DTMF LEDs dancing randomly

Discussion in 'General Electronics Chat' started by vinodquilon, Feb 7, 2010.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    See the attachment. On power up all four LEDs(1 TO 4) are ON.
    Now there is one problem exists when detecting initial ring vots as all four output LEDs dancing on and off for a duration of one second only, then comes into stable state corresponding to state of key press 5. After off hooking the test end, the LEDs obeys the normal truth table when pressing keys on remote and local phones.
    The outputs of the DTMF IC drives a decoder, which in turn drives the devices to be controlled from remote end (not shown on the attachment). So this initial dancing process seriously affects intended output performance. Pls give me a solution to overcome this dancing problem.
    I have made a trick, I deliberately disconnected the telephone lines on occurring first rings.Then I manually closed the line while calling from remote end only after occurring first rings. Then no dancing problems occurred, state remains to initial power on state of all LEDs on. But this is not a practical solution.
    States corresponding to all ON and key press 5 wouldn't be a problem as I am not planning to use the corresponding 4:16 Decoder outs. But the random dancing problem inversely affects all outs as they turn on and off randomly, thus the connected devices also. ​
  2. Audioguru


    Dec 20, 2007
    The two zener diodes allow a ringing signal to reach the IC that is much too high for it. Try using ordinary 200V diodes and maybe back-to-back instead of in series.