UV-badge

The new (UV printed) plexi cover is finally ready!!!

Excellent resolution!

The temperature, humidity and barometric pressure sensors have been changed to the Bosch BME280 integrated environmental sensor.

Reasons:

• One sensor instead of two components, means lower cost, needs less space, and consumes less power
• The energy hungry I2C interface has been changed to SPI
• The BME280 provides absolute pressure sensing (while the former LPS25 needs calibration)

In the original concept it was planned to 'estimate' the UV Index using an si1132 sensor. Long, torturous development lead to a divorce from this idea, however;

some reasons:

• si1132 measures visible light, then estimates the UV index; this can be very inaccurate in cloudy situations (according the datasheet as well)
• The new sensor (ML8511) measures true UV radiation
• The I2C interface should be dropped, as it's too energy hungry
• The si1132 needs a complicated driver which is memory and energy inefficient

More things about the si1132 operating method:

• The I2C interface is not static, it has a minimum clock frequency requirement...

Totally unacceptable! With a slower MCU, using software or semi­-software I2C interface this can't be used! Usually I2C and SPI interfaces are static, can be driven as slow as you want.

• The command operation expects a strange trial­error method. According to the datasheet you give a command, then you have to check whether it has been processed. If it has not been processed after several ms, it has to be repeated. What???? It's a very energy consuming process! Anyway we are talking about a digital interface which should process commands, but it acts more like a spoiled princess...

So say good by to si1132 (sorry Silicon Labs...)

Say hello to ML8511!

The version 2 of the device is ready!

Smaller then the previous one.

The design of the second version has been started.

The main reasons of the redesign are

• the touch button will be substituted with mechanical tactile button (because energy conserving reasons)
• without the touch button the case can be smaller

A preview of the facelifted 3D printed case:

The case is designed with FreeCad, and will be printed at Shapeways, as usual.

Sunburn can be controlled by observing the accumulated UV exposure. This measure can be described with the "UV index hour" unit.

Some explanation for the MED - UVIh conversion:

The general MED (minimal erytherma dose) values for the different skin types (Fitzpatric skin type scale) are the following:

Type I - 200 J / m2
Type II - 250 J / m2
Type III - 350 J / m2
Type IV - 450 J / m2
Type V - 600 J / m2
Type VI - 1000 J / m2

Conversion to UVIh:
J / m2 = Ws / m2
1 UVI = 25 mW / m2
1 UVIh = 25 mW / m2 * 3600s = 90 Ws / m2
UVIh = MED / 90

The firmware is nearly ready!!!

The equipment measures the following values in every 2 seconds:

• barometric air pressure
• air temperature
• relative humidity
• UV index

Additional features:

• amount of accumulated UV 'load' - ( UVIh - UV Index hour ) - more info coming soon...
• chart of historical values for the selected measure
• battery level

The program hardly fits the ATtiny AVR program memory...

The excellent quality 3D printed case is ready. The plexi cover is a laser cut 1.5mm thick plexi piece, with a simple white adhesive plastic mask.

The case was printed by Shapeways with SLS technology.

Custom contact connector clip, so only six pads needed on the PCB. Very space efficient!