Measure/Blog

Share your experiences with Measure Activity here!

(be sure to include as many pictures as you can)

Some of my ideas, thoughts, back-of-an-envelope sketches and musings regarding hardware interfaces to Measure:

A redrawn schematic of the differential amplifier interface as shown on Measure/Projects - it should be a bit clearer and easier to follow its electronic operation.

A redrawn schematic of the differential amplifier as shown on Measure/Projects - I've added a fuse and 5.1V Zener diode on the XO input, as you can see - this should, hopefully, be suitable for making these types of electronic interfaces for the XO robust and "noobie-proof".

This is all my half-baked, untested, back-of-an-envelope stuff - no promises it works! Peer review, comments, feedback, or criticisms are all welcome.

I don't claim all these ideas as unique or new - but I'm trying to take existing project ideas from the projects page and flesh them out a little more.

I don't have access to an XO-1 machine - if any developers who do might like to test my ideas, as they mature, I'd love to hear the results!

An example PCB layout for the above differential amplifier board. I'm sure someone might like to improve on my PCB design skills, or re-draw it using FOSS EDA tools.

I'm not sure if or how I can upload files in any old - non-image - file format. If so, I will upload - and thereby release under a Free licence - my EDA files for these designs in EAGLE's format.

I'd really like to have access to the schematics for the XO-1 for the microphone input socket - not the whole computer, but just the exact schematics and specifications for exactly what protection circuits or buffers or whatever there are between the input socket and the LM1888 audio chip.

Project ideas

Measuring daily changes in solar irradiance:

Using a suitable interface, such as the above, or even a simple voltage divider, to measure the DC voltage from a photovoltaic module, and log how it changes over a period of a day - I'm sure we could even use the OLPC photovoltaic charging module for this - it has a 12V DC output - just make sure your amplifier/buffer interface will be suitable for the 12V input, and will scale it down to 0-3V, and plug it in!

Optical barrier alarm:

Turn on the voltage bias, and connect a LDR. Say our LDR has 500k dark resistance and max 2800 ohms in light. (That was measured from a mystery one in my junk box.) Given 785 mV bias with 790 ohms effective impedence, we have then, 784 mV dark input voltage, and 612 mV light input voltage - quite detectable discrete levels.

1 mW 660nm red laser diode modules are cheap and easily accessible for experimenters - but remember, if working with children or young people, teaching prudent laser safety is a good idea, even at this power level.

Temperature acquisition:

Using the simple LM335 probe, described in detail at Measure, a simple classroom demonstration of Newton's Law of Cooling can be realised, using the computer as a datalogger.

Biomedical instrumentation / Demonstration-grade Electrocardiography:

This isn't intended to be a fully fledged, powerful, diagnostically suitable or useful ECG platform in the same way that TeleHealth is aimed at realising - but it should work, and it might even work well enough with the XO's integrated ADC to work as a proof of concept, or a demonstration.

It's a simpler, smaller, cheaper, non-diagnostic ECG, that might be valuable as a teaching or demonstration tool. It will certainly work, and show an ECG signal. At least, it should.

High-gain, high CMRR instrumentation amplifier back-end suitable for very basic ECG. Does not include AD conversion.

This only has two electrodes - and needs to be combined with an ADC - probably the XO's inbuilt ADC - the voltage levels will always be within 0-5 V.

As in the above schematic, a 5.1V Zener diode can be included on the input line to the XO to clamp it - along with a fuse to limit current into the XO at 100 mA; "just in case".

Also, as in the schematic shown above, power is also derived from the USB port's 5V rail.

Water tank level measurement

The easiest way to realize this would be using a small differential pressure sensor, connected to a plastic hose which is inside the bottom of the tank, and comes out the top to the sensor. The analog output from the sensor can then be interfaced, fairly simply, to the XO.

The Freescale Semiconductor MPX12GP looks ideal for this.

Milk Quality Sensor

As many of you will know, there's been a lot of talk about this interesting project in relation to the FabLab project in rural India. It would certainly be a cool application to port over to OLPC, for the developing world, with the XO hardware and software based on or derived from the Measure activity. Unfortunately, I've got no idea how the milk analysis works, or how it is implemented in hardware.

It could be done with an electronic pH probe, probably.

Pulse Sensor - Infrared Pulse Plesysmograph

An IR plesysmograph (That thing that you might have seen used in a hospital for example, where it is clipped on the finger and measures heart rate, by transmitting IR light through the tissue) can be constructed quite cheaply, using a 5mm IR LED, and an IR phototransistor, mounted on a standard wooden clothes peg, in conjunction with a simple interface circuit, similar to those shown above, based on an LM358 operational amplifier IC.