What is capacitive touch switch?

Have you ever wondered, “What is capacitive touch switch?” Capacitive touch switches have become increasingly prominent in various electronic products over recent years. From household appliances and computers to automobiles, capacitive touch technology provides a simple and intuitive interaction method. While many of us regularly use devices with this technology, most might not be aware of how capacitive touch switches operate. This article delves into the fundamental principles behind capacitive touch switches, exploring the self-capacitance principle, mutual capacitance principle, and true matrix principle.

In our daily lives, capacitive touch technology has become an omnipresent mode of interaction. When you lightly touch the screen with your finger, you are actually interacting with a sophisticated capacitive system.

Capacitive touch switches are also called:

• capacitive switch
• capacitive touch switch
• capacitive membrane switch

The core principle behind this technology is capacitance—a physical property that can store and release electric charge.

• Self-Capacitance
• Parasitic Capacitance in Self-Capacitance
• Field Propagation in Self-Capacitance
• Mutual Capacitance
• Parasitic Capacitance in Mutual

Self-Capacitance

The working principle of self-capacitive touch is based on a single electrode that can sense the proximity of a finger or another object. When an object approaches the plate, it changes the distribution of charge on the plate, causing a change in capacitance value. Self-capacitive systems have high sensitivity, but their primary limitation is the inability to implement multi-touch.

Parasitic Capacitance in Self-Capacitance

Parasitic capacitance is present in any signal source, such as signal lines, electronic components, and ground planes. Excessive parasitic capacitance can affect whether the touch is triggered, which can be fatal. For example, if the parasitic capacitance is 8pF, and the finger touch causes a 2pF capacitance change, then the MCU detects a 20% increase. But if the parasitic capacitance is 10pF, then the MCU detects only a 5% increase, which is hard to detect.

Field Propagation in Self-Capacitance

The electric field of self-capacitance projects 360° from the electrode, meaning that touches can also be made from the back of the capacitive touch membrane. Furthermore, the 360° projected electric field also directly affects the distance between buttons.

Mutual Capacitance

Unlike self-capacitance, mutual capacitive touch is based on capacitance changes between two electrodes. This system employs two sets of electrodes—transmitting electrodes (Tx) and receiving electrodes (Rx). When a user’s finger or another object approaches these electrodes, it disrupts the electric field between them, resulting in a change in capacitance value. Mutual capacitance is also known as projected capacitance. The advantages of mutual capacitance technology include tight electric field coupling, allowing for more flexible design. For example, keyboards can have closely grouped keys without worrying about cross-coupling. However, mutual capacitance also has its limitations, such as its measurement noise being generally greater than self-capacitance.

Parasitic Capacitance in Mutual

Capacitance Just like self-capacitance, mutual capacitance also has parasitic capacitance! However, mutual capacitance has two types of parasitic capacitances: between Tx and Rx, and between Tx and Rx and the ground, respectively. Unlike in self-capacitance, parasitic capacitance doesn’t affect the measurement of key capacitance. Due to charging and discharging reasons, it still affects the ability of the MCU to drive the electrodes.

Matrix Principle

• Self-Capacitance Matrix
• Mutual Capacitance Matrix
• Reliability

The true matrix capacitive touch technology, through its unique design, greatly enhances the flexibility and accuracy of touch control. This technology employs a matrix design, allowing a small number of pins to support a larger number of buttons.

Self-Capacitance Matrix

The self-capacitance matrix works with a combination of row and column electrodes. For instance, consider a self-capacitive touch matrix consisting of 4 row electrodes and 3 column electrodes. This design enables the control of 12 buttons with only 7 pins. Each row and column are scanned as independent electrodes, and the touch position is determined by combining the data from the rows and columns. However, this self-capacitive matrix design has a significant drawback: due to the ghosting effect, it cannot achieve multi-touch.

Mutual Capacitance Matrix

The mutual capacitance design differs from self-capacitance. In this design, columns are viewed as transmitting electrodes (Tx), while rows are perceived as receiving electrodes (Rx). This means that each row-column intersection represents a unique Rx-Tx combination and a distinct mutual capacitance, as shown in Figure 11. Since each node is a unique capacitance being measured, full multi-touch can be achieved.

Reliability

Because each Rx and Tx intersection is a unique node, the reliability of matrix capacitive technology is enhanced. Each node (i.e., intersection of Rx and Tx) is individually tracked in software. This is distinct from the self-capacitance method where row and column measurement data need to be combined to calculate potential button operations.

True matrix capacitive touch technology brings immense convenience and accuracy to touch control, with its design advantages particularly evident in mutual capacitance designs. This technology not only offers greater flexibility but also bolsters system reliability, making it hold great potential in various applications.

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Author: Haonan.Wang | WeGlow Technical Support