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Touch Sensitive Power on switch

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Voltz

Voltz

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I was wondering about how I would go about making a switch which is turned on when something (a hand) is passed over it, I initially thought about using an inductor and when it's field is distorted it powers on, but I wasn't sure if a hand would distort the magnetic field enough, any other ideas? (I plan to use it on a scratch build of a computer case)
 
Attached is the circuit I used for the display. T1 was connected to a aluminum foil sheet on the back of a sign on the inside the 1/4" thick glass display window. D4 is the LED in a solid state relay that controlled a motor. SW1 allows you to select a "Touch ON/Adjustable Timed OFF" or a "Touch ON/Touch OFF" mode. Putting you hand up to the glass at the sign turned on the display.

Ken
 

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Moyo2k said:
I was wondering about how I would go about making a switch which is turned on when something (a hand) is passed over it, I initially thought about using an inductor and when it's field is distorted it powers on, but I wasn't sure if a hand would distort the magnetic field enough, any other ideas? (I plan to use it on a scratch build of a computer case)
Click to expand...

When you say you plan to use it on a scratch build of a computer case I will venture a guess that you want to use it to turn a computer on? Should that be the case then the attached circuit may work for you.

The circuit uses a simple 555 configured as a one shot with an output duration of about 1 second. That would simulate the momentary push of the front panel power button. The circuit would be powered off the computers 5 volt SB (Stand By) power line. The circuit relies on stray AC (50 or 60 Hz.) in the air. Sometimes it will work fine by just touching the lead connected to pin 2 of the 555 or better like drawn bridging pin 2 to ground. You make the touch pad.

If something like this is what you have in mind for a PC I'll provide more details on the connections.

Ron
 

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Reloadron said:
When you say you plan to use it on a scratch build of a computer case I will venture a guess that you want to use it to turn a computer on? Should that be the case then the attached circuit may work for you.

The circuit uses a simple 555 configured as a one shot with an output duration of about 1 second. That would simulate the momentary push of the front panel power button. The circuit would be powered off the computers 5 volt SB (Stand By) power line. The circuit relies on stray AC (50 or 60 Hz.) in the air. Sometimes it will work fine by just touching the lead connected to pin 2 of the 555 or better like drawn bridging pin 2 to ground. You make the touch pad.

If something like this is what you have in mind for a PC I'll provide more details on the connections.

Ron
Click to expand...

Sounds about right, but here's the main problem, the scratch build of the case will be totally smooth (styled minimalistically) which is why I don't want to use a normal PC push button switch, so the switch will need to work through about 15mm of steel or so, basically I want to be able to brush my hand over a particular part of the case (which the switch will be under) and have the thing power on
 
Moyo2k said:
Sounds about right, but here's the main problem, the scratch build of the case will be totally smooth (styled minimalistically) which is why I don't want to use a normal PC push button switch, so the switch will need to work through about 15mm of steel or so, basically I want to be able to brush my hand over a particular part of the case (which the switch will be under) and have the thing power on
Click to expand...

Case modding huh? Good luck on having it below the steel case. Glass or Plexiglass as was suggested but steel is another animal. Another trick I have seen case modders do is a wireless remote using a key fob (like automotive). You got me as to touch or prox with a steel case unless the sensor is exposed.

Ron
 
I've made a really simple touch switch that works using just the resistance of your body for use in my power supply. It uses a 555 with a PNP transistor connected to the trigger pin, when your hand touches the 2 exposed wires that go to the base of the PNP, it applies a negative pulse to the trigger pin so the 555's output goes high.
 
Through metal impossible. I think the hifi people B&O got round this only recently (BC2) by making aluminium so thin that your finger depresses the surface a few um and they are pretty good at capacitive and similar touch sensitive circuits so if they couldn't do it you can't.

Could you not drill a hole and fill with insulated metal 'plug' that would work with 555 circuit above...? At least you will know where to touch! Also you could do something with tiny pin holes and pins maybe?

Maybe optical IR transmit / receive so you don't even have to touch the case just wave your hand in front? small holes required..

I think some smart design work could achieve a compromise between style you want and usability?
 
olly_k said:
Through metal impossible. I think the hifi people B&O got round this only recently (BC2) by making aluminium so thin that your finger depresses the surface a few um and they are pretty good at capacitive and similar touch sensitive circuits so if they couldn't do it you can't.

Could you not drill a hole and fill with insulated metal 'plug' that would work with 555 circuit above...? At least you will know where to touch! Also you could do something with tiny pin holes and pins maybe?

Maybe optical IR transmit / receive so you don't even have to touch the case just wave your hand in front? small holes required..

I think some smart design work could achieve a compromise between style you want and usability?
Click to expand...

Thanks for replies all, appreciating the suggestions so far, I thought about this idea but the problem I thought of is that if something waves infront of it triggering the switch it could become a nuisance turning it on or even worse off...

EDIT: Can anyone expand on how I could use my initial idea (of proximity detection) through plastic, if I can make it work through plastic then I don't mind that, I'll stealth it in some way,

EDIT(2): On the IR idea I've thought about it and it could work, if I position the two IR transmitters far apart then it should prevent accidental activation
 
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Yes you could toe them in which would give quite a steep angle but you would have to experiment I think.

I posted this circuit not so long ago but there is a circuit above that could also work. You will need to mess around quite a bit plastic thickness etc and a PC is a noisy environment.

touchsw..jpg
 
A few things you need to consider in this quest. Obviously starting a PC requires the momentary pressing of a normally open push button switch. How you choose to do that is up to your creativity and what I call case modding 101. :)

Inadvertent shutdown isn't likely as we know how we generally shut a PC down. Momentarily pressing the power button won't initiate a shutdown (depending on configuration) in normal circumstances. Holding the power button in for about 5 seconds will initiate a hard shutdown.

If you go the IR route, maybe consider a reflective transmitter (emitter) receiver (detector) package. Maybe it would be wise to think through the options and what case mod to accommodate what options and move along from there.

Ron
 
Well at the moment I'm leaning towards a IR LED proximity sensor embedded in the metal so that a hand wave in front of it will initiate start up, I can ignore the shut downs issues because its easy to disable the switch shut down in Windows, thanks all for your suggestions, so anyone got any schematics, if the switch is effectively a relay for the +5VSB being supplied to the ATX motherboard then basically all I need is a simple IR proximity detector so that current flows when my hand passes in front of it, this provides a new issue as the light research I've done only shows me IR proximity devices that switch OFF when your hand is passed in front of the LED, any suggestions?
 
Reloadron said:
....The circuit relies on stray AC (50 or 60 Hz.) in the air. Sometimes it will work fine by just touching the lead connected to pin 2 of the 555 or better like drawn bridging pin 2 to ground. You make the touch pad.

Ron
Click to expand...

Ron,

If you are referring to my circuit, it does nor rely on stray AC in the environment. Is works on the added capacitance of the hand to de-tune an internal oscillator in the Qprox chip. It's generally very reliable.
https://www.atmel.com/dyn/resources/prod_documents/qt113_105.pdf

OOPS, I missed your schematic (it was below the screen). Never Mind!. :(

Ken
 
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I'm gonna stick with an IR LED Proximity detector to switch the PC on, so anyone have an schematics?
 
if your after minimal involvemet you can use a touch pad where you place your finger of two contacts to make connection
 
Thunderchild said:
if your after minimal involvemet you can use a touch pad where you place your finger of two contacts to make connection
Click to expand...

Hmm I just had an Idea around that, what if (seeing as the case will be metal) There is an insulated zone so when I run my hand over it it will connect with the case and switch the PC on.

What sort of voltages are required to pass through 2 points on a human finger?
 
Moyo2k said:
Hmm I just had an Idea around that, what if (seeing as the case will be metal) There is an insulated zone so when I run my hand over it it will connect with the case and switch the PC on.

What sort of voltages are required to pass through 2 points on a human finger?
Click to expand...

To keep a friend entertained I once knocked together a "touch switch" on an old calculator case, using one of the original "buttons" a couple of transistors (to form a very high gain darlington configuration) I think a regular BJT and the darlington BDX53 or something like that, i used the original battery compartment with one AA battery,

I suggest you use two or even just one 2N2222, I coupled a pair together the other day in darlington configuration and just holding the base and VCC wires by the insulation got a led to light (so current through insulation was enough to get something) so I'd suggest one only but its down to trial and error really, of course the rest of the circuitry like a latch or whatever is up to you
 
Moyo2k said:
Hmm I just had an Idea around that, what if (seeing as the case will be metal) There is an insulated zone so when I run my hand over it it will connect with the case and switch the PC on.

What sort of voltages are required to pass through 2 points on a human finger?
Click to expand...

That was covered earlier in the thread, including the 555 circuit I posted as well as I think another circuit. Also, voltage does not pass through a finger, current does. The amount of current will be a function of the voltage at the two points.

Ron
 
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Reloadron said:
That was covered earlier in the thread, including the 555 circuit I posted as well as I think another circuit. Also, voltage does not pass through a finger, current does. The amount of current will be a function of the voltage at the two points.

Ron
Click to expand...

Ty So the schematic I need is above? Instead of a touch panel I was thinking more along the lines of an isolated insulated zone so that there is a bit of metal that is insulated from the rest of the case by a bit of rubber or something so that when it is shorted by something (in this case my hand) the 'switch' turns on

Its hard to explain, think of your hand gliding over the surface of something metal and connecting a bit separated by a circle of rubber to the rest of the metal... I think I need a diagram :p

**broken link removed**
Basically when a hand passes along the piece of isolated metal it connects up the charged metal case and the isolated bit of metal and activates the switch
 
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