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I made the circuit here to automatically switch on my workshop dust extractor when I use a power tool. It works fine when used with a drill press or router. However with a Dremel 4000 it only switches on when the Dremel is at its highest speed. I imagine this is because the Dremel has an electronic speed adjustment.

Is it possible to modify the circuit to work with the Dremel or should I look at using a different approach - perhaps using a current sensing transformer?

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It is not functioning for the Dremel tool because the Dremel is too small of a load.

For a larger load, the four diodes in the schematic allow about 1-2v of the AC mains into the transformer, which is wired up "backwards" intentionally, to boost this to >5v or so, so the solid-state relay can turn on. But the Dremel is such a small load, that insufficient electricity is making it through the transformer to "turn on" the solid-state relay.

Component values could be changed to allow this tool to trigger the switch, such as by changing the transformer or diodes used, but this will provide too much voltage to the relay when using larger loads, and could damage it. Additional design work could be done to mitigate over-voltage, but then it's no longer a simple and elegant solution and was basically redone from scratch.

As an easy fix, plug a work lamp and the Dremel into this together, and it should work as intended with no modifications.

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    \$\begingroup\$ Although now it will be the work lamp and not the Dremel which causes the switch to operate. \$\endgroup\$ – WhatRoughBeast May 19 '15 at 17:39
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    \$\begingroup\$ Thanks @rdtsc - that makes sense. I've worked around my problem by making the extractor the master and the Dremel the slave. So turning on the extractor starts the Dremel. Works well for me because the extractor switch is easier to get to when using the Dremel with its flexible drive shaft. \$\endgroup\$ – Keith Miller May 20 '15 at 7:31

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