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Project design help

Discussion in 'General Electronics Discussion' started by arjg89, Apr 15, 2013.

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  1. arjg89


    Apr 15, 2013
    Hello, I am building an alarm for a project on stripboard. The problem I have got is that I have only got a 1 tone noise coming from it, when obviously I should have a two tone noise - like an alarm. I have tested it on an oscilloscope, and multimeter. I have got power going to both of the 555 timers, but only got one square wave coming from the first of the 555 timers. I have tested the capacitor on the oscilloscope to see if it is charging and discharging which it is. What could the problem be? i have attached a picture of the circuit diagram and the schematic that I converted it to.

    Hopefully you will be able to work out where I have gone wrong on my circuit diagram. Its not the best diagram in the world but please appreciate that this is my first ever attempt to convert a schematic diagram to a circuit diagram. Any feedback is appreciated,


    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Have a lok at this circuit. Your first stage looks awkward, you don't even seem to use the output of the 555.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    That circuit should work fine. The connection between the two stages is the transistor, which acts as a buffer between the first oscillator and the second oscillator and modulates the frequency of the second oscillator through pin 5 of the second 555.

    You've already checked that your rising and falling control waveform is present on C1. Now check that it's present on the base of the transistor, and then on the emitter of the transistor. My guess is you may have the transistor connected the wrong way round.

    If the waveform is present on the emitter (it will be the same as the waveform that's on the base, but about 0.7V higher), then check that a similar but attenuated waveform is on pin 5 of the second 555. If not, check the connections to R6.
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