Friday, February 01, 2008

Make Controller Kit

The Make Controller Kit is a pair of boards that snap together, with a AT91SAM7X256 as the microcontroller. For communication it has USB, Ethernet, CAN, and JTAG, and supports the Open Sound Control protocol, which has interfaces for numerous programming and scripting languages. It offers eight analog inputs with 10 bits of precision over a 0-to-3.3-volt range. There are eight high-current digital outputs that can drive relays or two stepper motors or possibly solenoids. There are four servo controllers. It has an 8-position dipswitch. It costs $109 at the on-line store for Make Magazine.


amiya said...

Good and informative. So I prefer to bother you with this.
I am trying to construct a homemade EKG. Using Zeitnitz's software 'scope' to use my computer as the display device. Power supply by 2 9v batteries and CMOS IC TLO84 as amplifier. But there is too much noise and screening doesn't help much. I have even omitted optocouplers to avoid complexities. I will add a 50 Hz filter. Can you help me with some design ideas?

Will Ware said...

Optocouplers might make your noise problem better as they would isolate your analog electronics from the noisy digital electronics in your computer. But they would complicate things because they generally can't send an analog voltage directly, usually only a binary on-off signal.

An EKG doesn't need much bandwidth so you might consider doing pulse-width modulation through the optocouplers. Here's a schematic for a triangle wave oscillator. Select a frequency that's much higher than the highest EKG frequency. I would imagine 10 kHz should work.

Compare the triangle wave to your analog signal using a LM339 comparator or something similar. The amplitude of the triangle wave should be controllable by the ratio of R2 to R3. Try to make it just a bit larger than the largest EKG signal. The open-collector output of the LM339 should drive the optocoupler's input LED through a current-limiting resistor, about 1K for a +9 volt rail.

Both the triangle wave and (I presume) the EKG signal will go negative relative to the ground between the two 9 volt batteries, and you want the LM339's inputs to see only positive voltages. You can set up a voltage divider for the triangle wave and an iodentical one for the EKG signal so that they are smaller voltages, but always above ground.

To get the signal to the soundcard, you'll want to get the pulse-width modulated signal from the optocoupler output, which will look like a square wave of varying width, and low-pass filter it. Here's where the wide gap between the triangle wave frequency and the EKG's maximum frequency will be helpful. By setting the low-pass filter's cutoff frequency a little above the maximum EKG frequency, you should be able to eliminate the square wave almost completely and be left with the EKG signal as it went into the comparator, and you feed that into the soundcard. You can power the LM339 using the +5 and ground from one of your USB ports.

I hope this is all clear, and meets your needs. Feel free to contact me at "wware AT alum DOT mit DOT edu" if you have any questions. Sorry for the slow response.

amiya said...

Thanks a lot Will. I'll go with the optocouplers then. Your explanation about optocouplers couldn't have been better.