Mini PAR meter - minimum tool to get a good numerical sense of light intensity

Commands

Commands are sent as plain text terminated with \n. JSON mode is also supported — send a valid JSON object and the device will respond in JSON.

• hello
  • Response: MiniPAR,1.1 (plain) / {"device":"MiniPAR","version":"1.1"} (JSON)
• reboot
  • Restarts the device. Prints boot info ending with Ready.
• delay_ms,<value>
  • Example: delay_ms,500
  • Blocks for <value> milliseconds, then responds with the delay value: 500 (plain) / {"delay_ms":500} (JSON). Missing/negative value is treated as 0.
• battery
  • Response: NaN (plain) / {"battery":"NaN"} (JSON)
  • Battery measurement not yet implemented.
• i2c_scan
  • Response: comma-separated list of I2C addresses that ACK (e.g. 0x39)
• spec[,<count>]
  • Response: {"spectrometer":{"model":"AS7341","channels":{"f1_415":1.2345,"f2_445":6.7890,...},"par":123.45}} (JSON) / model,ch0,ch1,...,par (plain)
  • Returns basic counts per channel: raw_reading / gain / integration_time (counts per second per gain unit). Values are comparable across different gain and integration-time settings.
  • Also includes the calculated PAR value (same as the par command), computed from this same reading.
  • <count> repeats the measurement that many times (omit, or 1, for a single reading); each reading is printed as soon as it's taken. With <count> > 1: JSON response becomes an array of the single-reading objects ([{"spectrometer":{...}},{"spectrometer":{...}},...]); plain response is the single-reading lines joined with ; on one line.
• spec_raw
  • Response: same format as spec but raw unscaled integer counts (no normalization applied).
• spec_flash,<mA>
  • Example: spec_flash,10
  • Dark read → LED on → lit read. Response: dark:<values>;lit:<values>;diff:<values> (plain).
• spec_set_atime,<0-255>
  • Set spectrometer ATIME integration register. Response: the value set.
• spec_set_astep,<0-65534>
  • Set spectrometer ASTEP register. Response: the value set.
• spec_set_gain,<0-10 (AS7341) | 0-12 (AS7343)>
  • Set spectrometer gain index. Response: the value set.
• spec_status or status
  • Response: model=<name>,available=<0|1>,atime=<val>,astep=<val>,gain=<val> (plain)
• set_led,<mA>
  • Example: set_led,20
  • Response: {"spectrometer":{"led_current_ma":20}} (JSON) / actual mA set (plain)
  • LED current capped at 20 mA.
• par[,<count>]
  • Response: 123.45
  • PAR value in µmol/m²/s, scaled by linear calibration: y = a·par_raw + b
  • <count> repeats the measurement that many times (omit, or 1, for a single reading); each reading is printed as soon as it's taken. With <count> > 1: JSON response becomes an array ([{"par":1.2},{"par":3.4},...]); plain response is the values joined with ; on one line.
• par_raw
  • Response: 1.23
  • Weighted dot product of basic counts (raw / gain / integration_time) and per-channel PAR coefficients. Independent of gain and integration-time settings.
• cal_par_slope,<value>
  • Example: cal_par_slope,10.0
  • Response: {"calibration":{"slope":10.0}}
  • Set the a coefficient for y = ax + b scaling. Persisted to NVS.
• cal_par_intercept,<value>
  • Example: cal_par_intercept,2.3
  • Response: {"calibration":{"intercept":2.3}}
  • Set the b coefficient for y = ax + b scaling. Persisted to NVS.
• get_cal_par
  • Response: {"calibration":{"slope":10.0,"intercept":2.3}} (JSON) / slope=10.0,intercept=2.3 (plain)
  • Get the current a (slope) and b (intercept) coefficients for y = ax + b scaling.
• set_spec_coeff,<channel>,<value>
  • Example: set_spec_coeff,0,0.614975
  • Response: {"spectrometer_coeff":{"channel":0,"value":0.614975}}
  • Set per-channel PAR coefficient (channel 0–17). Coefficients are applied to basic counts (raw / gain / integration_time), so they must account for the desired normalization. Persisted to NVS.
  • Default coefficients are pre-scaled for the default AS7341 settings (ATIME=100, ASTEP=999, GAIN=2×); re-derive from calibration if defaults are changed.
• get_spec_coeff
  • Response: {"spectrometer_coeffs":{"channels":{"0":0.614975,"1":0.053037,...}}}
  • Get all 18 per-channel PAR coefficients.
• get_spec_coeff,<channel>
  • Example: get_spec_coeff,0
  • Response: {"spectrometer_coeff":{"channel":0,"value":0.614975}}
  • Get a specific per-channel PAR coefficient.
• set_name,<string>
  • Response: {"device_name":"MyDevice"}
  • Set custom device name. Persisted to NVS.
• get_name
  • Response: {"device_name":"MyDevice"}
  • Get current device name.

BME280 (optional temperature/pressure/humidity sensor)

Detected automatically at boot (I2C address 0x76 or 0x77); all commands below respond with error:not_available (plain) / {"bme280":{"error":"not_available"}} (JSON) if no BME280 is fitted. Temperature, pressure, and humidity are all enabled by default, at the highest oversampling (x16), IIR filter off, one-shot read mode.

• bme_status
  • Response (plain): continuous=<0|1>,continuous_active=<0|1>,temp_comp=<value>,temp_en=<0|1>,temp_osrs=<0-5>,press_en=<0|1>,press_osrs=<0-5>,hum_en=<0|1>,hum_osrs=<0-5>,iir=<0-4>
  • Response (JSON): {"bme280_status":{"available":true,"continuous_mode":false,"continuous_active":false,"temperature_compensation_c":0.00,"temperature":{"enabled":true,"oversampling":5},"pressure":{"enabled":true,"oversampling":5},"humidity":{"enabled":true,"oversampling":5},"iir_filter":0}}
• bme_set_temp_enable,<0|1>
  • Enable/disable temperature in bme_get_tph output. Temperature is still sampled internally regardless (pressure/humidity compensation depend on it) — this only affects reporting.
• bme_set_press_enable,<0|1>
  • Enable/disable pressure in bme_get_tph output.
• bme_set_hum_enable,<0|1>
  • Enable/disable humidity in bme_get_tph output.
• bme_set_temp_oversample,<0-5>
  • Oversampling: 0=off (clamped to 1, since temperature must always be sampled), 1=x1, 2=x2, 3=x4, 4=x8, 5=x16 (default).
• bme_set_press_oversample,<0-5>
  • Same scale as above; 0=channel skipped by the sensor.
• bme_set_hum_oversample,<0-5>
  • Same scale as above; 0=channel skipped by the sensor.
• bme_set_iir,<0-4>
  • IIR filter coefficient: 0=off (default), 1=x2, 2=x4, 3=x8, 4=x16.
• bme_set_continuous,<0|1>
  • 0 (default) = one-shot: every bme_get_tph call takes its own forced-mode measurement.
  • 1 = continuous: the chip free-runs in normal mode; bme_get_tph blocks only on the first call after enabling (waiting for that first conversion), then returns whatever the chip's background cycle last produced.
• bme_set_temp_comp,<value>
  • Set the temperature compensation offset in °C, added to every raw temperature reading (and folded into the pressure/humidity compensation math) to null out self-heating bias. Persisted to NVS.
• bme_get_temp_comp
  • Response: current temperature compensation offset in °C.
• bme_get_tph[,<count>] (alias: tph[,<count>])
  • Response (plain): <temperature_c>,<pressure_hpa>,<humidity_pct> — positional CSV, blank field for a disabled channel.
  • Response (JSON): {"bme280":{"temperature_c":23.45,"pressure_hpa":1013.25,"humidity_pct":45.20}} — null for a disabled channel.
  • Behavior depends on bme_set_continuous: one-shot mode (default) takes a fresh forced-mode reading every call; continuous mode waits for the first conversion only on the first call after being enabled, then returns the most recent free-running reading on every call after that.
  • <count> repeats the measurement that many times (omit, or 1, for a single reading); each reading is printed as soon as it's taken. With <count> > 1: JSON response becomes an array of the single-reading objects; plain response is the single-reading CSV rows joined with ; on one line.

Status LED (GPIO 10)

The LED on GPIO 10 provides a visual indication of spectrometer state:

LED state	Meaning
Off	No spectrometer detected at boot
Solid on	Spectrometer detected, all channels within range
Blinking 20 Hz	At least one spectrometer channel is saturated

Saturation is checked automatically on every spectrometer read (spec, spec_raw, par, par_raw, etc.). The LED returns to solid-on as soon as the next read finds all channels within range. Reduce gain or integration time (spec_set_gain, spec_set_atime, spec_set_astep) to clear saturation.

Protocol

• Command terminator: \n (\r is ignored)
• PAR units: µmol/m²/s
• Basic counts: raw_reading / gain / integration_time, where integration_time = (ATIME+1) × (ASTEP+1) × 2.78 µs. Used by spec, par, and par_raw — values are gain/integration-time independent.
• Default AS7341 settings: ATIME=100, ASTEP=999, GAIN=2× (integration time ≈ 280.8 ms). Per-channel PAR coefficients stored in NVS are calibrated for basic counts at these defaults.

FW Design (L. Grabowski, L. Caracciolo),

HW Design (L. Grabowski)