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Thread: Repurposing an ADSL filter for protecting Teensy 3 inputs

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    Repurposing an ADSL filter for protecting Teensy 3 inputs

    What do you think of using an ADSL filter to protect Teensy 3.1 I/Os from a photodiode connected to a 2 meter cable?

    I understand this will filter out the high frequencies. I don't really understand the implications of applying the filter designed for 48V to a lower voltage signal. I'm guessing it won't hurt anything, but might obliterate the signal entirely.

    I'm working on a very simple Teensy 3.1 data acquisition system intended to help with basic Physics lab experiments. I'm planning for the sensors to be connected to unshielded two conductor stranded phone cord about 2 meters long. From this article Protecting Inputs in Digital Electronics I see some of the bad stuff that can happen when microprocessor I/Os aren't protected, but I don't have a way of measuring subtle damage or monitoring the signals to see voltage undershoot, ESD spikes, etc.

    At this stage, I'm not attempting to make something that will take years of harsh environment abuse and I'm inclined to start with something fast, cheap, and easy. I'm just trying to make a working prototype and learn what I can along the way. I'm not sure how far to go. For now, I'm using a 200Ω resistor in series with the analog input. Some of the protection circuits get pretty elaborate, but I'm not expecting to make something that will survive a lightening strike...

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    A pair of Scottky diodes to 0V and 3V3 (such as BAT42) would take care of under/over-voltage.

    An ADSL filter does a very different job - it prevents the high frequency square wave used to transmit data from being audible on a phone on your analog landline (if you still use one).

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    Nantonos, thanks!

    I took your note as a gentle hint that the ADSL filter wouldn't be likely to help that much, so I did a teardown on a "2Wire DSL Filter" model no. LFT4-1 and learned a few things. It is a single-sided through-hole design containing the following:

    153J 50V capacitor, qty 2
    222J 50V capacitor, qty 1
    123J 250V capacitor, qty 1
    inductor, radial package, 4 pins, diameter 0.38", height0.43", no markings, qty 2
    4.5KOhm resistor, qty 2

    The ADSL filter has two wires in (TX/RX) and two wires out (TX/RX), so clearly there's no place for under/over-voltage to go. It might help with noise, but that's easy enough to handle in software for my application.

    A Teensy 3.x will fit in the case for this phone dongle thingy. The case might be the most useful part!

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