capacitive touch sense

[harps]

hello : )

i wonder if anyone can help? i've been trying to solve this for two days with partial success.

i am trying to make a touch sense circuit, so when I touch or go near a wire or metal touch plate a 5v logic out is created from it.

i made a simple little circuit a few weeks ago by accident with a couple of transistors it was a very effective touch sense circuit indeed, i did not even have to physically touch the wire for it to work ( not sure if it was a 2n7000 or a 2n2222?) but I just can't seem to replicate that circuit, no matter how hard i try. : /

i wonder if anyone knows of a simple circuit that can do this? I am sure many of you have had that funny experience when you are building on a breadboard and you go to put your hand near a wire and one of the led's begins to get brighter. : )

I am finding it so hard to replicate that type of circuit.
any help would be great
thanks : )

p.s i tried a monostable 555 timer, it works o.k , however the touch is not very sensitive and as the timer loops the 5v logic out flickers 'off' very briefly at the end of each loop point. i tried to fix this with a capacitor, but i am still experimenting on this.

 

[Jaguarjoe]

The newer Picaxe microcontrollers have touch inputs. The circuit for this would consist of one IC and one or two resistors. The program would be trivial.

PICAXE Forum - Powered by vBulletin

 

[CafeLogic]

You can't really make capacitive touch with a few transistors. I'm not sure what you would call the circuit by accident with the 2n7000. Anyway, if you want to replicate it, grab your 2n7000

1) connect the source to ground
2) connect the drain to VDD through a resistor (1k is fine).
3) connect the gate to VDD through a very high value resistor (1-10M)
4) connect the touch point to the gate and surround the touch point with ground so that the finger will bridge it, or make sure the person is touching ground somehow with some part of their body.
5) your logic out will be the junction at the 1k resistor and the drain of the mosfet. >4V for touch, <1V for no touch.

 

[Boncuk]

Hi harps,

here are a few more options:

MAX1441 (dual channel automotive touch sensor)

LM2907 (frequency to voltage - tachometer)

or

ELEKTOR 7-8/2008 (Touch Sensor).

The Elektor circuit requires circuit ground to be connected to mains protective earth for proper function using mains frequency for PWM.

Boncuk

 

[3v0]

There are several touch sensor chips made by various vendors. I expect that they are more complex then one would care to build from parts.

Microchip has mTouch
Cypress has CapSense
Silicon labs has QuickSense
etc

 

[colin55]

Nearly all capacitive touch sensors pick up mains hum.
The circuit you designed was possibly powered from a transformer and thus was connected to the mains. As soon as you power this type of circuit from batteries, it does not work.

 

[Jaguarjoe]

The Picaxe touch sensor detects differences in oscillator frequency to operate. It is not a mains sensing device. I would also think that automotive based touch sensors are not mains dependent either.

 

[3v0]

Perhaps collin was thinking of the DIY circuits.

 

[Mr RB]

Just use a high gain NPN transistor and connect it's base to ground with a 1 megohm resistor, or a 1 meg trimpot may be better. Then put a little plate on the lead on the base. The 1 meg resistor keeps it turned off, but the RF noise/hum etc from your finger will be plenty of signal to turn it on.

This simple circuit has been used in many cheap touchlamps etc and is quite workable for something that only needs a 10 vcent transistor.

 

[harps]

Just use a high gain NPN transistor and connect it's base to ground with a 1 megohm resistor, or a 1 meg trimpot may be better. Then put a little plate on the lead on the base. The 1 meg resistor keeps it turned off, but the RF noise/hum etc from your finger will be plenty of signal to turn it on.

This simple circuit has been used in many cheap touchlamps etc and is quite workable for something that only needs a 10 vcent transistor.

.
thanks everyone.


Mr rb ,thanks,
i saw this diagram below the other day and bought some bc549 'b' transistors believing them to be sensitive enough, but unfortunately they are not. maybe because they are a 'b' version?

**broken link removed**

i wonder which transistor would you use with your idea? I have a 1 m ohm fixed resistor. but shall buy a pot version like you mention.

thanks for your help.

 

[Boncuk]

Hi harps,

I recommend using a BC550C. It has an hFE of 420-800.

Some 40 years ago I made a circuit using six BC550Cs for cascaded amplifier stages connected to a short piece of wire (approx 10cm) as an "antenna".

Walking (slouching) on a carpet I could observe the static charge with a connected LED caused by nylon in the carpet.

You might also try two oscillator circuits, one with a fixed frequency and the other one variable via a touch plate. The frequency shift could be used for further signal treatment.

Boncuk

 

[atferrari]

You should be able to find some (old) designs based on CMOS inputs used as touch points.

At least two were in the "301 circuits" book from Elektor, more than 20 years ago.

 

[Boncuk]

You should be able to find some (old) designs based on CMOS inputs used as touch points.

At least two were in the "301 circuits" book from Elektor, more than 20 years ago.

Hi atferrari,

there is just a small problem: Elektor doesn't publish old articles (before 1997) for download.

Boncuk

 

[harps]

thanks boncuk

i have ordered a few of those transistors , i shall give it a go. thanks : )

 

[atferrari]

Hi atferrari,

there is just a small problem: Elektor doesn't publish old articles (before 1997) for download.

Boncuk

They sell that book...and so many others up to "309..." if I recall it right. That is why I mentioned it.

 

[harps]

i tried the BC550C, but the led did not light up at all.

i thought this would be simple to do , lol .

 

[colin55]

You need 2 or 3 cascaded transistors to get a "touch effect." Just moving this cirucit around the home will illuminate the LED as the probe picks up all sorts of electric fields in the air.

**broken link removed**

 

[colin55]

[FONT=&quot]

The Picaxe touch sensor detects differences in oscillator frequency to operate.

If you read the PICAXE forum, you will notice one members says the touch board is much more sensitive when the power plug is connected.
This is because the board does detect mains fields within a room, adding to the effectiveness of the inputs.
[/FONT]

 

[harps]

thanks colin i managed to make your diagram work : ) with a simple wire at the base ...

but when i attached a touch plate the led was on all the time. so i am still working on it. : P

 

[colin55]

The LED is on because there is so much radiation in a house that the circuit is turned on all the time.