6.6 KiB
ML8511
Arduino library for ML8511 UV sensor
Warning
Always take precautions as UV radiation can cause sunburn, eye damage and other problems.
Do not expose yourself to the sun as UV source too long.
When using artificial UV light (TL LED laser a.o.) use appropriate shielding.
Description
ML8511 - UV sensor - library for Arduino UNO.
Breakout
// +-------+--+
// VIN |o +-+| mounting hole
// 3V3 |o +-+|
// GND |o |
// OUT |o |
// EN |o S | Sensor
// +----------+
Operational
As the sensor / breakout is 3V3 one need to connect to Arduino 3V3. The library converts the analogRead() to voltages, and it uses a reference of 5.0 Volt == 1023 steps as default.
If one wants to use other ratio e.g. 3.3 volts == 4095 steps, one can set those with setVoltagePerStep().
ML8511 light(A0, 7);
light.setVoltagePerStep(3.3, 4095);
It is possible to always enable the sensor by connecting the EN pin to 3V3. The value of the enablePin in the constructor should then be omitted or set to a negative value;
When connecting to an Arduino UNO one can use the 3V3 of the Arduino to power the sensor. However it is not possible to connect the enable pin directly to the sensor. Use a voltage divider (10K + 20K) to convert the 5 Volts to ~3.3 Volts.
Interface
- ML8511(uint8_t analogPin, uint8_t enablePin = 0xFF) Constructor, if enable is connected to 3V3 constantly one does not need to set the enablePin parameter.
- float getUV(uint8_t energyMode = HIGH) returns mW per cm2, energyMode = HIGH or LOW. LOW will disable the sensor after reading.
- float voltage2mW(float voltage) returns mW per cm2 from voltage. To be used when one uses an external ADC e.g. ADS1115
- void setVoltsPerStep(float voltage, uint32_t steps) to calibrate the internal ADC used. Voltage must be > 0 otherwise it is not set and the default of 5 volts 1023 steps is used. This function has no meaning for an external ADC.
- float getVoltsPerStep() idem.
- void enable() manually enable.
- void disable() manually disable.
- bool isEnabled() get enabled status.
Experimental
WARNING: USE WITH CARE
- float estimateDUVindex(float mWcm2) input in mW per cm2, returns a value between 0 and ~15(?)
- void setDUVfactor(float factor) set the conversion factor
- float getDUVfactor() returns the set conversion factor (default 1.61)
See below how to determine the DUV factor for your sensor.
Note: The UV index can be very high, in La Paz, Bolivia, one of the highest cities in the world the DUV index can go above 20. See link below. This is really extreme and it is unknown how the ML8511 sensor (and this library) behaves under such conditions, and how long the sensor would survive. Datasheet goes up to 15 mW per cm2, with a default DUVfactor of ~1.61 the measurements could handle DUV of ~24 in theory.
https://edition.cnn.com/2021/11/03/americas/bolivia-heatwave-highlands-intl/index.html
Sensor sensitivity
Indoors there is very little or no UV light so use a known UV source like a black-light or go outside in the sun.
The formula to convert the ADC reading to mW cm^2 is based upon the graph shown in the datasheet. As I have no reference source to calibrate the library the accuracy is limited at best. If you know of a calibrated source please let me know.
The sensor has its peak sensitivity ( >80% ) from λ = 300-380 nm with an absolute peak at λ = 365 nm.
Experimental DUVindex
Note: this library is NOT calibrated so USE AT OWN RISK
The DUV index can be used for warning for sunburn etc.
DUV index table
Based upon https://en.wikipedia.org/wiki/Ultraviolet_index,
| DUV INDEX | Description |
|---|---|
| 0 - 2 | LOW |
| 3 - 5 | MODERATE |
| 6 - 7 | HIGH |
| 8 - 10 | VERY HIGH |
| 11+ | EXTREME |
Calibrate estimateDUVindex()
To calibrate the estimateDUVindex() function one needs to determine the DUVfactor. To do this you need an external reference e.g. a local or nearby weather station. You need to make multiple measurements during the (preferably unclouded) day and calculate the factor.
DUV from weather station
factor = --------------------------
getUV();
you do this e.g. once per hour, so you get multiple values. You can then average them to have a single factor.
Hardcode this found value in the library (in the constructor) or better use the setDUVfactor(factor) call in setup() to calibrate your sensor.
0.1.5 and before
The formula for the experimental estimateDUVindex(mWcm2) is based on the following facts / assumptions:
- (fact) The sensor cannot differentiate between wavelengths, so integration with different weights is not possible.
- (assumption) All the UV is radiated at λ = 300 nm. This is the most lethal the sensor can sense > 80%.
- (choice) All the UV is accounted for 100% for the whole value. (Erythemal action spectrum) As we cannot differentiate wavelengths, this is the safest choice.
0.1.6
The formula is simplified to a single factor that the user needs to determine. Below is described how to do the calibration.
External ADC
float voltage2mW(float voltage) can be used for an external ADC e.g ADS1015, ADS1115 or one of the (fast) MCP_ADC's.
https://github.com/RobTillaart/ADS1X15 https://github.com/RobTillaart/MCP_ADC
More about UV
https://en.wikipedia.org/wiki/Ultraviolet_index
Notes
- 3V3 Sensor so do NOT connect to 5V directly.
- do not forget to connect the EN to either an enablePIN or to 3V3 (constantly enabled).
Future
- refactor / reorganize readme.md
- test more
- get unit tests up and running
- investigate in calibration
- check performance
- investigate serial UV communication with UV led
- voltage2mW -> handle negative voltages by taking abs value?