Parts

• Severin T0O2034 1500W Oven
• Fotek SSR-10DA Solid State Relay
• MAX31855K cold junction compensated thermocouple reader, K-type
• HCPL-092J 1/3 bi-directional galvanic isolator and level converter for host serial port.
• RGB backlit LCD display, multiple line/row options in this range of displays, but custom made for adafruit :-/.

Design

Oven will be gutted of electronics and existing front panel. An Atmel 328P board will be substituted and will drive the new front panel and heater via an SSR.

The 328P will run at 3.3V in order to interface directly with the MAX31855 thermocouple reader.

The Host serial port shall be implemented with USB-B socket on the front panel, internally an FTDI or alternative USB to serial adapter will run on 5V supplied by the host and shall interface with the 3.3V 328P via the HCPL-092J galvanic isolator. The FTDI will not be powered when host is not present.

The 3.3V rail shall be derived and isolated from mains using a simple transformer and liner regulator arrangement.

The front panel will consist of a display of some sort, a rotary encoder, the USB-B socket and an earth jack. Probably mains switch for all input power.

There shall be an audible alarm indicating that it is time to open the door to cool down, this may be linked to a door switch.

Toaster Oven Teardown

What remains is some wiring and four elements wired in 2S2P, with the top and bottom elements having their own neutral wire and common live wire.

Initially we will put the SSR on the live wire and common the neutrals to control all four elements together.

Initial Performance Testing

The SSR works well and remains almost entirely cool, 328P can drive it directly.

The thermocouple's insulation cannot take 250C, replacement good to 350C ordered.

White wires are live, red are neutral, the SSR is currently on the neutral side of the elements.

Software

go here