Bill of Materials

External 5V Power supply

Minimum 1A (= 5W)

Why not an internal Power supply?

• I can't build a high quality AC/DC power supply for the price of a comparable commercially readily available product.
  You can use any type, just make sure it's got protection features and a regulated output. A cheap unregulated mains plug adapter probably won't work.
• The power supply is easier to replace or reuse, if it's not all on the same PCB board.
• You can decide to operate the Protection Relay from a DC source - simply use a DC/DC power supply.This might be the case if motorized water valves should close if there is no AC supply for whatever reason.
  

Examples

http://www.conrad.de/ce/de/product/514428/Dehner-Elektronik-SYS-1319-2005-T3-Tischnetzteil-5-VDC-4-A-20-Watt/2110212&ref=list
(Don't make the DC cable too long)

If there's a power socket nearby, you can use this:
http://www.conrad.de/ce/de/product/512723/Dehner-Elektronik-SYS-1381-1005-W2E-EURO-Steckernetzteil-Steckernetzgeraet-5-VDC-2000-mA-10-Watt/2110120&ref=list

If you like to include the power supply in the main case, use something like this:
http://www.conrad.de/ce/de/product/512731/TDK-Lambda-LS25-5-Einbau-Netzteil-5-VDC-5-A-25-W/2110485&ref=list

Solid State Relay SSR

Why not use internal triacs? With an earlier version, I used internal triacs, but they need to be mounted on a heatsink, and the heatsink carries line voltage, and if they fry, it's a pain to replace them.
External SSRs are much easier to handle: easily replaceable for different sizes of dump loads, no external parts carrying line voltage.
Just make sure you use SSRs with high voltage isolation between control voltage and switch voltage. Types with an LED are good for installing and monitoring, because you can see their switch state.

Zero crossing types are fine. With Random Turn-On Solid-State Relays, also known as “asynchronous” or
“instantaneous” types, you can control the frequeny a bit better (phase angle control), but at the price of distorting the waveform and producing HF noise, especially with low power systems combined with big dumploads.
Here's a pdf explaining the difference between zero crossing and random types
Alternative link

Also make sure you use a heat sink, if you want to use the SSR to its full rating.

The ELC outputs a control voltage of 4V, max. 300mA. So any SSR with a control input of 4V DC or less is suitable.

Ebay carries loads of them.
SSR example     Heat sink example 1   Heat Sink example 2


Case - suitable for a 160 x 100 mm PCB Printd Circuit board
Hammond Electronics BOXE


Microcontroller Unit MCU : PIC16F1939
http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en538148