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Optimise UI power using wake-on-approach (Part 3)

Posted: 05 Feb 2014     Print Version  Bookmark and Share

Keywords:Proximity sensor  wake-on-approach  PCB  interface  wire sensor 

In Part 2, we covered the basics of proximity sensor and hardware implementation details of wake-on-approach proximity sensor. In Part 3, we will cover different types of proximity sensor hardware designs, common challenges faced while designing and implementing proximity sensor, firmware implementation of wake-on-approach proximity sensors and some consideration in designing wake-on-approach proximity sensors.

Proximity sensor using wire
A wire is used as a proximity sensor. This method gives greater proximity detecting range and can be used when there are curved surfaces or where a longer PCB cannot be placed in the application.

Figure 1: Proximity sensor using wire.

Proximity sensor using trace on PCB
A long PCB trace can form a proximity sensor. The trace can be a straight line, or it can surround the perimeter of a system's use interface, as shown in figure 2. This method is appropriate for mass production, but it is not as sensitive as a wire sensor.

Figure 2: Proximity sensor using PCB trace.

Gang proximity sensor
In an application where the size of the PCB is limited, placing a dedicated proximity sensor would be difficult. Also, the application does not call for large distance detection. In such applications, all the buttons are ganged and scanned as single sensor; this method can be used when required distance is smaller compared to what we can achieve with wire or PCB trace.

So far we have seen how to design proximity sensors from the design requirement standpoint. Now let us see what are the common challenges faced while designing a proximity sensor.

Higher sensor parasitic capacitance (Cp)
The higher the proximity distance, higher should be the trace length on the PCB to achieve it. Higher trace length results in higher Cp. In general, it will take more time to scan sensors with higher Cp and this result in higher power consumption and slow response time.

This problem can be solved using shield electrode, which was discussed earlier. Shield electrode reduces Cp of sensor.

In general, the proximity sensor is tuned for high sensitivity to get maximum range. Any small increase in noise can cause a high impact on SNR (Signal to Noise Ratio). It is recommended to use software filters to overcome this.

Metal object
In the real world, the important factor that impacts the proximity performance is the presence of the metal object close to the proximity sensor.

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