Everything in today’s digital world is becoming just a touch away. Capacitive touch technology is widely used in modern touch-sensitive devices, whether filling a morning coffee mug, setting a washing machine timer, or turning on a PC display. Its fast response time and adaptability to different environments have made it a popular choice among manufacturers. Typical applications include water purifiers, kitchen appliances, vending machines, and touch-sensitive switches.
Most of us encounter capacitive touch sensors daily, but how often do we consider how they work? Capacitive displays detect touch by measuring changes in capacitance. Surprised? How can a component of two metal plates that store charge detect touch? This article explores the answer to that question.
Figure 1. A typical capacitive touch sensor.
The capacitive touch sensor
Figure 1 shows a capacitive touch sensor. The sensor is encapsulated in a durable black plastic material that holds an IP67 rating. The Ingress Protection (IP) rating system classifies the degree of protection a device has against solid objects and liquids. In this case, the first number after “IP” indicates dust protection, while the second number signifies protection against immersion in liquid.
Figure 2. The pin-out terminals are on the back of the capacitive touch sensor.
The pinout terminals
Flipping the sensor reveals the pinout terminals, which are made from brass (Figure 2). The two separated terminals marked 1 and 2, are the LED terminals that illuminate when the sensor is touched. The remaining 3, 4, and 5 terminals correspond to the supply voltage, ground, and output connections, respectively.
The lock, frame, and sctuator
Figure 3. The various parts of a capacitive touch sensor.
The mechanical lock and scales
Mechanical locks are provided on the sides to ensure a secure fit within water purifiers, kitchen appliances, vending machines, and switches (Figure 3). The scales assist in adjusting the device’s height to accommodate these applications.
Figure 4. The frame and actuator of a capacitive touch sensor.
The frame and actuator
By cutting the sensor’s frame into two parts, you’ll find the actuator. The mechanical locking mechanism between the actuator and frame ensures that the components remain securely intact (Figure 4).
The wire connections and the PCB
Figure 5. A closer view of the metal connection in a capacitive touch sensor.
The metal wire connection
A metal wire extends from the actuator to the printed circuit board or PCB (Figure 5). To understand its purpose, let’s take a closer look at the sensor’s internal structure.
Figure 6. The PCB inside a capacitive touch sensor.
The PCB
After cutting the sensor into two halves, a clear view of the PCB is revealed (Figure 6). The PCB hosts various soldered components, including the metal wire, which is securely soldered onto it.
An LED is centrally positioned on the PCB and connected to its respective terminals.
The fluorescent plate and electrode
A white fluorescent plate is enclosed beneath the glass surface (Figure 7). Light emitted from the LED passes through the actuator and reflects off the fluorescent plate, illuminating markings (if present) on the glass surface. The glass surface serves as a protective covering, ensuring durability and visibility.
Figure 7. The white fluorescent plate (left) and an electrode (right).
The electrode
A metal wire, the electrode, surrounds the fluorescent plate (as shown in the image). The electrode is the core component of the device, as it detects changes in capacitance and transmits this change as a signal to the PCB.
Iron in the human body acts as a string of capacitors aligned to the body’s surface. When these capacitors come close to the glass surface, they create a capacitance coupled to the ground, leading to a measurable change in capacitance, which is then interpreted as a touch input.
Figure 8. A closer look at the capacitive touch sensor’s PCB.
How the PCB works
Various soldered components can be observed after removing the PCB from its casing. The Touch Sensor IC, located at the center of the PCB, is responsible for sensing changes in capacitance.
To achieve this, the IC is connected to two reference ceramic capacitors, which help analyze changes in capacitance. The sensor electrode is also connected to the IC through a ceramic capacitor.
When a user touches the glass surface, the electrode carries the signal to the IC. This varying capacitance creates a frequency shift in the sensor oscillator circuit. The system oscillator circuit then reads this frequency change. It transmits it to the touch detection circuit, which accurately determines whether a valid touch has occurred and generates the corresponding output.
Filed Under: Insight
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