Tablets and mobile phones run on batteries and have stringent power requirements. These devices benefit from the multitude of ways designers use to optimize their power consumption and enhance their battery life. The usual method is to make applications running on these devices consume as little power as possible. Significant savings ensue if the different processes within these devices go to sleep when the device is not in use, turning on and starting to run only when the user activates them by using the device.
This implies the device has intelligence for detecting the start of its usage and starting the internal processes as required. One of the simplest ways usually followed by designers is to let only the User Interface (UI) run and let all other processes sleep until the UI senses an input from the user. This is usually in the form of a button press or touch, which makes the entire device wake up and start running all the necessary processes.
Advancements in the above method are the intelligence incorporated in the device to detect the approach of the user’s hand. Now, even the UI may be allowed to sleep, keeping awake only the sensor that detects the approach of the hand. Such proximity sensors can detect human hands or any conducting object coming near them without the need of any physical contact.
A device incorporated with such a proximity sensor operates in a low-power mode until the approach of a human hand. In the low-power mode, there is no other activity or process running in the gadget other than that scanning the sensor. Once the proximity sensor detects the human hand approaching, it wakes up the device, which enters an active mode. In this mode, the UI of the device is turned on. Proximity sensors that wake up the device from its low-power mode to an active mode are called wake-on-approach sensors.
Infrared and inductive proximity sensing techniques are commonly used in the industry. However, applications such as laptops, mobile handsets, white goods and home appliances that mostly sport touch panels for their user interface, use capacitive proximity sensing because of their greater reliability and aesthetics. As soon as the user’s hand approaches the interface panel, the proximity sensor wakes up the capacitive device from its low-power mode and it starts to scan all the buttons to detect a touch.
The panel of a modern TV or tablet PC may have buttons that are nearly invisible when the device is in low-power mode. The touch panels in use today have buttons that remain nearly hidden until they are highlighted by backlighting. However, backlighting requires LEDs that gobble up power very fast even when they are of the high-efficiency types. This reduces the battery life of portable devices such as PCs, tablets and mobiles.
Wake-on-approach proximity sensing and its level of control on the device is highly application dependent. Response time and power consumption are the two key parameters governing the optimization required, where response time is optimized by scanning the sensor repeatedly and in quick succession. However, optimization of power consumption requires the scanning to be slow to allow the device more time for sleeping.
