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Uses better LDR x lux model

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This commit is contained in:
rbaron 2021-09-22 20:40:31 +02:00
parent ba361e099c
commit a00559a98b
2 changed files with 16 additions and 18 deletions
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2
code/b-parasite/config/prst_config.h
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@ -6,7 +6,7 @@
// Built-in LED.
#define PRST_LED_PIN NRF_GPIO_PIN_MAP(0, 28)
// Photo Sensor
// Photoresistor pins.
#define PRST_PHOTO_V NRF_GPIO_PIN_MAP(0, 29)
#define PRST_PHOTO_OUT NRF_GPIO_PIN_MAP(0, 2)

32
code/b-parasite/src/prst/adc.c
View file

@ -1,6 +1,7 @@
#include "prst/adc.h"
#include <app_error.h>
#include <math.h>
#include <nrf_drv_saadc.h>
#include <nrf_log.h>
#include <nrf_saadc.h>
@ -136,31 +137,28 @@ prst_adc_photo_sensor_t prst_adc_photo_read(double battery_voltage) {
prst_adc_photo_sensor_t ret;
ret.raw = raw_photo_output;
ret.voltage = (3.6 * raw_photo_output) / (1 << PRST_ADC_RESOLUTION);
// This value needs to be calibrated.
// The photo resistor forms a voltage divider with a 10 kOhm resistor.
// The voltage here is measured in the middle of the voltage divider.
// Vcc ---- (R_photo) ---|--- (10k) ---- GND
// Vout
// So we can estimate R_photo = R * (Vcc - Vout) / Vout
const double photo_resistance =
1e4 * (battery_voltage - ret.voltage) / ret.voltage;
const float photo_resistance =
1e4f * (battery_voltage - ret.voltage) / ret.voltage;
// TODO: Now that we have the resistor value of the photo resistor, we need to
// estimate the brightness level. This needs to be calibrated with a real
// board in complete dark and in a super bright environment. This current
// value is just a placeholder.
// Dark resistance: 1 MOhm.
const double kDarkResistance = 1e6;
// Light resistance: 10 kOhm.
const double kLightResistance = 1e4;
// A value in 0x0 (dark) - 0xffff (light).
// A little better, but still not great.
ret.brightness = (uint16_t)UINT16_MAX * (kDarkResistance - photo_resistance) /
(kDarkResistance - kLightResistance);
// The relationship between the LDR resistance and the lux level is
// logarithmic. We need to solve a logarithmic equation to find the lux
// level, given the LDR resistance we just measured.
// These values work for the GL5528 LDR and were borrowed from
// https://github.com/QuentinCG/Arduino-Light-Dependent-Resistor-Library.
const float mult_value = 32017200.0f;
const float pow_value = 1.5832f;
ret.brightness =
MAX(0, MIN(mult_value / powf(photo_resistance, pow_value), UINT16_MAX));
#if PRST_ADC_PHOTO_DEBUG
NRF_LOG_INFO("[adc] Read brightness level: %d (raw); %d (brightness)",
ret.raw, ret.brightness);
NRF_LOG_INFO("[adc] Read brightness level: %d (raw); %d (lux)", ret.raw,
ret.brightness);
#endif
return ret;
}