Well, cannot see your circuit, but from others comments surely if you wanted
it to trigger at a temperature you would need it to suddenly come on at full
brightness not just come on slowly as the bias is altered.
I'd have expected some feedback in the system to make it happen like when
you need to make a relay go on hard.Schmidt Trigger or an IC that does
similar job.

Sorry I cannot spell today.
Brian

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This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"JoeJoe" wrote in message
o.uk...
See he

http://tinypic.com/r/qsqp1z/9

Question is: why would you want to put a variable resistor between Z and
Y, and not a fixed value one?

I could do this in my sleep years ago, honest...