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Optimal interface (with snubber) for a 4 ohm telegraph sounder?

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#1 remotewizard


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Posted 16 May 2014 - 02:20 PM

I'm finally taking Morse Code classes, trying to learn my dits and dahs.  So I naturally poked around the various Morse projects in the forums.  But now I have a more nuts-and-bolts kind of question.


I purchased a nifty antique telegraph sounder (basically a relay, when the clicking and clunking represents Morse dits and dahs).  There are several projects online to interface such a sounder with a *duino or a Raspberry Pi (such as http://sigalonhowtod...since=318825083).  What I'm looking for is the best design that will protect both my Netduino and my antique sounder.


The sounder has a 4-ohm coil, and (according to was designed to run off an approximately one volt 'Gravity Cell.'  The same article suggests running the sounder off a D-cell battery (current draw estimated at 200 mA).


As I said, there's a lot of suggestions online for how to connect the sounder.  Most use a simple transistor to switch a 5 volt power supply to energize the sounder's coil.  This seems a bit high to me, but since the sounder is built like a concrete bulldozer, I could risk it.


More to the point, what kind of snubbing configuration would you recommend to protect the Netduino from induced EMF.  Most folks seem to use a simple diode in parallel with the coil (which would shunt the induced current safely to ground).  Some add an LC circuit; a few suggest replacing the simple snubber diode with two Zeners (diodes wired in series, cathode-to-cathode, then attached in parallel with the coil).


Since I don't want to damage my sounder unnecessarily, and it's pretty much equivalent to a big low-voltage relay, I figured I'd ask the experts for the optimal solution.


Sorry to be so long-winded!

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