In the rapidly evolving world of keypads and electronic switches, the membrane switch has become a vital component in many modern devices. Membrane switches are prized for their durability, reliability, and versatility. Membrane switches are also economical and made from inexpensive readily available materials. Beyond their traditional role as simple on-off switches, these devices can be ingeniously adapted to use as resistive pressure sensors.
In this blog post, we'll explore the fundamentals of membrane switches, the concept of normally open switches, the role of conductive carbon ink, and how to measure and manipulate the resistance within these circuits.
What is a Membrane Switch and How Does it Work?
A membrane switch is a type of keypad device that enables communication between a user and an electronic device. Most membrane switches include small metallic concave discs called dome switches which act as buttons within the stack-up. Other types of membrane switches function as non-tactile switches that omit the domes altogether and use the spring action of the material layers that behave as a switch but lack the characteristic click of a dome switch.
Membrane switches are typically composed of several laminated layers, including a flexible top layer with artwork (the graphic overlay), a spacer layer that entraps components like LEDs and dome switches, and a bottom layer that contains the base circuit. These layers are laminated together to form a compact, sealed and resilient switch.
The working principle of a membrane switch is straightforward. When a user presses the top layer, it flexes pushing down the dome switch and makes contact with the bottom circuit layer. This action completes the circuit, allowing an electrical signal to pass through. The simplicity of this mechanism makes membrane switches ideal for a wide range of applications, from household appliances to industrial control panels.
But for the resistive pressure switches discussed in this post, the dome switch is omitted from the stack up leaving only a small air gap in between the two layers. This air gap is all that insulates the two layers from making electrical contact with each other, thus creating a normally open circuit. Once a small force is pressed atop the switch, the two layers make contact completing the circuit, registering as a switch closure event.
Example of a membrane switch keypad.
What is a Normally Open Switch?
A normally open switch is a type of switch that remains open until it is actuated or pressed. A normally open switch does not conduct electricity and is referred to be “open” since there is no contact between the terminals across the switch. In a normally open switch, the circuit is incomplete, and no current flows through. When the switch is pressed, the circuit closes, and current flows from one contact to the other.
Normally open circuits are used within membrane switches to enable the switch to function ‘ON” when the buttons are pressed. As soon as the button is let go, the dome springs back to position opening the circuit. For instance, in a control panel, pressing a button to start a machine would use a normally open switch to ensure the circuit is only completed when the button is actively pressed.
What is Conductive Carbon Ink and How is it Used in Membrane Switches?
Conductive carbon ink is a specialized ink formulated to conduct electricity. It is typically made from a mixture of carbon particles and a binding agent, which allows it to be printed onto flexible substrates and circuits. This ink is a crucial component in the construction of membrane switches- especially those that use PET as the base circuit material. Once applied, the conductive ink forms the circuitry and conductive paths that facilitate electrical connectivity within the switch.
In membrane switches, conductive carbon ink is often screen-printed onto the circuit layer to create the desired circuit pattern. Its flexibility and durability make it an excellent choice for the dynamic environments in which membrane switches are often used. The conductive paths formed by this ink are essential for the proper functioning of the switch, ensuring that electrical signals can flow reliably when the switch is activated. While most conductive inks are applied to simply create a circuit, other types of conductive carbon ink can be chosen for unique design challenges that include resistive pressure sensors.
PET circuit membrane switch with carbon ink and the graphic overlay removed.
Can the Resistivity of Conductive Carbon Ink be Changed?
Yes, the resistivity of conductive carbon ink can be altered. Manufacturers produce various types of conductive carbon inks with differing resistance values to suit specific applications. The resistivity of the ink can be adjusted by modifying the formulation, such as the concentration of carbon particles, the type of binder used, and the thickness of the printed layer.
By offering inks with different resistivities, manufacturers enable the design of membrane switches with completely customized electrical properties. This capability is particularly important when using membrane switches as resistive pressure sensors. Different resistance values can help in measuring varying degrees of pressure applied to the switch, or where the switch is pressed, thereby providing more nuanced and accurate readings for a variety of applications.
How to Measure the Resistance of the Circuit Within Membrane Switches
Measuring the resistance within a membrane switch circuit is a crucial step in ensuring its proper functionality and in calibrating it for specific applications.
Here are the basic steps for measuring resistance in a membrane switch circuit:
- Set Up the Multimeter: Use a digital multimeter set to the resistance (ohms) measurement mode. Ensure the probes are properly connected to the multimeter.
- Connect the Probes: Place the multimeter probes on the contacts of the membrane switch. These contacts are usually located on the tails of the switch, which are the extensions of the circuit layer designed for connection to external circuitry.
- Apply Pressure: To measure the resistance of a normally open membrane switch, you will need to apply pressure to the switch to close the circuit. This action simulates the operational state of the switch.
- Read the Measurement: Observe the resistance value displayed on the multimeter. This value indicates the resistance of the conductive path within the membrane switch.
- Evaluate and Adjust: If the resistance value does not meet the desired specifications, adjustments may be necessary. This could involve using a different type of conductive ink, modifying the thickness of the printed layer, or changing the design of the circuit pattern.
Applications Analysis
For applications that require a sensor to be designed to correlate a touch event to a location across its length, these types of resistive pressure sensors with carbon ink are a worthwhile choice. By calibrating the resistance value readings at various points on the switch, engineers can create a reliable, repeatable, and low cost sensor to help measure where something is touched.
Once the ink is applied, engineers can measure the resistance of the ink since it’s not a pure conductor. Measured as a resistance value per unit length, some designs can be created with linear resistance changes that can equal over 7K ohms per inch. If when pressed, the switch measures a value of 21K ohms, it can be deduced that the switch was pressed three inches from the end. Since the resistance values change linearly, if another press event measures a value of 63K ohms, this would equate to a touch distance of nine inches from the end. With a typical tolerance of +/-20% on the resistance, these designs aren’t terribly precise but can still serve as a proximity sensor when designed properly.
By following these steps, manufacturers and engineers can ensure that the membrane switch circuits, and pressure sensors meet the required specifications for their intended applications. Accurate resistance measurement is especially important when using membrane switches as resistive pressure sensors, as it ensures reliable and consistent performance.
Summary
Membrane switches are versatile components that offer more than just simple on-off functionality. By leveraging normally open switch designs, conductive carbon inks, and precise resistance measurement techniques, these switches can be adapted for use as proximity measurements as resistive pressure sensors. This capability opens up new possibilities for innovative applications in various industries, from consumer electronics to industrial automation. Understanding the principles and techniques discussed in this post will help engineers and designers create more effective and reliable membrane switch-based pressure sensors.
