555 monostable , relay, transistor, cd rom motor

Discussion in 'Homework Help' started by regal006, Sep 19, 2012.

  1. regal006

    Thread Starter New Member

    Jul 14, 2012
    so, i m quite having a proble, its deadline is 13 hours from now.

    i have 3 monostable cascaded with . 1st MS triggers from another circuit, then its output is to 2nd monostable, then to 3rd.

    i have 2 relays used to reverse the cd rom motor.

    1st monostable drives the transistor that drives a relay, opening the cdrom
    2nd monostable is waiting time.
    3rd monostable is to drive another transistor-relay that will close the cdrom.

    my monostable circuit is working fine with no relays but after i add transistors and relays, abnormalities starts to occur,

    sometimes 1 monostable skips, sometimes , 2 monostable is high, sometimes it turns to a loop which should not be.

    what suppose is the problem..
  2. m1ch43l

    Active Member

    Aug 16, 2012
    How much current is at play within the whole circuit; monostables, relays and transistors included? I suspect that the relays (if you are using the common 'mechanical' ones with coils) are wreaking havoc due to their collapsing magnetic field when 3rd monostable is turning off, thereby inducing a back-emf against the monostable circuit. I had a problem similar to this whilst using timer circuits but operating in exactly the same way trying to power a fan. The pulsing circuit went wild and kept the fan at constant speed despite varying the output frequency. Put a freewheeler diode across the relays if you are indeed using these.
    At least this diode helped in my case... I was playing around with 100mA by the way.
    A motor, irrespective of the size or type is an inductive load.
    I'd consider replacing the relays with FETs. Alow power FET should also do the trick. Hope you are using fets instead of normal transistors.
    Last edited: Sep 19, 2012
  3. regal006

    Thread Starter New Member

    Jul 14, 2012
    thanks for the reply.

    i have a 3A power supply so it should not be the case here..

    and i have all my relays paralleled with diode,.

    my transistor is 3904 , relays are 6v SPDT


    the transistor relay motor circuit works fine with switch as input in the base of the transistor..

    my supply voltage is around 12 volts
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Your using 6V relays on a 12V supply, this will cause voltage spikes and give false triggering put a resistor in series with the relay coil (the same resistance as the coil),or try to use 12v relays and use a 5v regulator for the 555 timers,this should help.