3.2.1 Click board™ Modifications

Before the Click board can be used in this demo, it needs to be modified by removing the op amp gain stage and some other components.

The PIR Click board is used for demonstration purposes. The original Click board has an op amp solution for amplifying the signal, which is shown in the figure below. This needs to be removed to demonstrate that the tinyAVR® 2 family microcontroller can amplify the raw signal from the PIR sensor using the integrated PGA and differential ADC.
Figure 3-2. Original Click board™ and Schematic

Unnecessary components are removed from the Click board showed by the red Xs in the figure below. New connections and components are in blue. U2, R9, R10, R1, and PWR LED are not part of the original signal chain but are removed to minimize the power consumption.

Figure 3-3. Planned Modifications to the Click board™ and Schematics

Resistor R5 is changed from 30 kΩ to 470 kΩ. R5 and C8 (100 nF) form a low-pass filter with a cutoff frequency of 3.38 Hz. This filter will transfer the full AC signal from the PIR sensor in addition to the DC bias and filters out high-frequency noise.

Resistor R11 is changed from 100 kΩ to 470 kΩ, and C1 is changed from 10 µF to 2.2 µF. When R11 is combined with capacitor C1, it forms a low-pass filter with a cutoff frequency of 0.154 Hz. This filter will block most of the AC signal from the PIR and only transfer the sensors DC bias to the negative input of the ADC.

Figure 3-4. Modified Click board™ and Schematics

The cut-off frequency for both filters should be selected based on the application needs and what type of movement the system is designed to detect.