The Si1133/5x Optical Sensor EXP board now has an example project running on EFM32PG12 MCU to demonstrate our latest Si1153 for proximity/ambient light sensing and Si1133 for UV sensing. The example project will require an EFM32™ Pearl Gecko PG12 Starter Kit(SLSTK3402A) to plug in the Si1133/5x Optical Sensor EXP board through the 20pin expansion header.
The source code of the example project is provided in the attachment.
We also have a nice video describing all the features included in the demo. Please make sure to check it out before getting started.
1. I2C address
How many different I2C addresses does Si1133 support? How to set them?
Si1133 supports 2 different I2C addresses. If the AD pin is high during the power-up, the sensor will come up with the default I2C address 0x55. If the AD pin is pulled low during the power-up, the sensor will use the alternate I2C address 0x52.
2. I2C communication failure
Why does the host fail to communicate with the sensor through the I2C interface?
There're a couple of possible causes:
a. Pin 7 MUST be pulled high during the power-up. If it's not, the sensor won't be powered up properly.
b. Check the I2C address and make sure that the correct I2C address is used.
c. Check the I2C timing specifications and see if that meets the requirement listed in Table 8.3 in the datasheet, especially the I2C frequency and rise/fall time.
3. Power-on reset
Why could the host fail to communicate with the sensor upon power-on reset?
During the power-on reset, Vdd must drop below 0.5V for the sensor to properly reset itself. If the Vdd is held between 0.5V to 1V, the sensor will enter an unknown state and fail to power up normally.
4. UV index accuracy
What's the UV index accuracy of Si1133?
With diffuser, the UV index accuracy can reach +/-0.75UVI. Without diffuser, the accuracy is about +/-2UVI.
5. UV spectrum response
What's the UV spectrum response of Si1133 look like?
Si1133 is responsive to both UVA and UVB wavelength. Silicon Labs does not provide spectrum response information of Si1133.
6. Optical Design
Why do Si1133 require a diffuser?
All standard UV instruments have diffusers and require the sensor to be facing straight up vertically regardless of where the sun is. Without a diffuser, the UV sensor will have limited view angle. This is the same for competitors’ product.
7. Sensor output
Does the Si1133 output UV Index directly?
No, the ADC readings need to go through a set of coefficients, and the results will be the UV index. There are two sets of coefficients - one for diffuser designs and one for designs with no diffuser.
Can Si1153/33 sensors be used for ambient light sensing? How to convert ALS results to LUX values? Any example code available?
Only Si1153-AA00-GM and Si1133-AA00-GM parts can measure ambient light. Si1153-AA09-GM and Si1153-AA9X-GM parts cannot measure ambient light due to the on-die 940nm filter.
There's no simple equation to convert ALS measurement results to LUX values by any means. The only solution is to perform tests under certain light sources and calibrate ALS results against LUX values read from a LUX meter. Then find the ratio or formula to estimate LUX values based on ALS measurement data. Since it's an estimation, the accuracy won't be anywhere close to a LUX meter. Si1153/33 sensors CANNOT be used in LUX meter type of applications, but can still be used in applications that only require an approximate LUX level.
We've built a model to use 3 different channels' ALS measurement results to estimate LUX values. The example code is attached. However, the model is overly complicated and we recommend the customer to only use that as a reference and develop their own equation to estimate LUX.
Silicon Labs doesn't support ALS calibration for any applications.
How to calibrate Si1133? How is UV index calculated?
To achieve the best performance, calibration is essential for Si1133 because of sensor-to-sensor and unit-to-unit variation in sensor's placement with respect to the diffuser (or window opening), as well as the variation in the material of the overlay and the diffuser. We recommend the customer to perform calibration on the prototype after the optical design is completed.
There're 2 options for the light source in the calibration procedure, one is to use the sun and the other is to use a solar simulator. A commercial UV index meter is also required as the reference. Here're the basic steps for calibration. First of all, set up a test such that the Si1133 sensor and the UV meter can be placed at the same location under the light source. Secondly, log multiple readings of both the sensor and the UV meter across different UV index levels (0 ~ 10 UVI). Lastly, fit the result to a second-order polynomial equation:
UV_index_reading = k(m × Si1133_raw_data ^ 2 + Si1133_raw_data)
The coefficients k&m can be later used to calculate UV index based on Si1133's readings.
Silicon Labs provides default configuration and coefficients to calculate UV index as well as optical design recommendations in the UV application note AN968:
The software example code is also attached.
Is there an example project working with the Si1133/5x-OPT-EXP evaluation board on 8bit MCU? If so, where can I get it?
The only example project we have for Si1133/5x-OPT-EXP evaluation board is built on the EFM8 BusyBee STK. You can download it in the attachment. The project can be imported into the Simplicity Studio v4. For any project related questions, please submit support tickets to the 8-bit MCU support team.