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Old transistor magnetometer

Discussion in 'General Electronics Discussion' started by Nick Sparkes, Apr 15, 2014.

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  1. Nick Sparkes

    Nick Sparkes

    Apr 15, 2014
    Hi everyone.

    I have recently resurrected an old school project of 40 years ago. It's a magnetometer that we never got around to building.

    A nearby changing magnetic field is supposed to trigger the relay, which latches on until reset.

    I know there are much better designs now, but for nostalgic reasons I'd like to build it in its original form. I've started, but not getting very far...

    The circuit uses (unconventionally drawn!) PNP germanium transistors and I have them in my workshop - all tested and working.

    TR4 appears to be drawn in error with the collector/emitter reversed. The pick-up coil I'm using at present is an old Post Office 2000Ω solenoid.

    I've replaced the 50Ω relay with a 50Ω resistor for test purposes and running it from a 9 volt battery.

    The first stage appears to be working more or less like a conventional amp, but that's about all that's happening. At the Darlington pair TR2/3 everything seems to stop! No current flows through the 'relay resistor', even with a volt or so tapped onto C1.

    Is there something more seriously wrong with the circuit as drawn?

    Thank you.


    Attached Files:

  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Nick and welcome to Electronics Point :)

    Wow! It's a loooong time since I've seen transistors drawn like that!

    I think there's something wrong with TR4. Its base can never be pulled "below" 0V so its collector shouldn't connect to the 0V rail unless it's an NPN, and the symbol and parts list clearly show a PNP.

    Assuming TR4 was an NPN, the relay circuit should work, I think. When TR2/3's collector pulls "low" in response to magnetic signal pickup, TR4 will pull its emitter to 0V. That will be coupled through C5 into the relay coil, and the relay will close.

    Assuming that the vertical line with the dot at the bottom, below the relay contact marked RL1, is supposed to connect to the 0V rail, then the relay should latch on, via the contact marked RL2. Breaking the circuit by pressing the normally closed pushbutton marked "Reset" would unlatch the relay.

    When the relay is latched, R6 feeds the low voltage back to TR5 and removes the supply from TR2/3 to keep the relay latched ON through TR4 (still assuming it's an NPN) which should be unnecessary since the RL2 contact already keeps the relay energised.

    I can't see the reason for D1 or TR5 at all.

    If you just want to make it work, this is what I suggest. Change TR4 to an NPN. Replace C5 with a short. Remove TR5 and replace D1 with a short. Connect the D1 diode across the relay coil (anode to -9V rail) to protect TR4 against back EMF.

    That should work, I think. When TR2/3 pulls its collector towards 0V, TR4 will act as an emitter follower and pull its emitter down towards 0V. The relay coil will be energised, and the RL2 contact will latch it ON. Breaking the circuit will unlatch it.
  3. Nick Sparkes

    Nick Sparkes

    Apr 15, 2014
    Hi Kris and thanks for your insight.

    Yes, TR4 is drawn upside down. I also noted no back-EMF diode across the relay.

    I'll have another play with the cct and see if In get any more joy.


  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I don't think TR4 is drawn upside down. You mean with the collector and emitter swapped? I don't think the circuit would work at all if you swap the collector and emitter. Good luck!
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