Shop Vac Auto Switch (no Arduino Needed)

As many hobbyist woodworkers, I have a shop vacuum attached to my table saw and each time I want to perform a cut I have to turn it on before I turn the saw on. This may look silly but it's a pain in the neck to turn the shop vac on and off as many times as the table saw.

There is an existing solution to that out there: a "Shop vac automatic switch". This is a device where you plug your table saw and your shop vac. When the master device is turned on (the table saw in this case) it lets power flow in the slave device (the shop vac).

You'll notice that there are lots of DIY projects to make that auto switch by yourself. You'll just need a current sensor, a relay and an arduino. ... Wait, using an arduino to perform such a simple control ... wouldn't it be like using a bazooka to kill a fly? Maybe.

In this ible I propose a simple, yet effective, way to build yourself that same device, without the need of an Arduino!

Disclaimer: I am not an electronic engineer and for sure the circuit I designed can be optimized. Please don't hesitate to post a comment :)

Step 1: Supplies

You will need the following components:

an enclosure with at least 2 female wall plugs and one male connector (I upcycled an old "power filter");
one ASC712C current sensor module;
one relay module;
one comparator (I used a MAX903);
several resistors: 330Ω, 4.7kΩ, 2 x 1kΩ;
one potentiometer (any value will do);
two 470µF electrolytic capacitors;
one NPN transistor (the famous 2N2222 will do);
one 5V DC power supply (I upcycled a phone charger);
one small perfboard (you can also print your own circuit);
solder, electrical tape, shrink tubing, wires etc.

And some basic tools:

soldering iron;
pliers;
etc.

Step 2: Prepare the Enclosure

I decided to upcycle a "power filter" because the enclosure is the right size, it has already 4 power outlets, one inlet and a rocker on/off switch.

I first removed all the useless electronic from the inside (I tossed it in the "might be useful later" bin).

Gave it a bit of cleanup.

Changed the inlet plug with one having higher wire gage.

Step 3: AC Wiring

First prepare the 5VDC power source, solder a pair of 18 gage wires to the inlet of the wall adapter and protect the connections with shrink tubing or electrical tape, cut the phone connector and peele the end of the wires, mark the positive one.

Make sure the N wire of the enclosure's inlet is connected to the N of both slave outlets, master outlets and to the 5VDC power source. Also make sure the GND wire of the inlet is connected to both grounds of every outlets and to the metal case. The L wire of the inlet needs to be split into 3 wires: one will go to the current sensor, one to the relay and one to the 5VDC power source.

A wire needs to come out of the master outlet's L, later on we will connect it to the current sensor

And a wire needs to come out of the slave outlet's L, it will be connected to the relay.

I used the existing rocker switch to add a feature: manual override. It will allow me to turn on the slave outlet manually whenever I need it. It is connected in parallel with the relay.

Step 4: The Theory

According to the datasheet, the current sensor ACS712C outputs 100mV/A having VCC/2 representing 0A.

Since we are working with Alternating Current (AC), and VCC is supposed to be 5V, the sensor will give us a 60Hz sine-wave voltage centered on 2.5V with an amplitude proportional to the current drawn by the master appliance.

To be able to convert that signal into an action we need a few steps:

compare the voltage with a reference, for that we will use the comparator MAX901 and the reference will be given by a variable voltage divider (a potentiometer). The output of the comparator will be 0V when no current sensed and a 5V 60Hz square wave otherwise;
convert the square wave into an almost-linear curve using a first order RC filter;
smoothen even more the "almost-linear curve" with a second order RC filter;
negate the signal with a NPN transistor (NOT function) because the relay module is active when it's input is low (0V).

I have set quite high RC values intentionally because they are going to perform a desired effect: a delay. In this situation, the relay activates a bit more than a second after a current is sensed, and it deactivates the same amount of time after no current is sensed.

Think about when you turn on a powerful machine such as a table saw, during the time the blade goes up to speed it draws the full amount of power. It is better to wait for the blade speed to settle, and the consumption to go down before starting up a second heavy engine like the shop vac, this way you reduce the chance of overloading your AC circuit.

And, when we turn off the table saw, it is preferable to have the shop vac working a bit more time to suck all the remaining dust.

Step 5: Build the Circuit

You can test the circuit on a breadboard if you like.

Soldering the components shouldn't represent a big challenge.

Connect everything together, the board, the sensor and the relay and turn it on. It is important to set the correct reference/threshold value for the comparator by rotating the potentiometer until the very moment the relay goes off (make sure no appliance is connected to the master outlet). This way you "let the comparator know" when it can "consider" that no current is being drawn.

Test it: connect an appliance to the master outlet (a hand-drill for instance) and another to the slave outlet (a desk lamp with the switch turned on for instance). Run the master device, one second after the slave device should turn on.

If it's not working as expected you can try to troubleshoot with a voltmeter. Assumption: you are powering the circuit with 5VDC.

Test	Expectation
Test	when master is off	when master is on
Voltage between the "IN -" (reference/threshold) and the "IN +" (output of the current sensor) of the comparator	0.00V	> 0.00VAC (voltmeter in AC mode)
Voltage between GND and the output of the comparator	0.00V	2.50VCC (voltmeter in CC mode)
Voltage between the output of the first order RC filter and GND	0.00V	> 0.00VCC
Voltage between the output of the second order RC filter and GND	0.00V	> 0.00VCC
Voltage between the input of the relay module and GND	5.00VCC	0.00V