The Cuisinart Air Fryer Hack

Background

Our toaster was toast.  The Cuisinart Toaster Oven Air Fryer TOA-60 died after about 1 year and a half of active use.   The function selector had burned out due to arcing.  The door switch also showed signs of arcing.  Cuisinart cheerfully replaced the unit under warranty, but that left the old stainless steel hulk sitting around.  This project rebuilt it using a Raspberry Pi and allows it be controlled by Alexa from across the Internet.  Feel free to skip the disassembly and discovery in Part 1 and read the new design in Part 2.  Click on any picture to expand it.

Part 1: Understanding the Toaster

Disassembly

The first goal is to get the stainless steel cover off.  We need a 10" long Phillips screwdriver, a large flat head screwdriver, a set of Allen hex keys and a headlight.  This process takes about an hour.  Click on the hyperlinks below to see pictures.

1. Remove the grill and the crumb trays.

2. Remove the back.

a. There are more than 12 screws holding the back.

b. One of the screws needs an Allen key for removal.

3. Remove the left plastic foot assembly. 

a. There are 3 conical head screws.

4. Remove the right foot assembly.

a. Pop off the 2 rubber covers on the right assembly.  Use the handy Allen key.

b. Remove the 2 round head screws inside the foot.

c. Remove 1 conical head screw in the middle.

d. Keep these screws separate from the others.

5. Next reach through the back opening to remove 8 screws that are on the front.

a. You will need the long Phillips head screwdriver.  Magnetic tips will help.

6. There are 3 spring clips on each side of the stainless steel cover.  They attach the cover to a flange in the front.

a. The clips are built into the outside cover.

b. Use a flat head screwdriver to gently force the cover backward off the clips.

c. The cover is now free. The components are exposed.

7. Put the 2 plastic feet back on to avoid scratching your dining table.

a. Note that the screws for the left and right feet are different.

8. Next, you could grasp the function knob and pull it straight out.  It may be tightly fit.

a. You can see the 2 screws that mount the rotary switch.  It just brings you the satisfaction knowing that if you had a replacement part, you could fix this thing.  Cuisinart may not sell a replacement.

b. If a replacement is not available, don't bother with this step.  You can leave the switch as a non-functional ornament.  Removing it will leave an ugly hole.

9. Here are some pictures

a. the left side,

b. another left side,

c. right side,  this is where all the action is,

d. top, another top, yet another top,

e. light bulb assembly,

f. light switch,

g. door switch, and another of the door switch.

Tracing the wiring

The red, white and blue colors of the wires seem to be chosen without a strategy.  Tracing the spaghetti reveals the following schematic.  Personally, I would have placed the thermostat along the wire marked "Timer gated power", allowing the heater elements to be grounded directly to neutral.  But no big deal.  The Microtemp thermal fuse is rated to blow at a specific temperature (marked 216°C, 420°F).

Rotary function selector

The function selector gadget seems to be a proprietary Cuisinart specified part that is sourced by a Chinese vendor.  A replacement may not be easily available.  Similar parts are listed on eBay with the same part number, but they are nowhere near usable.  The original part can be described as a 4 pole, 7 position, multi throw rotary switch, and it is the heart and brains of this toaster.  It comprises two rotary modules stuck together concentrically, and each module has 2 switches.  Let's call the 2 modules Front and Back.  Front contains switches A and B, and Back contains C and D.  Each switch has one input and two outputs, except B has only one output.  Depending on the position, each input connects to none, 1 or 2 of its corresponding outputs.

The following table shows its connectivity.  There are 4 inputs: A, B, C and D.  There are 7 outputs: A1, A2, B1, C1, C2, D1 & D2.  The check marks indicate if the input is connected to the output at that position.  Heater elements are in red, fans are blue, timers are black.  For example, in the position Toast (position 4), input A connects to both A1 &A2, input B connects to B1, and input D connects to D2.  This effectively turns on A1 (fan in low), A2 (upper outer heating element), B1 (lower heating element) and D2 (engages the toaster timer).  The 'Warm' setting activates the lower heater element.  Interestingly, 'Bake' & 'Toast' are identical, as are 'Broil w/fan' & 'Air-fry'.

Switch à	Front Module			Back Module
Input à	A		B	C		D
1. Warm			ü			ü
2. Broil	ü	ü		ü		ü
3. Broil w/fan		ü		ü	ü	ü
4. Toast	ü	ü	ü				ü
5. Bake	ü	ü	ü			ü
6. Bake w/ fan		ü	ü		ü	ü
7. Air Fry		ü		ü	ü	ü
Output à	A1	A2	B1	C1	C2	D1	D2
Targetà Component	Fan Low	Up Out	Low Htr	Up In	Fan High	Oven Timer	Toast Timer

Air Fry function

The Air Fryer has a two speed fan to circulate hot air inside the chamber.  The fan also expels some air from the chamber, thus cooling it down.  Extra heat is needed to maintain the temperature while the fan is on.

Power Measurements

After they were disconnected, the 3 heater elements were measured: Upper Outer=16 Ω, Upper Inner = 17.4 Ω and Lower = 17.3 Ω.  At most two elements will be on at any time.  The fan consumes 35W.  So the maximum device consumption is about 15 amps, or 1800W, as specified.

Temperature Control

The original toaster uses a thermostat.  This is a mechanical device that consists of a dial to select the desired temperature, a temperature sensor and an on/off switch.   Its job is to control power to the heater elements so that the oven reaches and maintains at the desired temperature setting. The following graph shows its operation at a set point of 300°F.  The red line displays the actual temperature swinging between 250° and 400°F.  The green trace shows the heater elements coming on and off.  While the average temperature is 300°F, the temperature swings are excessive and lead to food getting burnt.

Part 2: The New Design

Requirements for a New Design

Engineers always list the requirements before starting a design. It's the marketing guys that keep changing the requirements.  Here is the initial list:

1. Backward compatibility with current functionality using the existing mechanical timers.  It should work intuitively for the non-geeks.  The timers and temperature control should work as expected.  The function selector, which is broken, is exempt from this requirement. 

2. Use a digital thermometer since the original thermostat is inaccurate and causes large temperature swings.  Temperature excursions should be limited to ±5°F.

3. Add more heating modes for Warm and Slow cook functions.  The three heating elements can be configured to provide fine temperature controls.

4. Multi-step program operations, like: Preheat, then Bake at 350° for 30 minutes, cool down to 250°, then Broil for 5 minutes and keep warm till 6PM.

5. Delayed start and Pause options.

6. Browser-based GUI to select oven function, temperature, duration, etc.  Integration with external applications via a REST API.

7. Alexa integration for voice control and voice notifications.

8. SMS text notification, for completion of each program step.

9. A nominal buzzer, although Alexa will provide voice responses.

10. Touch-sensitive LCD screen for control, though a web page will also be available.

11. Moisture sensor, to prevent burning and to implement a dehydrator.

12. Camera for visually checking the doneness of food, say "Toast till brown", or "Broil till bubbly".

13. Smoke detector, to help decide when to give up the toast and automatically order a pizza, just kidding.

New Hardware Design

At a high level, a Raspberry Pi Zero 1 W uses 8 of its GPIO pins to control 8 relays.  The relays drive the toaster's three heater elements, fan and lights.  Besides turning on each element at full power, the relays can also configure them in series to allow for gentle warming.   Five other GPIO pins are used to sense the state of timers, thermostat and switches.  Three more GPIO pins for the thermometer.  A web server on the Pi listens for commands via Wi-Fi.  It also drives an LCD screen and serves up REST APIs for basic commands.

The control part is fairly straightforward.  A 8 channel current booster (ULN2803A) is used to drive the relays.  This wonderful chip contains all the necessary transistors and no additional components are needed.  It requires the +5V lead (COM) to dump flyback currents from the relays.  The ULN2803A connects very nicely to the relay module.

The low voltage section for sensors is wired separately.  A 5 Vdc power supply is added inside the toaster shell.  The Raspberry Pi is installed on the fan housing.  The metal oven outer casing will be replaced to avoid blocking WiFi signals and to allow better cooling for the new components.  It will also show off the cool tech.  Ribbon cables connect the Pi to relays and sensors.

Temperature Measurement

A thermocouple is added to measure the actual instantaneous temperature.  It consists of two components: a MAX6675 IC and a K-Junction thermocouple.  The first thermocouple I tried had problems.

· Measurements showed that the thermocouple had a very poor response time and lagged the actual temperature.  This was because the thermocouple was firmly bolted to the toaster walls and effectively had a large 'thermal mass'.  The metal surrounding the thermocouple took a long time to reach the temperature of the air in the oven. 

· The following graph illustrates this. The oven is already warm and is maintaining the set point of 200°F. The green plot is the heater element turning on and off, and the red plot is the thermocouple reading.  The heater goes on at about the 865 sec mark, but the reading is still dropping.  The reading starts to rise around the 910 sec mark.  The heater goes off at the 940 sec mark, but the reading continues to increase till the 1020 sec mark.  Again, this measurement used the thermocouple for temperature control.  Note that the average temperature is higher than the expected 200°F and the range is between 190 to 225°F.

· A smaller thermocouple was used and it has a much better response.

· The temperature is measured each second.  The ambient temperature is tracked while the over is not being used.

· A MAX31856 converter was tested, but the benefits were not clear.  A MAX6675 with Hardware SPI was adequate for the job.  Relay based control (deprecated)

The relays were initially wired as shown below.  Two of the relays (R1 & R2) were used to string heater elements in series.  This allowed for heating with lower wattages.  The table below shows the power and relay settings for each element combination.  There are 11 combinations, of which 4 are suitable for broiling.  However, this element chaining feature was not sufficiently effective, and there was a lot of relay clicking.

The relays also allow three settings for the fan: high, low and off.  The fans greatly affect the heating and cooling rates and affect the temperature control logic.

Combination	Resistance (Ohms)	Power (Watts)	Relays	Mode	Function
Lower + Upper Out	8.31	1732	R4, R5	Oven	Bake
Lower + Upper In	8.67	1660	R4, R5	Oven	Toast
Lower + (Upper Out -> Upper In)	11.40	1264	R5, R6, R1	Oven	Brown
(Lower -> Upper Out) + Upper In	11.43	1260	R5, R2, R4	Oven	Roast
Lower -> Upper Out	33.30	432	R5, R2	Oven	AirDry
Lower -> Upper Out -> Upper In	50.70	284	R5, R2, R1	Oven	Warm
Upper Out + Upper In	8.34	1728	R4, R6	Air-Fry	Broil
Upper Out	16.00	900	R6	Air-Fry	Brown
Upper In	17.40	828	R4	Air-Fry	Toast
Upper Out -> Upper In	33.40	431	R6, R1	Air-Fry	Warm
Lower	17.30	832	R5	Grill	Grill

Solid State Relay (SSR) and the PID Controller

The temperature stability was not satisfactory when chaining the combinations of heater elements.  Also, switching the relays on and off creates loud clicks, which is uncouth if not distracting.  A 20 Amp Solid State Relay was added in line with the heating elements.  While this $10 relay is probably too cheap to be reliable, it is good as a proof of concept.  The SSR is directly driven by the Pi’s GPIO pins; ~3.0v seems to be adequate.  The Pi sends a Pulse Width Modulated (PWM) signal to control the power sent to the heaters by varying the duty cycle of the wave.  The SSR turns the AC power on and off only when the voltage is close to zero. A 60 Hz AC line crosses the 0 volt line 120 times per second.  In the figure below, the vertical black lines indicate when the PWM signal is high, indicating that the SSR should be conducting.  However, the SSR will start conducting only when the instantaneous voltage nears 0V.  Similarly, the SSR will stop conducting at the next zero crossing after the modulating signal turns off.  In the figure below, the cyan colored parts of the AC power is conducted.  By turning the power on and off as desired and vary the duration (width) of each pulse, i.e., the width can be varied from 0 to 100% of the PWM wavelength.  This allows a precise control the wattage being delivered to the heating elements.

The PWM modulating wave frequency is a very low 0.6 Hz, or a period of 1.67 secs.  This allows for 100 AC cycles and 200 zero crossings for the SSR.  There really is no need to drive heater elements at a high frequency.

The relay wiring was modified as follows.  The power indicator lamp pulses during operation to indicate the SSR operation.

A software PID controller drives the SSR to provide fine control for the heating.  The goal is to use PID to select the correct power to get quickly to the desired temperature and stay there.  The following calculations are performed each second to determine the SSR duty cycle.

1. Temperature Error (in degrees F) = Set point temperature - Current oven temperature (aka Process value).  The last 10 error values are saved in a list.

2. Proportional Wattage = K_p * Error

3. Integral Wattage = K_i * sum(errors over last 10 readings)

4. Differential Wattage = K_d * (Current error - average(error over last 3 readings))

5. DeltaT = min(Current oven temperature, Set point temperature) - ambient temperature.  This value helps calculate the adjustments for heat leakage.

6. Conduction heat leakage  = K_Conduction * DeltaT.  Strictly speaking, the conduction value should use the current oven temperature, not the Set point, but that would encourage overshooting.

7. Convection heat leakage  = K_Convection * fan speed * DeltaT.  The thinking here is that the fan is ejecting some heated air and replacing it with outside air at ambient temperature.

8. Required wattage = Proportional + Integral + Differential + Conduction heat leakage + Convection heat leakage 

9. Max available wattage = depends on current relay settings (see table above)

10. SSR Duty Cycle = Required wattage / Max available wattage. The duty cycle value computed is limited to a range of 0 to 1.

The error history list, typically the last 10 error values, is cleared when the set point is changed.

The SSR is also turned off momentarily while the relays are changing positions.  This may help prevent arcing in the relays.

The SSR has a bit of a leakage that causes the neon lamp to glow even when the SSR is turned off with a zero duty cycle.  A 22k resistor is added to drain the leakage and drop the voltage so that the neon light does not come on while the toaster is not operating. 

Tuning the PID

Tuning involves setting values for K_p, K_i and K_d  with a lot of trial and error, mostly errors.  Making temperature charts helps develop the needed intuition.

· The Proportional coefficient needs to be large enough to reach the set point swiftly, but not overshoot it excessively.

· The Integral coefficient also needs to be large enough to reach the set point, but not cause oscillations.

· The Differential coefficient helps stabilize the temperature near the set point.

· The power needed to maintain a steady state at various temperatures was used to estimate K_Conduction . 

· Similarly, the heater power needed to maintain steady state at the three fan settings was used to estimate K_Convection.

· The coefficients are stored in a configuration file and can be modified via a web API. 

· There was an issue with the oven tray blocking heat from the lower elements.  The tray extended end to end and created a separate thermal zone below it, where the temperature was much higher than above it.  This caused the PID system to have significant overshoots during warm ups.  The PID system would try to home in to the correct temperature, but the extra heat from below would cause an overshoot.  The chart below shows the problem.  Using a smaller tray on the grill, or turning the fan on, equalizes the temperatures and eliminates overshooting. 

Pinouts

It is important to plan ahead to map out the pins needed to control all the sensors and devices.  The information below is not useful, but note the level of detail recorded.  The Pi dedicates a few pins to specific functions, like SPI and PWM.  These pins are allocated first.

Relay Module
GPIO	Pin	Ribbon	Function
22	15	Brown	serializer: Upperin + Upper out
23	16	Red	serializer: Lower + Upper out
24	18	Orange	Inside light
25	22	Yellow	Heater, upper inner
12	32	Green	Heater, lower
16	36	Blue	Heater, upper outer
20	38	Purple	Fan, high
21	40	Grey	Fan, Low
GND	6	Black	Relay Board
GND	20	White	ULN
VCC	4	Brown	ULN 5vdc
Solid State Relay
GPIO	Pin	Ribbon	Function
18, PWM0	12	Red	SSR +
GND	14	Orange	SSR -
Sensors
GPIO	Pin	Ribbon	Function
2	3	Grey	Door
3	5	Purple	Light Switch
4	7	Green	Toaster Timer
17	11	Yellow	Oven Timer
27	13	Orange	Thermostat
GND	9	Blue	Common to sensors
Thermometer 6675
GPIO	Pin	Ribbon	Function
vcc	17	White	+VDC 3.3
10	19		MOSI, NOT USED
9	21	Blue	MISO, SDO
11	23	Grey	Clock
8	24	Purple	Chip select 0
GND	25	Black	GND
7	26		Chip select 1, NOT USED
Piezo:
GPIO	Pin	Ribbon	Function
19, PWM1	35	Red	Piezo
GND	39	Black
Capacitors
GPIO	Pin	Ribbon	Function
Vcc	1	xx	3.3v
Vcc	2	xx	5v
GND	6	xx	Gnd

The New User Interface

The toaster can be operated in ‘compatible’ mode by manually turning on either of the two dial timers.  This will start the oven or the broiler at the temperature set on the dial thermostat.  The oven light will come on while the timer is active.  The light button has been repurposed to control the fan.  Double clicking this button will run the fan at High.  The GUI based functions described below are enabled when the manual timers are not engaged.  Turning on the timer while another function is running will set off a horrible alarm.  The temperature will be controlled by the dial thermostat and continues to be bad.

The toaster has a web based GUI that connects using an API.  It has two views: a Standard functions and a Program view.  The Standard view shows all the normal oven function, like Toast, Bake, Air Fry, etc. 

Touch any function button, and the selected function will change color to green, and the “Standard” button will change to “Start”.  Touching a different button will select that function instead.  After a minute, the function will be cancelled and de-selected.  Select the temperature and duration.  Touching the “Start” button will start the selected function and the “Start” button will change to “Stop”.  Long-pressing a function button will enable the “Silent mode” and the button will be colored yellow; this suppresses the beeping at the end of the step.  The oven fan will come on automatically based on the selected function, though it can be overridden.  Each function also has an associated fan speed, which can be overridden.  Each function has an associated time and temperature which is remembered from previous usage.  These can be changed by moving the slide controls. 

The handle on the sliders display the set points for the temperature and duration, while the current temperature and the elapsed time are indicated by the red column.  The sliders can be modified at any time during operation.

Toast and Bake activate one upper and the lower heating elements.  Broil and Air Fry activate both upper heating elements.  The table of functions is shown below.

Function	Fan	Usage	Elements
Toast	Off	Max heat from above and below	Lower + Upper in
Bake	Off	Steady heat from above and below	Lower + Upper Out
Warm	Off	Gentle heat from above and below	Lower + Upper Out
Broil	Off	Max heat from above	Upper Out + Upper In
Air Bake	Low	Convection baking	Upper Out + Upper In
Air Fry	High	Convection frying	Lower + Upper Out
Air Dry	High	Dry meats and vegetables	Lower + Upper Out
Preheat	Off	Reach set temperature, up or down	Lower + Upper Out
Pause	Off	Wait for specified duration or time of day	None
Grill	Off	Heat from below	Lower

 A sequence of steps can be scheduled using the program buttons on the lower blue panel. 

Swiping left or right will lead to the “Custom” view.  This view allows for alternative settings.  It supports three main heating modes: Bake (heat from upper and lower elements), Broil (heat from upper elements only) and Grill (heat from the lower element only.)  The fan can be set independently to two speeds.  Swipe left or right to go to the Standard view.  This view allows programming multi-step sequences of functions.  Touch any function button, the selected function changes color.  The “Custom” button changes to “Start Step n”.  Select the temperature and duration and then touch Start.  The Start button becomes the Stop.

The new functions are driven by the control panel shown below.  It will be displayed on the LCD screen and on the browser.  The dial thermostat will be ignored while the panel is being used.  The buttons on the panel will change color when each is activated.  The light will come on while any function is active, including Pause. 

It allows the device to be programmed.  Each program can have multiple steps. 

· Pause allows a waiting period specified by the time control.  It can be used for Delayed starts.  However, the device can also be programmed and started remotely.

· Cool allows a waiting period till the temperature drops to the set point

· The Start button starts the program.  It will change to Pause when the device is running.  Pressing pause again will cause the toaster to stop.

· Step settings, like temperature and fan speed, can be changed while the program is running.

· Pressing the Next or Previous buttons while the program is running will display settings for that step.  Pressing the Start button will cancel the current step and start the displayed step.

· This is not your granny's toaster.

· There will be ways to save the programs for later use and to restore them.

· Long-press on the “Standard” button to display the configuration screen.  Set the time zone, preferred language, temperature scales, mobile number for SMS, buzzer tones, passwords, etc.

Add 5 mins to current step button

Software

The software (Python/JavaScript) is available in GitHub.
<<<tbd>>>

Conclusion

Given the temperature control and programmability, this gadget would be suitable as a solder reflow oven.  However, the temperature rise and fall rates may be slow at about 0.8°C/sec.