Yoghurt making is easy: Heat milk to 85°C, let cool down to 42°C, add yoghurt culture (any available yoghurt), and keep it all by 38°C for seven or more hours. You will find plenty of more details on the internet.
For keeping the 38°C for 7 hours, little yoghurt machines exists. However, they support only up to one litre. That is not enough for families.
A light bulb provides enough heat to keep an thermo box heated to 38°C (and more). To prevent overheating, a raspberry pi together with a thermo sensor is used as closed loop controller. Together, the temperature can be kept constant by a margin of less than 0.1°C
Step 1: What You Need
- A light bulb (not energy saving, something like 25W is perfekt)
- A solid state relay, e.g. http://www.amazon.de/Solid-State-Relais-Halbleiterrelais-24-380V/dp/B009SXHJD6/ref=sr_1_2?ie=UTF8&qid=1451221337&sr=8-2&keywords=solid+state+relay
- A thermo sensor type DS18B20
- Resistors: 4 x 500 Ohm
- LED Green and red
- Transistor 2N3904
- Some cables – what ever you find in your home
- A raspberry pi – any model will work
- A thermobox - the size that fits your needs
Step 2: Assembly – the Box
For better stability, I glued a board on top and on bottom of the lid. Two holes bored for cables. The inside of the lid got a bulb holder.
First cable connects the light bulb. Second cable brings the thermos sensor inside the box.
Some cushions inside the box are useful if you do not use all available space.
Step 3: Assembly – the Electronics
I use as temperature sensor the DS18B20 which is sealed in a waterproof head/cable. The three wires are connected to ground, 3.3V, and an available GPIO pin from the raspberry. Best work yourself though this tutorial: https://learn.adafruit.com/adafruits-raspberry-pi-lesson-11-ds18b20-temperature-sensing/overview
The solid state relay needs some more juice than the 3.3V, therefor an additional transistor is used to beef that up (see http://www.elinux.org/RPi_GPIO_Interface_Circuits#Using_an_NPN_transistor ).The high voltage side of the solid state relay is connected to power and an outlet. That outlet is connected to the bulb inside the thermo box.
For details, best see for the wiring diagram.
Step 4: Assembly – Software and Operation
As minimum we use two python scripts to control the temperature in the box by switching the lamb bulb on and off (thermostat.py – control loop which sources readTemperature.py – that reads out the sensor).
After assembly, start up sudo python thermostat.py and after a couple of minutes, the temperature inside the box will have reached 38°C. The more stuff inside the box, the more constant the temperature will be.
Dear reader, please provide feedback if you need more details on any step. I am not familiar with the level of expertise of the audience here and might be too short on details sometimes.