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PIR (PASSIVE INFRARED) DETECTOR USING ST7FLITE05/09/SUPERLITE

2.1 SIGMA-DELTA IMPLEMENTATION

Sigma-Delta (as shown in Figure 7) needs an integrator, comparator and digital filter for 1 bit

DAC.

Figure 7. Sigma-Delta Conversion concept

Note: Refer to AN1827 for more details on Sigma-Delta implementation

These components are implemented in the application as follows:

– Comparator: On-chip ADC is used

– 1-Bit DAC: I/O Port PB2 is used

– Integrator: External Resistor and capacitor.

The Sigma-Delta software procedure is as follows:

1. Fix a threshold level for the transistor output (measured by the microcontroller’s ADC

input). This is selected as 3V (96h) in our application.

2. Configure the AR Timer to generate an overflow interrupt every 52 µs.

3. Read the value of the transistor Q1 output signal in ISR (interrupt service routine) using the

microcontroller’s ADC input.

4. Make the PWM signal HIGH if the ADC input is less than the threshold level.

5. Make the PWM signal LOW if the ADC input is greater than the threshold level.

By reading the output signal of transistor Q1 with the on-chip ADC, the biasing signal (1-bit

DAC) is adjusted by software.

On motion detection, the signal from the sensor tries to change the threshold voltage level. But

as the threshold voltage is kept at a fixed level by the biasing signal, this leads to a change in

the duty cycle of the biasing signal. This variation in the duty cycle can be detected by calcu-

lating the number of times the biasing signal (calculated by variable low_var as also shown in

flow chart) is made LOW in a fixed time, 52msec (1000 times the periodic interrupt) in our soft-

ware.

Digital

Filter

1 Bit DAC

+

+

-

-

Integrator

Comparator