Spec

1. The On/Off duty cycle should be continuously variable between 0% and 100%
2. The period should be switchable between a very short time and quite a long time

I chose, arbitrarily, the shortest period to be 5 seconds. The Electrim bin has a 2kW kettle element in it and, although I used a 16A relay with a life expectancy of 10⁷ operations, it seemed to me that any shorter period would be a bit frenetic.

I bought all the bits from BitsBox and they had a nice 12 position switch which I felt compelled to fill up with options; so I have:

• 5 Seconds
• 7 Seconds
• 10 Seconds
• 15 Seconds
• 20 Seconds
• 25 Seconds
• 30 Seconds
• 40 Seconds
• 50 Seconds
• 60 Seconds
• 70 Seconds
• 90 Seconds

I've never used anything other than 5 Seconds!

3. An override switch to supply continuous power to the boiler. This is just in case the 100% duty cycle setting is a little bit less than 100% which would be a waste of Relay life.
4. An indicator for the duty cycle
5. An indicator for the override