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Ring detector circuit, transistors not fully activating

Discussion in 'General Electronics Discussion' started by galaxywide, Jan 10, 2013.

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  1. galaxywide


    Jan 10, 2013
    I'm not sure if this is the right place to post this, if it is not I apologize and ask the mods to please move it :)

    I am attempting to build a ringer interface circuit, and I’m running into problems that are over my head.

    The purpose of the circuit is to buffer the output of a central telephone system that goes to several ringers spaced throughout the building. The reason for the circuit is that whenever the line rings, it overloads and needs to be reset, thus this circuit will buffer the line and offer a lot more output capability.

    The schematic is below:

    NOTE: R7 goes to 24V, not 12V. My mistake.


    Now, my problem is this: when everything is on, and the line is ringing (4N27 triggered), I only get 12V out , instead of 24. I went through and checked voltages, and it seems like none of the transistors are turning on (or off as the case may be) completely. When I tie the base resistor of the first (closest to the 4N27) signal transistor to ground (turning it 100% off), I then get the correct 24V out.

    Also, it exhibits the same problem with a 5V supply, so I don’t think that’s it.

    Earlier, I got around to hooking up the 555 timer circuit as per the schematic, and all that happens is that the IC gets super hot (too hot to touch comfortably) in a very short time. No output whatsoever. I tried it on the bench, and it triggers reliably with voltage as low as 1.7v when turning on an LED. I measured the voltage at the collector of Q1, and it reads 2.5v unloaded with the line ringing, haven't checked it with any load yet.

    I honestly have no idea what is going on here....this was supposed to be just a simple interface circuit!

    Anyone have any ideas as to what may be causing this? I’m a bit of a novice at circuit design, so any and all help is much appreciated.

  2. galaxywide


    Jan 10, 2013
    So I've been doing some testing, and I think I found the problem: the opto-isolator is not being turned on completely! I checked it by hooking pin 1 to +12V through a 10K resistor, and pin 2 to ground; I then observed that all the downstream transistors were working correctly and turning on and off like they should.

    Next question: how do I fix this? How do I make the AC signal more fully turn on the 4N27?
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    One major problem is that your circuit cannot turn Q3 off.

    At the very least, R7 needs to be connected to 24V, not 12V.

    Another issue is that the opto is turning off Q1, and to do that it has to pull the base very firmly to ground. It may not be able to do that. A better option is to have the opto turn the transistor on (it only needs to conduct a little to do this).

    The easiest fix is to replace Q1 with a PNP transistor. Base connected to the 1k resistor, emitter to 12V, collector to the trigger of the 555, and a 10k resistor between he collector and ground. Also remove the 10k resistor from the trigger input to +12V

    Knowing what Rnotsure and Cnotsure are would be helpful.
  4. galaxywide


    Jan 10, 2013
    Thanks for pointing me in the right direction, I'll try this and see how it goes.

    R(not sure) is 33K ohm, and C(not sure) is 47uf, to give a monostable period of around 1.7 seconds.

    As for R7 being connected to 12V, that was a typo in my schematic, and I noted it as such in my OP (I was in a bit of a rush at that point, and unfortunately didn't have time to fix the schematic). It is in fact connected to +24V like it should be. But thanks for catching that!
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Why is Cnotsure connected to pin 5 of the 555? That side of that cap is supposed to be grounded.
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