the surface of the circuit boards rather than installed through holes.  Some of the communication types and chips for many of the sensors and devices require enough pins (or just that ultimately are not available any other way) that surface mount is the only option.  Many of these devices are extremely small and the simplest way of mounting them involves Infrared pre-heaters or hot air soldering and desoldering techniques.
5. Real Time Clock chip.  Used for realistic good time stamping and tracking of the roast including the actual date for…
6. Ethernet chipset and connectors. To allow a log tracking the roast information to be accessed from your computer including details form the …
7. K or J Thermocouple and management chips to enable the PIC to know the various temperatures logging and responding to them from the air, the bean mass and other locations to control the heat and fan speeds as well as to…
8. Display the results on a LCD Screen in real time to allow you to access a….
9. Control interface such as buttons or touch device to configure the roast or select  a stored profile speeding things up or slowing them down or changing the temperatures etc in real time as well as access profiles or other details in
10. Built in Flash memory or possible….
11. SD Memory slot to transport profiles to the roaster or to archive them.
12. Various power control parts such as TRIACs, IGBT, etc for phase control (dimming/speed control)
13. PCB manufacturing materials such as photosensitive boards, chemicals, trays, transparency, etc…
14. Small precision drill press to place any required through holes in circuit board too to allow “test” prototypes to be built
15. PCB Vendor to produce a permanent solution once the prototypes are complete for me to transfer everything to since I need a couple of these for my roaster, my girlfriend’s roaster, and probably one for my landlord who is addicted to good coffee now and will end up getting her own eventually when I need to move out 😛
16. Eventually I’ll need to figure out what sort of box to put it all in.

At this point I’m pretty confident I’ve identified all of the required actual components at least in general.  I need to finalize the power control requirements with a multimeter figuring out what all the actual roaster parts need/use and select those final parts and do some testing with lightbulbs and fans before I start connecting them to the roaster as well as do some testing of actual temperatures being put out by the SR500 as it comes from the factory.  Once those are finished I need to finalize the method of configuring the profiles using the final controls and display and I’m pretty much set.   I have not interfaced SD into the circuits yet but the communication systems are already there for other sensors.  The “file system” will be a bit of a problem programming FAT/FAT16/FAT32 into it due to some apparent Microsoft thing *makes rude gesture* so I need to look into that some more.

Theoretically I have a functioning controller in simulator.  It does not currently access AC voltage directly in the simulator though because I still need to determine the actual voltages being used to establish the amperage requirements of the various parts.  One area that is confusing is on the bottom of the roaster it shows 1600watts.  Well it’s a small fan and a heater coil and a TINY bit of electronics that uses less than an amp for that last part.  The fan seems to be deriving power from a plug marked 100W as shown in the “Left of High Voltage Board” photo.  Where the real problem comes in is with the heater causing the most concern.  You would assume the heater was 1400 or 1500 watts on its own with the unit showing 1600 watts on the bottom.  When you look at the board though….

1000W to heater coils?

Oh… and that device marked Q5 below it.  There’s a similar one next to it to the left.  That is one of the devices I have not determined exactly what it is yet.  There appears to be no identifying marks on it for a part number.  It might be creating DC voltage as a regulator or it could be one of many types of TRIAC.  The white stuff is a typical lithium thermal grease like the use on computer CPUs to make sure they don’t overheat by having a good thermal connection to the heatsink.  In this case some regulators depending on the voltage and amps or and pretty much most TRIAC dimmer devices generate a LOT of heat.  I’m thinking it’s a TRIAC due to the Q numbering being used since a voltage regulator typically uses a U number pattern.