Issues with a touch switch circuit

Thread Starter

Ams Sma

Joined Feb 11, 2017
17
Hello
I made a touch switch in a PCB based in the following schematic
1628268988496.png


When touching the plate with a finger the BJT turns on and the 555 trigger pin goes LOW activating the pin 3 (OUTPUT).
The time the circuit is on depends of the 100K resistor and the 10u capacitor

The circuit works pretty well with a power supply with 9V.

When I use a 9V battery the circuit works well if I’m not isolated from the earth (touching the floor with a bare foot)
If I’m isolated from the earth (using shoes) the circuit stops working after 3 or 4 tries and only works again if I touch the earth.

If i use an oscilloscope to probe the trigger pin of the 555 the circuit works with no issues.

How can I improve the circuit to work to prevent this behavior?

Thanks
 
Last edited:

Thread Starter

Ams Sma

Joined Feb 11, 2017
17
The touch plate is picking up AC electricity hum and radio signals from your touch if the 0V of the circuit is at earth ground and you are insulated from earth ground.
Thanks for your reply
I was aware of what you said. Is there any way to improve the circuit to work when i'm isolated from earth?
 

LowQCab

Joined Nov 6, 2012
1,189
Can You divide the Touch-Plate in to 2-Parts ? (split down the center).
If You can, then just add a high-value Resistor from the Input-Pin to the 6-Volt-Rail,
and ground the other half of the Plate.
Probably no Transistor required.

If You use the CMOS version, (7555), you don't need the Transistor at all,
but You will need to add a Zener-Diode to protect the Input from Static
which might destroy the Chip, and a very small Capacitor to absorb Electrical-Noise.
.
.
.
 

Thread Starter

Ams Sma

Joined Feb 11, 2017
17
Can You divide the Touch-Plate in to 2-Parts ? (split down the center).
If You can, then just add a high-value Resistor from the Input-Pin to the 6-Volt-Rail,
and ground the other half of the Plate.
Probably no Transistor required.

If You use the CMOS version, (7555), you don't need the Transistor at all,
but You will need to add a Zener-Diode to protect the Input from Static
which might destroy the Chip, and a very small Capacitor to absorb Electrical-Noise.
.
.
.
Yes i can divide the touch plate.
I will try your suggestions.

I'm using the CMOS tlc555 for optimize power consuption
What value do you recommend for the zener? VCC?
Thanls!
 

LowQCab

Joined Nov 6, 2012
1,189
The Zener may be as high as the max supply Voltage, which I believe is around ~18-Volts,
but there is no reason to make it higher than the Rail-Voltage You are using.
5-Volts would be ideal.

You will also need a ~1M Pull-Up-Resistor.

Keep in mind that the Zener will probably turn the 555 into an AM-Radio-Receiver,
so adding a 1nf to 10nf Capacitor should calm things down.
.
.
.
 

Sensacell

Joined Jun 19, 2012
2,908
Touch switch circuits are tricky, that one is also not a very good design.
It relies on external AC fields to work, it does not include its own source of AC signal voltage.

The concept always relies on the capacitance between the circuit and the object being sensed.

The fact that there appears to be a single wire sensing electrode belies what is really going on, the circuit path must always be a closed circuit, a loop.
When you connect your scope to the circuit, the GND connection provides a return path for capacitively coupled signals- the circuit works fine.

If you power the circuit with a battery, and 'float' the circuit - no external connections and located away from conductive objects, it will stop working - try it.
This is because there is no longer a loop for the current to flow. If you place the circuit on a large conductive surface, (insulated of course) it will work again because the capacitance between the GND of the circuit and the object creates a path for current- the transistor is triggered by current flow.


I once tried to make a 'touch box' that would sound an alarm when you touched the metal box, it all works great when the parts are laid on the desk, but as soon as you put the circuit INSIDE the box- nothing! it totally doesn't work.

This again is because there is no longer a loop or path.

Consumer products that have capacitive touch buttons are designed such that there is always a reference GND electrode (chassis or case) near the buttons, your fingers influence the capacitance BETWEEN the electrodes and the local GND, again, it's a closed circuit made of minute capacitance between the electrodes and REFERENCE GND.
 

Ian0

Joined Aug 7, 2020
3,533
All beginners seem to start with the "hum pickup" type touch switch - probably because of its novelty, but it's never reliable. Then they try the "bridge two contacts" type, which is better, but if badly affected by either variations in users' skin resistance or moisture.
It doesn't take many more parts to make one that really does work.
This one detects capacitance by utilising what is normally a logic problem - the "race hazard".
The 555 produces a 100Hz clock, which is delayed slightly by the preset+capacitor network, and goes to the clock input of the latch. The touch pad (circle with a cross in it) puts a variable delay in the D input, so that the output is either high or low depending on the capacitance connected to the touch pad. If the edge on "clock" arrives first the output is low, if D arrives first it is high.
I have drawn a Schmitt trigger on the clock input - Nexperia includes a Schmitt trigger in their 74HC74. Other manufacturers (Texas, for instance) don't.

Once you have the oscillator and the preset+capacitor variable delay, you can make dozens of them just by using multiple D-latches, such as 74HC173 (four) or 74HC574 (eight). You can also use the OE pin on the HC574 to multiplex output and read them with a microcontroller, or with a 74HC597 you can read them serially. The most I have ever made on one panel is 72.5266B5FE-E80A-4F2A-A119-3FEDCB75774F.jpeg
The output is very stable, doesn't bounce and rarely picks up any interference. You can even gain extra noise immunity by using a 50Hz derived from the mains* instead of the 555, so that the edge on which it senses corresponds to a mains zero crossing.

*Isolated, of course.
 

Thread Starter

Ams Sma

Joined Feb 11, 2017
17
All beginners seem to start with the "hum pickup" type touch switch - probably because of its novelty, but it's never reliable. Then they try the "bridge two contacts" type, which is better, but if badly affected by either variations in users' skin resistance or moisture.
It doesn't take many more parts to make one that really does work.
This one detects capacitance by utilising what is normally a logic problem - the "race hazard".
The 555 produces a 100Hz clock, which is delayed slightly by the preset+capacitor network, and goes to the clock input of the latch. The touch pad (circle with a cross in it) puts a variable delay in the D input, so that the output is either high or low depending on the capacitance connected to the touch pad. If the edge on "clock" arrives first the output is low, if D arrives first it is high.
I have drawn a Schmitt trigger on the clock input - Nexperia includes a Schmitt trigger in their 74HC74. Other manufacturers (Texas, for instance) don't.

Once you have the oscillator and the preset+capacitor variable delay, you can make dozens of them just by using multiple D-latches, such as 74HC173 (four) or 74HC574 (eight). You can also use the OE pin on the HC574 to multiplex output and read them with a microcontroller, or with a 74HC597 you can read them serially. The most I have ever made on one panel is 72.View attachment 245144
The output is very stable, doesn't bounce and rarely picks up any interference. You can even gain extra noise immunity by using a 50Hz derived from the mains* instead of the 555, so that the edge on which it senses corresponds to a mains zero crossing.

*Isolated, of course.
I'll try your solution
Thanks
 
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