On Fri, 16 Dec 2005, Alan B. Pearce wrote:

>> I'm starting to think that this is getting a
>> bit overkill, seeing as Peter's bridge-rectifier/
>> opto seems neater..
>
> I know that this arrangement is used to detect the presence of a train in a
> block on model railways.

Yes except it works well mostly for low loads (say <1A). Beyond that it 
starts making a lot of heat for a small and unobtrusive circuit it is 
supposed to be. A Hall current sensor is not so expensive by the time 
you need to measure go/no go 7A and it has to take the inrush of 
presumably 40-50Aish gracefully (i.e. even if the filament decides to go 
just then). I'd still look into a simple thermistor siliconed to the 
insulated side of the triac with extra shrink tubing to increase 
insulation (you are using a triac, no?).

Peter
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