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Step 13The DS Auto Switch
So we want a switch that turns the DS on automatically when we start up our power tool, and then turns it off again when we are done making dust. Ideally it will switch on just after the tool, so that the starting current of the power tool is not exacerbated by the simultaneous starting of the dust extractor. And when the power tool turns of, we want the DS to stay on for a few seconds so that the hoses are cleared of any remaining dust.
There are a load of different ways to approach the auto switch circuitry. Here, for example, is an auto switch that uses commercial current detector. The cheapest I could find the toroidal sensor component for this was for $50 so this was out as far as I was concerned. I wanted to make it without buying anything, using the odd bits and bobs I had knocking about, so my design was a bit, hum, unusual. There are a load of alternative ideas for this in the resources section at the end.
If you want to try out my design, instead of using a current transformer, which seems the standard approach, we use a reed switch, activated by a very small coil in the live power cable that supplies the power tool. The reed switch activates a relay, which in turn energises the heavy duty contactor, which is hefty enough to cope with switching both vacuums on or off at once. If you want to do it this way follow the circuit diagram below, nothing is too complicated, expensive or difficult.
To make the coil, just wrap some 16 AWG (or fatter) magnet wire round something thin that is a similar size as the reed switch. I used the blank end of a drill bit, but be careful not to scratch the enamel insulation (something plastic or wooden is better). To begin with I didn't even use magnet wire, just standard insulated wire, as you can see in the pics, and it still worked OK. This way is not as sensitive though, because of the insulation gap. So if you want to use it with lower current draw tools as well, magnet wire is better. About 13 turns is all you are likely to fit on the reed switch - that's fine.
The capacitor bank in the 6V relay circuit adds a delay to the switch, so that the DS stays on for a few seconds after the power tool is turned off. Having a capacitor bank like this though, means that we need to add a resistor (or around 8KOhm) in series with the reed switch, to protect it from the inrush current when it is switched on (without it the reed switch will weld shut). If you wanted to be a bit more elegant you could put together some kind of 555 time latch circuit, but I didn't have any 555s to hand.
If you just want a very quick and dirty solution, just having the reed switch activate the AC contactor worked OK when I tested it. You will not get any delay, and the motors will start together with this one, but it is very simple and it works (though how long the reed switch would last I can't say).
Components (circuit diagram below click the 'i' in top left to get full size):
Reed Switch - just one of a bunch I had laying round. It is about 1" in length, glass body, the coil goes tightly round this.
D1 - rectifying diode
D2 - rectifying diode
TR1 - a small step down transformer (to 6V) - time to use one of those 'wall warts' you have been saving.
C2 - 6.3V 4700uF (but just use what you have in your scraps box)
C4 - 6.3V 4700uF
C5 - 6.3V 4700uF
DC Relay - 6V DC relay, a smallish low current thing is what you want.
Contactor - Heavy duty contactor, I found this on a thrown out saw - these are useful for NVR applications.
R2 - 8.2KOhm resistor, important for protecting the reed switch.
C3 - a 0.22uF 275V AC capacitor
R1 - 330 Ohm resistor
B2 - a suitably beefy bridge rectifier
There are a load of different ways to approach the auto switch circuitry. Here, for example, is an auto switch that uses commercial current detector. The cheapest I could find the toroidal sensor component for this was for $50 so this was out as far as I was concerned. I wanted to make it without buying anything, using the odd bits and bobs I had knocking about, so my design was a bit, hum, unusual. There are a load of alternative ideas for this in the resources section at the end.
If you want to try out my design, instead of using a current transformer, which seems the standard approach, we use a reed switch, activated by a very small coil in the live power cable that supplies the power tool. The reed switch activates a relay, which in turn energises the heavy duty contactor, which is hefty enough to cope with switching both vacuums on or off at once. If you want to do it this way follow the circuit diagram below, nothing is too complicated, expensive or difficult.
To make the coil, just wrap some 16 AWG (or fatter) magnet wire round something thin that is a similar size as the reed switch. I used the blank end of a drill bit, but be careful not to scratch the enamel insulation (something plastic or wooden is better). To begin with I didn't even use magnet wire, just standard insulated wire, as you can see in the pics, and it still worked OK. This way is not as sensitive though, because of the insulation gap. So if you want to use it with lower current draw tools as well, magnet wire is better. About 13 turns is all you are likely to fit on the reed switch - that's fine.
The capacitor bank in the 6V relay circuit adds a delay to the switch, so that the DS stays on for a few seconds after the power tool is turned off. Having a capacitor bank like this though, means that we need to add a resistor (or around 8KOhm) in series with the reed switch, to protect it from the inrush current when it is switched on (without it the reed switch will weld shut). If you wanted to be a bit more elegant you could put together some kind of 555 time latch circuit, but I didn't have any 555s to hand.
If you just want a very quick and dirty solution, just having the reed switch activate the AC contactor worked OK when I tested it. You will not get any delay, and the motors will start together with this one, but it is very simple and it works (though how long the reed switch would last I can't say).
Components (circuit diagram below click the 'i' in top left to get full size):
Reed Switch - just one of a bunch I had laying round. It is about 1" in length, glass body, the coil goes tightly round this.
D1 - rectifying diode
D2 - rectifying diode
TR1 - a small step down transformer (to 6V) - time to use one of those 'wall warts' you have been saving.
C2 - 6.3V 4700uF (but just use what you have in your scraps box)
C4 - 6.3V 4700uF
C5 - 6.3V 4700uF
DC Relay - 6V DC relay, a smallish low current thing is what you want.
Contactor - Heavy duty contactor, I found this on a thrown out saw - these are useful for NVR applications.
R2 - 8.2KOhm resistor, important for protecting the reed switch.
C3 - a 0.22uF 275V AC capacitor
R1 - 330 Ohm resistor
B2 - a suitably beefy bridge rectifier
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http://www.leevalley.com/en/wood/page.aspx?p=63013&cat=1,240,41065
$45CND, nice package, no fuss.