Project Goal & Requirements

Develop a low-cost, open source home automation system using off the shelf components.

To promote interoperability, an xAP bridge should be implemented to allow standardised automation software, such as MisterHouse, to interact with the modules.

Current Status



Required Modules

  • Tap Timer
  • 4 Channel (up to 4 channels) 240V/2A Mains Relay Board (use with CFLs), with 4 switch inputs
  • 4 Channel (up to 4 channels) 12V Relay Board (use with LEDs, Halogens), with 4 switch inputs


4 Channel Mains Relay Board

Current Requirements

  • XBee module 3.3V  50mA
  • Relay coil current 6V 4 x 8mA
  • Switch current 3.3V 4 x 1mA

Our power suppy must be able to deliver 60mA at 3.3V, and 40mA at 6V, so we need to pull 100mA from mains. At first glance, this would mean we would be pulling around 24W, but we are a capacitive power supply, so the real power we are drawing is a lot less (we have a low power factor). We could correct for this, but since we are such a low power device, we don't significantly distort the voltage waveform. Indeed, our capacitive suppy is (slightly) correcting for the inductive switchmode PSUs in CFLs and halogen transformers.

Following from the PSU brief:

Irms = 220 . 2 . pi . 50 . C

C = 100mA / (220 . 2 . pi . 50)

C = 0.00144686311mF

C = 1.45uF

Since we only get half the current as we will be rectifying it, we must double the capacitance.

C = 2.9uF

So a 3.3uF 450V capacitor will do for the PSU.