Back in November of 2014 I posted a project showing the system I had made to control my shop-vac for automated dust collection. You can see the project here:
After posting, there were several people asking me for the schematics so they can build their own. So instead of only posting the schematics I drew up, I though it would be a good idea to provide a little additional explanation for everyone else that might not have as much experience with electronics. I don’t believe its a complicated design and there is no special code to write. Its a pure analog circuit that I hope can help simplify the use of your dust collection system.
Now for two disclaimers, one serious and the other not as much…..
- Here it is, you knew it was coming…...Danger: Hazardous voltage 120 Volts not only startles you but can kill you. The sole purpose of this system is to turn on and off your dust collector. It does this by applying 120VAC and hence there are several parts of the circuit that are HOT with 120 volts. If you do not feel comfortable with building these types of circuits or know general practice of working with electricity I would not recommend you attempt to build this system.
- Now the second disclaimer, I am an engineer and this is my first blog post. I am good with numbers and not so much with words. So I apologize up front if I am not explaining something clearly or well enough. I always despised writing papers in college and always preferred the math proofs or diff eq side of things. So with that said, I will try to be as detailed as possible but please tell me if I didn’t explain something enough.
Update on the system. Since the project posting, I have moved into a new house and have since purchased a new Powermatic PM1300TX dust collector. It is worlds beyond the shop vac in terms of power and suction but sadly due to its infrared remote control and timer box, it isn’t compatible with my automatic dust collection system. Once I complete this write up of the first system my plan is to design a second version that can integrate into the control box of the PM1300tx and control by means of 220v.
Concept of the System
What we want to do is turn on the dust collector when a tool turns on. It may be a black box to some, but if we start at what we know and break it down its not too mysterious. So first we must determine the inputs and outputs to our system. The inputs are to “know” when the tool turned on and the output is to turn on the dust collector. Sounds simple right. The first part, “knowing” when the tool turned on can be accomplished by detecting the current that is drawn when the tool is operating. That is the one “flowing” quantity of electricity that is somewhat easily detected. The second part, is to apply power to the dust collector. This is accomplished by using a relay to apply 120v to the dust collector. Just like when you press the Green “ON” button on the shop vac, the relay applies 120v to the shop vac motor.
For the input to the system, detecting when the tool is turned on, I decided to use a hall effect current sensor package which can be found on amazon for about $10. The IC part number is ACS712 and looks like this:
Tip: I recommend the sensor that has screw type terminals instead of compression pin terminals. They are much more secure and carry the current more safely.
I wanted a voltage that is proportional to the current being drawn by the tool. The current sensor is just a little IC that does exactly that by means of the hall effect. Read more here if you’re interested in the details. The sensor is bi-directional and therefore can measure current in both directions. We only need measurements in one direction but this has to be taken into account to interface to the next step…the “processing” of the signal. The output is 0-5 volts DC and is proportional to the current being drawn from -30 to 30 amps. So when the tool is off and no current is being drawn it will put out 2.5 volts. Alternatively when 30 amps is being drawn it will output ~4.5 volts and when -30 amps is being drawn it will put out ~0 volts. And in between those values the voltage output follows a linear path. See below graph (snap shot taken from ACS712 data sheet) :
For the output of the system, where we apply voltage to the shop vac, a simple electro-mechanical relay will due the trick. A relay is just a electronically controlled switch. Read more about them here. I happened to have a Omron relay from my electronics stash which worked perfectly. The part number I used was G2RL-1ATP5-E. It has a 12VDC coil and contacts that can handle 16A at 250VDC. Looks like the following:
The way it works is if you apply 12VDC (or whatever the coil voltage is) to the coil, the contacts are connected together. When no voltage is applied to the coil, the contacts are not connected. Therefore we can apply voltage to the shop vac motor by applying 12 volts on the relay coil.
At this point we know when the tool is on and how to turn on the dust collector but there is still a bit of “processing” we need to do to interface the input to the output. That will come in the next post. I’ll post schematics in the end tying it all together. For all those who made it this far, thanks for reading!