Rick's b.log - entry 2018/12/24

Playmobil Advent Calendar

See them all in the Advent Calendar 2018 playlist.

Tree

Photos

And in a land far far away, a small playground and olde village shoppe.

Weirdo food

Perhaps my Christmas dinner of veg (peas, crunchy peas, runner beans, and broccoli), rize (koshihikari), and Frey Bentos "Just Chicken" might not sound terribly appealing. Fair enough. The Frey Bentos is a tradition guilty pleasure... but I just can't imagine sitting down to a nice hunk of Zebra after a Crocodile starter...

No comment

Pulse oximeter

A similar method exists where it is possible to shine a bright light directly at skin and record subtle colour variations reflected off the skin using a camera. This is the principle used by those apps that claim to take your heartrate using just your phone's camera. It is also the method used by pulse detectors such as those on various Samsung Galaxy phones (though it's a special optical sensor rather than a general purpose camera).
In theory the same sort of mechanism could be used by simply pointing a video camera at somebody. The subtle variations apply also to skin, such as the face. There will be tiny differences with each heartbeat. The problem with this is that the variations are miniscule (go stare at a significant other and see if you can notice it, you probably cannot) and while a video camera may have a degree of accuracy that would allow it to 'see' such tiny differences, the main factor here is that conditions are far from ideal - people moving, makeup, lights that aren't bright enough, and mains lights spewing out 50Hz or 60Hz pulses...

Of course, this sort of job sounded perfect for a cheap and simple microcontroller - and it seems like the microcontroller of choice these days is one of the ARM Cortex M series (probably an M0 core), and the device in question quite possibly one of the STM32 family...

There we go, an STM32F030C8. An ARM Cortex-M0 core with 64K Flash and 8K SRAM. Other onboard hardware is: One advanced (PWM), one basic, and five general purpose 16 bit timers; up to six serial protocols (2 SPI, 2 IIC, and 2 USART); a 12 bit ADC; up to 39 GPIOs; up to 48MHz clock rate; RTC (if 32kHz crystal provided); etc.

Of course if you're paying attention you'll have noticed the TX and RX pins just above the MCU in the above photo. I believe some of the slightly more expensive devices contain Bluetooth support, so this may be a serial datastream to send to a Bluetooth module? Might be worth investigating at some time.

Here's the other side showing the sensor, an op-amp, and power conditioning (power input is 2 AAA cells, nominally 3VDC). This photo hasn't been flipped, the text is printed back to front.

And finally...

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