Fingerprint scanning is often used in electronics development, and many modern IoT applications rely on fingerprint sensors to improve security and identify users. Fingerprint sensors are widely used in smartphones and other wearable devices, as well as in the smart industry and smart home applications to identify and secure data.
The two most commonly used fingerprint sensors today are optical and capacitive sensors. In this article, Enable Micro to explain how they work.
I. Optical Fingerprint Sensor
Optical fingerprint sensors have been used for a long time. The working principle of optical fingerprint sensors is to shine a bright light on the fingerprint and take a digital photo. The photosensitive microchip creates a digital image by looking at the peaks and valleys of the fingerprint and changing them to 1's and 0's to create the user's own personal code. Figure 1 shows how the light source reads the fingerprint and where that information arrives. Although unlikely, the downside of this is that digital photos can be copied.
II. capacitive fingerprint sensor
Capacitive fingerprint sensors are more common in the market and can be found on cell phones. Similar to capacitive touchscreens, it measures the finger by using a person's electrical conductivity, creating an electrostatic field and creating a digital image based on that field.
Principle of Capacitive Fingerprint Sensor
For greater accuracy, a capacitive fingerprint scanner uses a miniature capacitor array circuit to track fingerprint details. It uses fingerprint prints placed on a conductive plate to change the charge stored in the capacitor, while peaks and valleys (air gaps) keep the charge on the capacitor constant. An operational amplifier integrator circuit keeps track of these changes, which can then be recorded by an analog-to-digital converter in which this digital data can then be analyzed. Figure 2 shows the physics behind this.
It is difficult to get around the technology because the image cannot pass through a capacitive fingerprint sensor and requires other materials that will record different changes in charge on the capacitor. It is more expensive, but also more sophisticated and secure.