what's 10V pwm purpose in this Cap_sense circuit?

Thread Starter

Younes Thabet

Joined Jan 9, 2019
17
How does this circuit function (PWM!, Double-Diode!, Capasitive spring"the square") and can it work with 5/3.3V pwm and a GND instead of -5V!?
is there a simpler circuit where we don't use a pwm?

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KeithWalker

Joined Jul 10, 2017
1,224
That is an unusual circuit. I can not make any comment on its function because there is not enough information.
What is the square symbol in the bottom left of the diagram connected to? Is it the capacitive sensor ?
What load is connected to the output?
What is the frequency of the PWM signal?
Is the PWM signal +10V referenced to ground (which is not shown)?
Regards,
Keith
 
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Thread Starter

Younes Thabet

Joined Jan 9, 2019
17
That is an unusual circuit. I can not make any comment on its function because there is not enough information.
What is the square symbol in the bottom left of the diagram connected to? Is it the capacitive sensor ?
What load is connected to the output?
What is the frequency of the PWM signal?
Is the PWM signal +10V referenced to ground (which is not shown)?
Regards,
Keith
exactly, an unusual circuit!
the square is the capacitive touch spring ...
the load is MCU pin .. probably ADC!
frequancy IDK TBH!
yes the PWM is refrenced to GND.
 

MisterBill2

Joined Jan 23, 2018
7,035
Really, I do not see how this circuit can provide any useful function. If you already have a PWM signal this is probably not the circuit to get anything useful from it.
It is, however, the sort of circuit that I would ask whoever proposed it to explain what it wouild achieve and how it works. Tha is my handy way of getting folks to see the flaws in their thinking without having to explain that they are wrong, since they usually discover the problem while explaining how it works.
 

ronsimpson

Joined Oct 7, 2019
940
the square is the capacitive touch spring
PWM10V Lets say this is a 10V square wave.
Square is box is a touch "thing" like a capacitive touch key board.
On the right is a analog input to the micro.
If touch is open, (nothing is there) the PWM will pull up on C3 with 1075k and pull down down with 150k resulting in a voltage on the right being 1 volt (more/less). If you touch the touch spring you will add capacitance. The square wave will charge up the capacitor in your finger to about 9 volts. (peak detector) This voltage will pass through to C3.
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So I really don't know what it does but I tried.
 

MisterBill2

Joined Jan 23, 2018
7,035
Since now I am aware that the square represents a touch plate, for analysis it can be replaced by a small capacitor and large value resistor , both to ground. Then the result will be some current flowing in the diodes on both the positive and negative parts of the PWM wave. For figuring out what happens we need to guess that the source impedance of the PWM signal is zero, and that the input impedance of the device reading the C3 voltage is infinite. Now you can run the circuit on a simulator because all of the parameters are known. You will also be able to see the effect of changing that negative 5 volts to zero.
The result will be a voltage that varies with the duty cycle of the PWM source and also with the values of resistance and capacitance from that square block to ground. Some sort of analog touch sensor , it appears.
 

ronsimpson

Joined Oct 7, 2019
940
Using a 10khz PWM signal. Running 10X faster will allow you to measure smaller capacitors. Running at 1/10 the frequency is for measuring much larger capacitors.
1pF of touch capacitance "C2" =1.58 volts out. (10khz 50% duty cycle)
10pF = 1.7V
100pF = 2.13V
1000pF = 6.83V
1603044099670.png
 

KeithWalker

Joined Jul 10, 2017
1,224
What is the purpose of the the PWM signal? If the circuit is a capacitive touch sensor, all you need is a fixed frequency square wave.
 

ronsimpson

Joined Oct 7, 2019
940
What is the purpose of the the PWM signal? If the circuit is a capacitive touch sensor, all you need is a fixed frequency square wave.
I think "PWM signal" is just a source of a clock. Some computers have many PWM and no/little clocks.
But then we don't really know anything on this project.
 

MisterBill2

Joined Jan 23, 2018
7,035
I think "PWM signal" is just a source of a clock. Some computers have many PWM and no/little clocks.
But then we don't really know anything on this project.
Exactly! The toy processors need everything added on "shilds" so that the "sketches" can do something useful. And a PWM wave would allow the output voltage to be adjusted to some specific level with no touch..
 
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