Renesas RL78/I1D User Manual page 48

DETECT-IT RL78/I1D detector boards kit
• Second stage AC coupled amplifier (Ch0 operational amplifier):
The second stage is non-inverting and has a gain of 2. The upper frequency (cut off/corner) is ~8 KHz.
At time "B", an ADC conversion samples (and saves) the reference Ch0 voltage level, which is proportional to the
Photodiode current amplified by Ch1 and Ch0, with no Infrared LED energy emitted, so only ambient room infrared
energy is measured.
At time "C" the IR LED is turned on and it takes approximately 160-180µs for the resulting trans-impedance result to
settle at Ch0 output.
Then at "D", a second ADC conversion reading is taken (and saved).
At time "E", the IR LED and both operational amplifier channels are turned off.
A smoke detection algorithm is implemented that looks at the increase in Infrared Photodiode detection current from
ambient condition (Infrared LED OFF) to Infrared LED ON condition. A running average of the "delta" level (ADC
measurements at timeframe D – timeframe B) is calculated. The most recent delta level has the highest weighting, and
the algorithm looks at both the magnitude in Photodiode current change (delta), as well as the rate-of-change. This is
done to make the smoke detector react quickly without responding to IR Photodiode output random changes due to
ambient Infrared noise energy.
Note : The RED waveform is the output of Ch0 operational amplifier
The BLUE waveform is the Infrared LED enable signal (high level indicates LED is on). Horizontal (time)
reference is 100µs per division.
• Inputs comparators with low level and high level DC threshold:
The comparators are not used in this smoke detector, since the ADC is used to measure the output of Ch0 for Smoke
level threshold. Therefore Jumper J3 which powers the IVREF0 and IVREF1 resistor bias chain can be removed to save
current drain.
UM-YDETECT-IT-RL78 V1.30
Figure Waveforms of Smoke detector sampling
Kit User Manual
Page 48 of 59