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Archive | June, 2012

Pulse Sensor II: Getting the Lights On

Analog vs Digital Sensors

When it comes to sensors I was always an analog guy. Analog sensors put out voltages or currents that are easy to verify with a multimeter. Or if you really need a pretty picture then you can hook up an oscilloscope and see a very complete story. Analog sensors tend to be simple, and have datasheets that tend to be under ten pages long. However, these days, most of the time, analog voltages are going to get run into A/D converters for some further digital domain processing and decision making.

So the chip makers have decided that if the signal needed to be digitized, why bother running the signal into a microcontroller, and perhaps degrading the signal along the way? Why not instead build the A/D converter into the chip and give the customer a signal that has been pre-sliced and pre-diced. And while the chip makers are adding an A/D converter, how about some signal processing and adaptive sensing? Why not build a whole small processor into the sensor to try to give the user options. Perhaps 200 or so options? So simple sensor chips are leveraging their digital interfaces and turning into swiss army knives of options and functionality

And to help the user get acquainted with all those high-powered options, the chips need to have datasheets that explain the options. Sometimes 78 pages long. So you might ask why a 78 page datasheet is necessary  just to read some photodiodes with accompanying LEDs? And I guess the answer would be: Yes, we are going to turn on some LEDs and read some photodiodes, but with lots of options and flexibility for various users and applications.

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Pulse Sensor I

Making a Pulse (SPO2) Sensor

Many electronic hobbyists and people who are interested in science are aware that the human pulse may be detected by shining a light through a finger or earlobe. The technique is formally called photoplethysmography generally, but also more simply pulse oximetery and PSO2 sensing. It is the basis for the PSO2 (pulse and oxygen saturation instruments) that you may have experienced in a hospital. Inexpensive (~$50) consumer grade devices are now also available in drug stores and online.

Although the discovery of the principles involved in pulse oximetry dates to 1935, the development of smaller, more simple machines didn’t happen until the 1980s (following LED and photo-detector development). Pulse oximetery then revolutionized oxygen-level monitoring during anesthesia, and has since become common in other parts of hospitals and found several other medical applications. These include non-invasively sensing respiration, affective state (emotion), and general fitness. More on that later.

Electronic hobbyists who have tried this technique themselves are often frustrated or disappointed due to some of the challenges in sensing the signal. The optical signal itself is not large, in the range of one tenth of one percent ( 1 / 1000 ratio). For electronic hobbyists, one way to think about this signal is a 1 volt AC signal sitting on a 1000 volt DC offset. This makes it necessary to shine as much possible light as possible through the finger or earlobe, while taking care not to saturate (max out) the photodetector.

Several forms of noise also can degrade the signal. Below is a short list of major forms of noise and error in the signal, and their causes. Because of the small signal size of the signal in the illumination domain, any movement of the sampled body part in relationship to the sensor, can appear as an optical signal many times as a large as the desired optical signal. Any light that leaks into the sensor from outside will also register as a noise source. Finally, once the signal is translated into an analog electrical domain, high-gain analog amplifiers often couple AC hum (50 / 60 hz) in power lines into noise that appears in the output signal.

There are some additional challenges in constructing PSO2 sensors that will work successfully on a wide range of people and conditions. Even when using the same LED illumination, the amount of light reaching the sensor can vary significantly with finger size (ever shake hands with a football lineman?), skin color, and even things like nail polish. This usually makes it necessary to control the amount of “raw” illumination, as well as the gain of the photo-detector. PSO2 sensor makers have also noted very large changes in signal when measuring cold hands, because the body can shut down peripheral capillaries in the cold. (Or maybe it’s just MY problem). Finally, pressing too hard on the sensor results in (a “white knuckles effect” – only it’s your fingers). The medical profession just calls this “limited perfusion” (aka, no bloodflow). The medical PSO2 equipment makers have spent a fair amount of time and research trying to ameliorate some of the problems listed above.

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Full Spectrum Engineering 40W Hobby Laser

At Modern Device, we construct a lot of surface-mount printed circuit boards. The process involves putting paste solder on PCBs, and for that, you need stencils.

We were using the Epilog Mini 24″ laser at (Providence’s) AS220 Fab Lab, but it wasn’t convenient to go to the lab every time we needed a stencil, and it was more than inconvenient when the stencil was flawed in some way.

So we took what we knew would be something of gamble and purchased a 40W FSE hobby laser for about $2500 from Full Spectrum Engineering about a year ago.

We excitedly unboxed it, filled a bucket with water to cool the tube, hooked up the air compressor for air assist, aligned the head, fired it up, and… pushed the pulse button and got a large splat.

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June Sale

It’s summer!

Modern Device is having a sale in tandem with our European collaborators JeeLabs for the whole month of June.

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During the month of June, everyone who has previously purchased something from the Modern Device shop (i.e. before June 1st 2012) can use discount code “MD2012” to get 12% off all items.

The sale will last through June 30 both at Modern Device and at JeeLabs in Europe:

The sale will end June 30th – sale ends at midnight.

One quirk is that you have to have bought something earlier to get the discount. This is to reward our old-time customers and others who have found Modern Device and JeeLabs designs useful. It applies to everything in the shop. Once you buy anything, you’re eligible for future sales forever.