DTMF LEDs dancing randomly

[vinodquilon]

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.

 

[betwixt]

That schematic looks plain wrong to me.

You should NEVER ground one side of the telephone line, the incoming wires are balanced and both above ground potential. I'm surprised the telephone line still works at all.

1. Use an isolated power supply or alternatively, use an isolated phone line transformer so you have isolation from the phone lines. This is not only important from a functional point of view, it might save your life if there's a static storm or direct lightning strike on the phone lines.

2. Look at the data sheet for the IC, it shows how to connect it's input circuit correctly. The present design seems to use differential inputs but then grounds one side which makes no sense. Single ended input uses fewer components too!

Brian.

 

[vinodquilon]

That schematic looks plain wrong to me.

You should NEVER ground one side of the telephone line, the incoming wires are balanced and both above ground potential. I'm surprised the telephone line still works at all.

1. Use an isolated power supply or alternatively, use an isolated phone line transformer so you have isolation from the phone lines. This is not only important from a functional point of view, it might save your life if there's a static storm or direct lightning strike on the phone lines.

2. Look at the data sheet for the IC, it shows how to connect it's input circuit correctly. The present design seems to use differential inputs but then grounds one side which makes no sense. Single ended input uses fewer components too!

Brian.

I haven't connected ground point to Tip or Ring lines.

 

[cdh7]

Hi,

It looks like pin 10 of that DTMF decoder IC can be used to tri-state the output buffers. What if you build a simple off-hook detector circuit and use it to activate the output only after the telephone has been picked up? That way you won't see any strange outputs while the phone is ringing.

Are you building some kind of remote control? Then you'll need a circuit anyhow that listens for the ring and automatically answers the call. Maybe you could use that to enable/disable the DTMF outputs.

Regards,
Chris

 

[nostalgiax]

plz help me.....
hello, can u help me?
i want to generate tone using mt8889 8051 controller.
if u dont mind, can u help me to interface it?
i appreciate that...
CAN U TELL ME HOW TO GENERATE pulse or dtmf tone dialing? plz........
i use intel 8051 controller..