I built one using exactly the same principle 9 months ago. I reported on it here.
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forums.reprap.org]
I have used off the shelf parts using an Aico Ei141 ionisation type smoke alarm with an integral Ei128R base unit that houses a switching relay. A latching relay is required for the circuitry and this is housed in a separate box in exactly the same way.
Does anyone know if he is a member of this forum?
