This key for morse code practice was built by En’s grandpa. After her uncle passed she was able to get this from her grandparents’ house.
Amazingly, even with it’s age and almost bursting batteries it’s still able to make some feeble sounds.
The last time I saw batteries like these was sometime in the early 80s. They still put out a bit over 2V after all this time.
At this point I’m still in the process of reverse-engineering the circuit so I’ll leave that to another week… but it’s an interesting thing to look at. It’s old-school hardware hacking with the leads flying every which way in a salvaged radio case without a back.
I especially love the 69-ing 9V jacks to connect the soldered-in batteries!
The total bill of materials for the active parts of the circuit:
- 2x AA batteries
- .05uF (I think) cap
- 10K resistor (though on the body is printed, strangely, 100,000)
- 2N438 NPN transistor
- TO3 transistor (I have a hunch it’s PNP based on probing)
- Switch (key)
My guess is this is all out of his parts bin. :-) There is no reason you’d need to use a big can TO3 power transistor for a 3V circuit like this!!
UPDATE: I did a bit of disassembly and the TO3 transistor is a 2N175 PNP transistor so my guesses based on hitting the transistor with the diode test mode of the Fluke. Test equipment FTW!
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