How to make a momentary latching switch – House of Hacks

Sunday, January 29, 2017

How to make a momentary latching switch


Description

A pair of momentary switches become a latching on/off switch as Harley expands on a previous video about remote controlling a shop vac. This is the first of several in a modular switching system to remote control shop equipment using the PowerSwitch Tail II.

The central part of this system is the PowerSwitch Tail. It contains an electronically controlled switch to turn things on an off. There are a large number of ways to control this. In this episode, we introduce a modular system to allow different types of switches to be used to control the shop vac (or any other type of appliance).

PowerSwitch Tail II (Amazon affiliate link)

How to easily make a low-voltage, remote shop vac switch

For a written transcript, go to House of Hacks: How to make a momentary latching switch

Music under Creative Commons License By Attribution 3.0.
Intro/Exit: "Hot Swing" by Kevin MacLeod at http://incompetech.com
Special effects: livingroom_light_switch by AlienXXX at http://freesound.com

Transcript

Last year I showed an easy way to remote control shop equipment using a PowerSwitch Tail, a couple batteries, a switch and some wire.

Today at the House of Hacks I’m going to show how I made a push-on/push-off switch that mimics the way a lot of shop equipment are controlled.

[Music]

Hi Makers, Builders and Do-it-yourselfers. Harley here.

Just a quick reminder, if you haven’t done so already, subscribe to the House of Hacks channel to get notified of future videos.

Last year I made a video responding to a comment by Rob about how I made the remote control switch on my central shop vac system.

In that video, I showed the core design element: the PowerSwitch Tail and how to use it with a simple battery operated switch.

Today i’m going to show a different way to control the same PowerSwitch Tail by eliminating the batteries and using a switch with two buttons: one to turn the tool on and one to turn it off.

This is similar to how many shop tools are controlled. It also has the additional feature of being able to be expanded upon in the future.

If you recall, the PowerSwitch Tail requires 3 to 12 volts DC applied to these two connectors to cause the tool to turn on.

Batteries are of course one source of power for this but they need to be replaced on occasion.
Since I didn’t want to deal with replacing batteries, in my application I decided to use a surplus wall wart style power supply. I had a bunch of these lying around and figured this would be a good application for one of them.

I plugged it into the same outlet I plug the PowerSwitch Tail into.

I connect the low voltage power supply to two connectors on an RJ-11 jack.

Then I connect the other two connectors on the RJ-11 jack to the two connectors on the PowerSwitch Tail.

This allows me to use a phone wire as an extension cord.

For the switch's end, I put another RJ-11 jack in a project box. This project box can now have any type of switch mechanism in it I want and provides a nice modular way to use different types of switches.

For example, I could put in a toggle switch just like I showed in the last video.

Simply wire the negative side of the power to the negative input on the PowerSwitch Tail and wire a switch between the positive side of the power and the positive input for the PowerSwitch Tail.

However, since we have power in the project box, we aren’t limited to just a simple mechanical switch.

We can build circuitry that controls the PowerSwitch Tail.

The first thing I’ve made is a simple latching switch.

Similar to the switches on many tools, like my drill press and my bandsaw, I press the green button to turn on my vacuum and push the red button to turn it off.

Inside the box is a simple flip flop.

A flip flop is a type of circuit with two inputs, called Set and Reset. It also has two outputs, called Q and bar Q, or also known as not Q. It’s just the inverse of Q.

The inputs receive momentary pulses.

If the pulse is on Set, then Q goes high and bar Q goes low.

If the pulse is on Reset, then Q goes low and bar Q goes high.

If we consider just one output, Q, we can see Set causes it to turn on and Reset causes it to turn off. It just flip flops between the two positions.

Flip flops can be made with a variety of different circuits ranging from discrete components to various types of integrated circuits.

I happened to have a Quad 2-Input NOR gate chip in my parts bin so I used that.

But I could just as easily have used NAND gates, a chip with a dedicated flip-flop circuit in it, or a couple of transistors and resistors.

Once I had the circuit built, all I had to do was put it in the box and wire it up.

The switches are wired with pull down resistors. This allows the inputs to be normally low and go high when the button is pressed.

The green button connects to the Set input. The red button connects to the Reset input.

The negative input to the PowerTail Switch goes to the negative power connector.

Since I’m switching the positive side of the power, I’m using a PNP transistor.

Its base connects to the flip-flops Q output.

The PowerSwitch Tail’s positive input goes to the transistor’s collector.

And finally, the transistor’s emitter connects to the positive power connector.

In this configuration, the transistor acts as the switch for the PowerSwitch Tail’s power.

When it’s all put together, pushing the green button turns on the appliance and pushing the red button turns it off.

Since this switch system is modular, I have plans to build other switches too.

The next one is a current sensing switch so the vacuum will automatically turn on when a tool is in use and will turn off, after a short time delay, when the tool is turned off.

I’d love to know in the comments below if the level of detail I presented here was too much, just right or too little.

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Sometimes this manifests through making things with a technical or mechanical bent.

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