Replicate unknown simple circuit

Discussion in 'The Projects Forum' started by Jag140mc, Feb 13, 2012.

  1. Jag140mc

    Thread Starter New Member

    Feb 12, 2012
    3
    0
    Gentlemen, IMG_8635.jpg

    IMG_8636.jpg

    I have a device that I am trying to duplicate. It gives an audible tone when the test leads are connected, and when there is a very small change in the resistance between the two test leads, the frequency of the tone changes. It is 3 volts (2 x AA), has a speaker, 2 transistors, I believe (one marked, one not) a resistor (330K ohm), and a capacitor (unmarked). Pretty simple, but I can't figure out how I can replicate it. I'm guessing it uses the small change in resistance from the leads to change the current to the base of one of the transistors and thereby amplifies the change in current flow, but that's as far as I can get.
    I’ve attached pictures of the small circuit board... my diagram of the circuit with measured voltages with the test leads disconnected and connecter is too large, I can send it by email. My email is <SNIP>.

    The N33 6814 is certainly a transistor… NPN...?
    Not so sure about the TO3 case unit... a PNP transistor?
    Looks like the collector of the N33 feeds the base of the TO3 unit?
    Anyway, any thoughts appreciated. I want to duplicate the circuit, and don't know where to start. Thanks so much for any help… very much appreciated. I’m happy to provide additional information if needed. Michael Riley Riverside, CA
     
    Last edited by a moderator: Feb 13, 2012
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,662
    633
    It looks like a Darlington connected pair with a 33k 1/2 watt resistor in series with the collector of the first (smaller) transistor to limit the maximum current through the smaller transistor. Where did you get this relic of early transistor circuitry?

    More on Darlington transistors:
    http://en.wikipedia.org/wiki/Darlington_transistor
     
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  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Boy that is an old one.

    I do not see any circuits. But just a darlington on a heat sink.

    Care to show more of the circuitry...

    What you want is do-able. But we need to know more about the circuitry.
    As in show some more details.

    It would make job a lot easier.
     
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  4. Jag140mc

    Thread Starter New Member

    Feb 12, 2012
    3
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    Dick,

    I purchased a "buzz box" in the 1970's. It is used to time magneto fired engines like Maico, Husqvarna. The change in resistance from points closed to open is miniscule, but this device senses this and changes the frequency of the audio tone output. It still works!

    R!f@@ I'm able to attach my diagram of the circuit as a PDF file. The tab on the N33, and the assymetric location of the connections on the TO3 cased unit are supposed to indicate their function... (B,E,C?) and I've tried to show that in the diagram.

    I appreciate your comments and suggestions! Thanks so much!
    Michael Riley
    Riverside, CA
     
  5. mcasale

    Member

    Jul 18, 2011
    210
    12
    It looks like you have a good old-fashion oscillator here. See attached [redrawn] schematic.

    I am guessing that the change in resistance at the test leads changes the operating point of the PNP guy, and that's why the frequency changes.

    I'm a little rusty on this sort of design.

    The capacitor provides some positive feedback from the base of the NPN to the Collector of the PNP at certain frequencies.
     
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  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Blimey, it's a tone generator of some kind.

    U can duplicate quite easily.
    Have u found a replacement for the transistors.

    I believe any Audio frequency amp transistor will suffice.
     
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  7. Jag140mc

    Thread Starter New Member

    Feb 12, 2012
    3
    0
    Gentlemen,
    Thanks so much for your help! Mcasale, thanks for the clean circuit Diagram. I plan to use a "general purpose" NPN transistor, an audio output PNP transistor, and if I can't measure/estimate the capacitor value, trial and error on that component. I'm having fun with this, learning some new things, and am most appreciative of your help!
    Kindest Regards,
    Michael Riley
    Riverside, Ca
     
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