Ingredients list

Hardware required for the Silvia-Pi build includes: 1x Raspberry Pi 2 1x Raspberry Pi 7-inch touchscreen 1x Rancilio Silvia (or any coffee machine that could do with better temperature control) 1x Solid State Relay (SSR) 2x Double Pole Single Throw (DPST) relays 2x transistors & diodes (for the driver circuits on the DPST relays)

1x integrated power supply (your country’s input V AC in, 5 V DC out) 1x k-type thermocouple 1x thermocouple amplifier (1-wire MAX31850K)

Why are you using an SSR? Why don’t you use a mechanical relay to control the boiler? Well… This is because of the potentially high switching rate of the controller. In actual practice the switching rate is relatively low, and some low-end machines do use mechanical relays. However, mechanical relays typically fail open. SSRs typically fail closed. This is definitely something to keep in mind when thinking about safety on your application. In addition, mechanical relays are only good for a specified number of cycles. What’s more, mechanical relays make a noise, while SSRs are quiet. For this application a good SSR to use is the Kudom KSI240D10-L SSR: 10A 240V AC, 4–32V DC.

The K-type thermocouple temperature range is typically between –250ºC to 1250ºC, and accurate to ± 1ºC. For signal processing purposes the K-type is easy to accommodate. There are numerous varieties of Integrated Circuits that are packaged with amplifiers, filtering, cold-junction compensation and analogto-digital converters that are specifically built for the K-type thermocouple and are low cost too. Because of this the K-type is perfect for this coffee machine application.