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I have built a DTMF decoder which offers relay closures corresponding to the digits received. Everything works great, except the equipment I am interfacing to is slow.

The office telephone system gives a pulse of tone whenever a digit on the telephone is held down. From what I've read, the pulse lasts 300ms. The equipment my DTMF decoder is interfacing to cannot detect a pulse that quick.

I am working on getting the tone pulses extended to 1s, but need a plan B. Can anyone think of a way to take a 300ms pulse and extend it to 1s?

I've tried using a 74LS123 (retriggerable multivibrator) but I dont think the IC is designed to go that long.

I've thought of using a JK Flip Flop to latch in the signal that eventually closes the relay contacts, but haven't worked it out yet. I have a limit on the amount of real estate I can use.

I'm looking at some kind of RC charge/discharge circuit to hold the relays on. But I cant figure out a way to have the capacitor charge in 300ms and discharge over 750ms - 1second.

Any input would be appreciated.

Very simplified schematic attached.

enter image description here

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  • \$\begingroup\$ What kind of pulse is this 300ms pulse? \$\endgroup\$
    – abdullah kahraman
    Commented Jan 24, 2012 at 21:38
  • \$\begingroup\$ I would go with a small microcontroller. \$\endgroup\$
    – abdullah kahraman
    Commented Jan 24, 2012 at 21:40

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Do you want the relay to latch on or just stay energized for the length of the tone?

If (a) then just use any of different latches: could use a toggle F/F if you want it to toggle on and off with tones, or capture the rising edge from the decoder and use it to clock an LS74 flip flop.

For (b) you didn't say what problem you ran into the the LS123. I find it hard to believe that it won't configure as a 1-second one-shot, but if that's really the case, you can make very long one-shots using an NE555.

Also, do you really need two levels of isolation from the phone line? You have both an optocoupler and a relay in your circuit. Better yet, back off and take a 30,000 foot view: what exactly are you trying to build?

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  • \$\begingroup\$ No you are correct that I dont need the relay. The reason they were there from the beginning was to be monitored by another piece of equipment. This other piece uses a small current loop to sense a contact closure. However the optocoupler on its own would probably work just as well. In the case of the ls123 chip, I must be connecting my external components incorrectly, and will look at it again. \$\endgroup\$
    – Michael
    Commented Jan 24, 2012 at 23:18
  • \$\begingroup\$ I'm accepting your answer because the 74LS123 does work. Lesson 1, always VERIFY your test equipment works! Thanks for the input. \$\endgroup\$
    – Michael
    Commented Jan 25, 2012 at 1:40

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