How Do Capacitive Touch Sensors Work?

Capacitive touch interfaces gained immense popularity after their successful implementation in the Apple iPod, which featured a touch-sensitive rotary wheel. This breakthrough led to the adoption of touch buttons in a myriad of household appliances, industrial equipment, and automobiles. In this article, we delve into the fundamentals of capacitive touch technology and the obstacles involved in ensuring its reliable performance in practical applications.

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The touch-controlled user interface of the iPod was a key factor in its success. Beneath its sleek ring and illuminated buttons were

The Role of Capacitance in Circuits

Individuals with an electronics background have likely noticed how a finger impacts a circuit. For example, audio and radio circuits may change their output pitch or reception channels when a finger approaches. Anecdotes circulate about someone who found their radio receiver functioning properly simply by touching one of the valves. As a humorous fix, they resorted to using a sausage in place of their finger, providing both temporary functionality and astonishment to the technician tasked with restoring the radio later.

Capacitive touch circuits operate based on the alterations in capacitance caused by a finger or another body part interacting with the circuit. When a finger touches, it introduces a parallel capacitance to ground, thereby augmenting the capacitor's value in the circuit.
  When in contact with the circuitry, the human body contributes a small parallel capacitance.
One potential method is to utilize capacitors within an oscillator. For this purpose, we can vary the value of C2 between 1 µF and 7 µF in 1.5 µF increments to imitate the introduction of a parallel capacitance.
  An astable circuit can incorporate one of its capacitors as a touch sensor. This simulation illustrates the variation of C2 and its effects.
  With increasing capacitance, the frequency diminishes. This measurement could serve as a means to detect a finger touching the capacitive sensor.
At this juncture, we begin to establish a touch detection circuit. The resulting output may be connected to a counter. By counting the number of pulses within a defined time frame—say, 100 ms—and establishing a count threshold, the distinction between a touch and no touch can be effectively determined. Alternatively, one can monitor changes in on-pulse widths.

Real-world implementation becomes relevant as we strive to execute such circuits. The human body introduces capacitance ranging from 100 pF to 200 pF. By replacing C1 in our circuit with a 300 pF capacitor and varying C2 between 100 pF and 200 pF, the output frequency shifts from 356 kHz without touch to 284 kHz when maximum human-body capacitance is present. It's important to note that this remains a simulation, and the selected transistors might not function accurately at these frequencies in reality. Nonetheless, we are equipped with the essential components of a capacitive touch sensor.
  Utilizing the minuscule capacitance of the human body, our oscillator functions near 350 kHz prior to being touched.

Types of Capacitive Sensors: Self-Capacitance

Capacitive sensing can be categorized into two types: self-capacitance and mutual capacitance. Self-capacitance, as previously detailed, involves the touch sensor forming a capacitor to ground, with the circuit gauging changes in its value upon a finger's placement. Instead of using the capacitor in an oscillator, touch circuits frequently employ methods that facilitate precise charge control applied.

An example can be seen in the
  Microchip CTMU, which combines a current source with a switch to discharge the sensor capacitor.
  The CTMU registers lower voltage levels when a touch is detected at its sensor.
  More advanced sensors, such as sliders (top), may yield inconsistent results. Interleaving these sensors (below) can enhance output smoothness.
Capacitive sensors are typically situated behind plastic covers. Greater material thickness results in reduced sensor sensitivity. Furthermore, many surfaces—such as those found in washing machines—are curved, while printed circuit boards are generally flat. While flexible PCBs made of polyimide can resolve this, they tend to be expensive. As an alternative, the gap between the touch surface and the PCB can be bridged with springs or conductive foam. Although this approach resolves mechanical issues, additional testing and fine-tuning of both circuitry and software remain essential to guarantee reliable performance.

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Types of Capacitive Sensors: Mutual Capacitance

Mutual-capacitance sensing techniques utilize a capacitive sensor linked between two pins of a touch chip or microcontroller, measuring charge changes both with and without finger presence. Essentially, the finger extracts charge from the capacitor, similar to being placed between two plates. Consequently, a touch manifests as a capacitance drop. This technique facilitates the formation of a capacitor grid across flat surfaces, enabling touch screens, commonly referred to as projected capacitive touch or PCAP. A method involves sequentially charging the X rows of the sensor area while evaluating capacitances with the Y rows, cycling multiple times per second. This method is predominant for smartphone displays and laptop touchpads.
  Mutual capacitance employs capacitive sensors positioned between two pins of the microcontroller or sensor IC (left). Touchscreens and touchpads utilize a grid system (right) to create touch-sensitive surfaces.

These applications employ dedicated chipsets, such as

Incorporating Capacitive Touch into Arduino and Raspberry Pi

Capacitive touch ICs ready for use can be sourced from

 

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DIY: LTspice Code

 
Astable circuit @ 70 Hz:

Q1 N002 N004 0 0 BC547B
Q2 N003 N005 0 0 BC547B
R1 N001 N002 100R
R2 N001 N003 100R
R3 N001 N004 10K
R4 N001 N005 10K
C1 N005 N002 1µ
C2 N003 N004 {X}
V1 N001 0 5V
.model NPN NPN
.model PNP PNP
.lib C:\Users\<USER>\Documents\LTspiceXVII\lib\cmp\standard.bjt
.tran 15s startup
.step param X 1u 2u 0.5u
.backanno
.end

Astable Circuit @ 350 kHz:

Q1 N002 N003 0 0 BC547B
Q2 OUTPUT N004 0 0 BC547B
R1 N001 N002 100R
R2 N001 OUTPUT 100R
R3 N001 N003 10k
R4 N001 N004 10k
C1 N004 N002 300p
C2 OUTPUT N003 {X}
V1 N001 0 5V
.model NPN NPN
.model PNP PNP
.lib C:\Users\<USER>\Documents\LTspiceXVII\lib\cmp\standard.bjt
.tran 1ms startup
.step param X 100p 200p 25p
.backanno
.end

CTMU Model:

CTouch_Pad PAD 0 10pF
CFinger PAD 0 {X}
I1 0 PAD PULSE(0 5.5u 2u 0 0 5u 100u 1)
M1 PAD N001 0 0 NMOS
V1 N001 0 PULSE(0 5 12u 0 0 50u 100u 1)
.model NMOS NMOS
.model PMOS PMOS
.lib C:\Users\<USER>\Documents\LTspiceXVII\lib\cmp\standard.mos
.tran 30u
.step param X 0p 200p 20p
.backanno
.end

At one point, mechanical buttons were the norm. Then, in a dramatic turn, the Apple iPod featuring an all-touch interface was introduced. It seemed as though product marketers worldwide were taken aback, as every device from washing machines to car door handles demanded a touch interface. Much of the iPod's triumph can be linked to its touch-controlled user interface. Its capacitive sensors registered changes in capacitance, indicating a finger's presence for activation, whether pressing or rotating. The principle at play here was not novel, as the idea of human body capacitance impacting electronic circuits dates back to when Leon Theramin employed it to regulate a heterodyne oscillator in his eponymous instrument. Individuals familiar with electronics may have noted how a finger influences a circuit, such as audio and radio systems responding to nearby fingers by altering output pitch or tuning. A humorous tale is told of someone discovering their radio operated correctly simply by placing a finger on one of the valves. Rather than maintaining this method for a long time, they substituted a sausage, leading to unexpected repairs for the technician. Capacitive touch circuits function from the capacitance variations caused by a finger or other body parts interacting with these circuits. The introduction of a finger contributes a parallel capacitance to ground, heightening the circuit's capacitor value. One approach involves the use of capacitors within an oscillator. By varying the value of C2 between 1 µF and 7 µF in 1.5 µF increments, we can simulate the introduction of this parallel capacitance. Examining the output via FFT (Fast Fourier Transform) reveals the frequency starting at 70 Hz, dropping to lower frequencies as capacitance increases. It is crucial to bear in mind that the output is a square wave, obscuring the analysis due to harmonic distortion.

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