555 Timer One Shop Stays High

Discussion in 'The Projects Forum' started by PointyUK, Dec 23, 2013.

  1. PointyUK

    Thread Starter New Member

    Sep 22, 2013
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    0
    Hi,

    Although I am a novice I can normally troubleshoot stuff OK but this is driving me mad.

    The following circuit portion is supposed to do 2 things when it gets a low signal on Q1 , sound a buzzer for 5 seconds before switching on the relay. Q1 turns on the 556, Timer1 goes low for 5 seconds and then high, while Timer2 goes high for 5 seconds and then low. Yes I know there are probably better ways to do it and I am going to redesign this part, I can't remember the reason I used 2 timers now, as it was designed a while back, but I would like to know why this doesn't work properly...

    [​IMG]

    The problem is that the circuit seems to work fine on the breadboard, but in the actual circuit the buzzer part just stays high. If I connect my multimeter up to the output of Timer 2, it shows 5v for 5 seconds and then 0v, as if it's working correctly. Why is it when I connect a load that it locks the output high?

    TIA

    Les
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    I am not clear on what you mean that it works on a breadboard, but not in the actual circuit. However, if it has been built twice and one assembly works and the other does not, then there is a wiring difference (error) between the two circuits. Thus, posting the schematic is not sufficient information; you would need to post photos of both assemblies, and in a circuit with this many connections, even that may prove difficult.

    Or maybe I don't understand your post.
     
  3. sheldons

    Active Member

    Oct 26, 2011
    616
    101
    you may need an additional transistor to switch your buzzer on the second timer output..
     
  4. PointyUK

    Thread Starter New Member

    Sep 22, 2013
    9
    0
    Sorry for being a bit vague.

    That part of the circuit works fine in a PCB that was isolation routed months ago. I decided to make some additions to the board, but other than the layout and the fact I changed from a NPN to PNP transistor, that part of the circuit is the same.

    Anyway in the new version the buzzer refused to turn off, but using the same components on the breadboard it worked fine.

    I have since changed the design again so it's irrelevant really, but I would like to know what I was doing wrong.

    I had similar problems today when experimenting with just a single 555 timer. I thought I would do away with the 2nd timer and just use one. I just connected the buzzer + to +5v and the buzzer - to the 555 output. Similar to this... but replacing the bottom LED with my transistor to relay and the top LED with the buzzer.

    [​IMG]

    This didn't work, but if I put 2 LEDs in place instead, it worked fine. I also tried using 2 transistors one NPN & one PNP, but that didn't work either.

    So how would you drive 2 devices from one 555 output, one on the LOW output and one on the HIGH output?

    Hope this makes sense.

    Les
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,783
    1,103
    Is your 5V power supply adequately rated for the buzzer plus relay-coil current?
    Where are the supply decoupling cacacitors for IC1 and IC2?
     
  6. PointyUK

    Thread Starter New Member

    Sep 22, 2013
    9
    0
    Yes, the relay is about 40ma and the buzzer 30ma. It's running from my bench supply at the mo but the built in PSU will provide 1amp.

    Ooops!
     
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