Inductive Touch - Circuit and Name?

[Spadez]

Hi,

Im looking for a circuit or information regarding a simple inductive touch system. To put it simply, i have a block of aluminium (its quite large), and i would like to have a system in place, where the user touches the aluminium and this turns on an LED.

These are my questions:

Is "inductive touch" the correct word?
Is this possible with aluminium or does it have to be a material like steel?
How does it actually work? Is it based around picking up your bodies voltage?
Can you point me in the direction of a simple circuit for this if possible (if not, dont worry)

Thanks!

 

[ericgibbs]

Hi,

Im looking for a circuit or information regarding a simple inductive touch system. To put it simply, i have a block of aluminium (its quite large), and i would like to have a system in place, where the user touches the aluminium and this turns on an LED.

These are my questions:

Is "inductive touch" the correct word?
Is this possible with aluminium or does it have to be a material like steel?
How does it actually work? Is it based around picking up your bodies voltage?
Can you point me in the direction of a simple circuit for this if possible (if not, dont worry)

Thanks!

hi,
This is the sort of switch that would work.
Very Simple Touch Switch Using 4050 CMOS Buffer | Free Circuit Diagram

 

[Spadez]

Thank you for the link Eric.

I might be wrong, but in the link you posted, it seems like you would place your finger over two contacts (touch probes) next to each other, and it would sense the difference?

What im actually trying to do is have it so there is a lump of aluminium, and touching the block will trigger the LED. So really I only want one touch probe, which is in my case the entire lump of aluminium.

Could your posted circuit work in that situation? I also found this on the same site, this might be a bit more applicable to me, since it only has one touch probe?:
Simple Touch Switch Using 555 IC | Free Circuit Diagram

Also, is aluminium a suitably good electrical conductor to work for this kind of touch switch?

 

[ericgibbs]

Thank you for the link Eric.

I might be wrong, but in the link you posted, it seems like you would place your finger over two contacts (touch probes) next to each other, and it would sense the difference?

What im actually trying to do is have it so there is a lump of aluminium, and touching the block will trigger the LED. So really I only want one touch probe, which is in my case the entire lump of aluminium.

Could your posted circuit work in that situation? I also found this on the same site, this might be a bit more applicable to me, since it only has one touch probe?:
Simple Touch Switch Using 555 IC | Free Circuit Diagram

Also, is aluminium a suitably good electrical conductor to work for this kind of touch switch?

hi,
Look at this pdf.

Is this the type you are looking for.?

EDIT: this is a 555
https://www.circuitstoday.com/touch-switch-circuit-using-ne-555

 

[Spadez]

That looks good. Apparently for my project the best technique is to create a "hum" inside the aluminium, and measure the decrease in amplitude when I touch the unit.

Is the 555 timer used for this, or is it only used for measuring difference in impedance. Have I got this right?

 

[ericgibbs]

That looks good. Apparently for my project the best technique is to create a "hum" inside the aluminium, and measure the decrease in amplitude when I touch the unit.

Is the 555 timer used for this, or is it only used for measuring difference in impedance. Have I got this right?

hi,
There is usually enough 'hum' mains pick up by the human body to switch the input of a centre biased CMOS gate.

I have seen devices that have an outer simple coil powered with a low voltage 'ac' which produces an electric field that the users hand/finger capacitively couples to the plate.

What are you going to control by this switch.??

 

[Spadez]

Thank you for the reply. I'm actually using this to go into my PIC and use it as a trigger for my assembly code.

My concern is partly that this isn't a 1mm small plate of aluminium, it's a lump the size of a house brick with a wall thickness of around 5mm-10mm. Any potentional hum produced by me might get diffused and not picked up.

 

[Nigel Goodwin]

In my experience touch switches (of any type) are incredibly unreliable - I wouldn't consider using one.

 

[ericgibbs]

Thank you for the reply. I'm actually using this to go into my PIC and use it as a trigger for my assembly code.

My concern is partly that this isn't a 1mm small plate of aluminium, it's a lump the size of a house brick with a wall thickness of around 5mm-10mm. Any potentional hum produced by me might get diffused and not picked up.

hi,
Its not the thickness thats a concern, its the total surface area, it could pick up to much ambient hum that swamps the hum from your body/hand.

The best way would be to build a quick working mock up and prove the concept when using the large plate.

I would not use this type of switch for any 'critical' application, they can pick up unwanted signals which could cause an unwanted action.

 

[Spadez]

Damn, I thougt it was quite reliable...it was going to be for a critical application. I should be able to have a play with some components next week.

I could probably boost my chances somewhat by finding out the likely hum of the body signal, and use lowpass filters to reduce interferance of signals that arnt from the body. Would that make it more reliable?

 

[ericgibbs]

Damn, I thougt it was quite reliable...it was going to be for a critical application. I should be able to have a play with some components next week.

I could probably boost my chances somewhat by finding out the likely hum of the body signal, and use lowpass filters to reduce interferance of signals that arnt from the body. Would that make it more reliable?

hi,
Body pick up is usually the local mains frequency.

To see the effect use a scope, put your finger on the end of the scope probe on *1, scope set to 10mV/cm Vertical and about 100mSec X sweep.

When you see the mains hum on the trace, touch the 0v screen of the probe with a finger on the same hand, the hum should almost disappear.

When you see the hum on the scope, move your hand back and forth close to the probe free end and the hum amplitude will vary.

A short length of wire clipped to the end of the probe will make a simple aerial.

OK.?

 

[audioguru]

I think static electricity will quickly destroy both simple circuits.
There is a circuit on the web that uses a high frequency and the person's finger changes capacitive-coupling of the high frequency.
I think it uses Cmos devices with a high value input resistor and diodes as voltage clamps.

 

[Spadez]

Eric, ill try that when I can get back to the uni electronics lab, probably early next week.

AudioGuru, the capacity was an option, but the thickness of the aluminium (~1cm) may create a problem for this? I can try and get the thickness down, if it will make the circuit more effective ? Im a little out of my league here, so any advice on the best approach is much appreciated. At this point in time, the best option i've come up with is this:

**broken link removed**

 

[crutschow]

A dedicated IC for proximity sensing such as SparkFun Electronics - Capacitive Touch Sensor IC or **broken link removed** may work better than a home-brew circuit. Google "touch sensor ic" for more devices.

Edit: The thickness of the plate has little effect on capacitive sensing.

 

[audioguru]

Static electricity will destroy the input transistor unless a series resistor is used and maybe at least one diode.
Pin 2 of the 555 needs a pullup resistor.
There must be a source of interference nearby for the transistor to amplify when a finger passes it.

 

[Spadez]

I may be wrong, but in the circuit for the IC linked to (sparkfun) it has two metal contacts and the user is meant to put their finger over both. I will only have one contact, which will be the aluminium brick. Did I misunderstand the circuit?

Is the capacitance still a better solution for my situation? Does the surface area of the metal have an effect, because that is going to also be quite large (the aluminium is the size of a house brick).

 

[Hero999]

There should also be a pull-up resistor on pin 2 in case some noise activates it randomly.

 

[Spadez]

Ok, thank you for the information. Ill draw a new schematic.

However, before I persue this concept too much, I really need to figure out what technique is going to work for me the best. Should I be going down the capactiance route, or should be going down the "hum" route with the 555.

 

[Spadez]

I also found this thread, which talks about another technique:
https://www.electro-tech-online.com/threads/touch-activated-switches.102791/

 

[audioguru]

The Touch Activated Switch is the high frequency capacitance one I talked about.