Astable 555 timer help

Discussion in 'General Electronics Chat' started by someone, Jul 21, 2007.

  1. someone

    Thread Starter New Member

    Jul 20, 2007
    2
    0
    Hello all, I am building a 555 timer circuit based on this schematic to turn a relay on and off, however the 555 behaves as if it were a one shot. I am using the following values:

    R1 = 10K
    R2 = 100K
    C1 = 10uf

    Thanks in advance.
     
  2. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    Could you provide more information on your troubleshooting steps? Have you tested the 555 without the realy connected? How are the voltages on pins 6 and 7 behaving?
     
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    In addition to nanovate's advise, tell us more about your relay and you might want to consider drawing your circuit out and posting it.
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
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  5. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    It might be behaving like a monostable multivibrator if you don't have the discharge pin connected (pin 7). The discharge pin will discharge the capacitor once the voltage across the capacitor is 2/3 of Vcc. Also, check if the trigger pin (pin 2) is connected along with the thresold pin (pin 6). If not, the output won't be triggered high again, after the capacitor is discharged to 1/3 of Vcc.

    However, it is a good idea to post your circuit diagram, so we can help you with more detail.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    The replies thus far may prove useful to you but it will be difficult to provide you meaningful assistance regarding your problem until you supply us with a schematic of your specific circuit hookup.

    hgmjr
     
  7. someone

    Thread Starter New Member

    Jul 20, 2007
    2
    0
    Thanks for the replies, I have tried replacing the relay with an led and I have tried replacing the 555 timer itself but it still did not work. When I checked the voltages on pins 6 and 7 as suggested I found that the capacitor was not discharging. Any suggestions would be appreciated.

    Edit: here is a drawing of my circuit

    [​IMG]
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    There is no schematic.

    hgmjr
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Do you know the "frequency" of your output signal ... or what it's suppose to be?

    What instruments are you using to measure pin 6 and pin 7?
     
  10. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    I would use a lower supply voltage since 18V is the absolute max. The recommended operating votage goes to 16V.
     
  11. hgmjr

    Moderator

    Jan 28, 2005
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    Most likely the 10 ufd cap is leaky and that fact together with the very low current developed by the 100K resistor being used to charge the cap is preventing the cap from reaching the trigger voltage level needed to sustain oscillations.

    hgmjr
     
  12. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Are you using 9V or 18V? It seems that you wanted to use 9V, evaluating the LED resistor, but you have 18V depicted there. I think you meant +9V and 0V (or GND).

    Your circuit seems to be OK, so if you are having misbehaviour, check the connections that you are making. Check for bad joints.

    Your circuit should have the LED on for 0.76s and off for 0.69s, so you should see the LED blinking if you follow your schematic. Formulas are:
    Ton = ln (2) x (R1 + R2) x C (charge will be done through R1 and R2)
    Toff = ln (2) x (R2) x C (discharge will be done through R2)
     
  13. jdcruz

    New Member

    Nov 27, 2007
    3
    0

    The schm works. I've tested it with the said RC values and Ton(fosc)=0.79s(approx)
     
  14. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Also, check if you have a protection diode in parallel with the relay coil. This diode should be reversed. If your relay doesn't have a protection diode built in, you might need this external relay. Without this diode, the 555 timer will be triggered by the back emf generated by the coil, thus behaving erratically. I know this because I happened to have this diode disconnected in a similar circuit. Eventually, you may damage the 555 timer (the LMC555, since it is a CMOS IC, would be damaged pronto).
     
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