# DTMF decoder kit



## robp790 (Jan 8, 2008)

I just purchased a "Sweet 16" DTMF decoder kit. In reading the instructions it says to use two wires from a phone to test the kit after building it. I had intended to use a file from audacity, which should be the right side of a stereo recording. Does any one know if the signal generated is the same? It seems as like the phone would be creating current and the audacity recording is creating sound output. I am having doubts if this kit will do what I want, which is control an "asylum door"


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## HomeyDaClown (Oct 3, 2009)

This board has a single channel audio input to be fed by either the receiver (part of an old style telephone handset) or from just about any line level audio source (like a PC sound card).

That's the easy part.
To make use of it like others have done for asylum door props, you'll need to add some flip flops or a quad flip flop in order to get the outputs to latch on.
That's if you want action like a clap-on clap-off deal so that the dents in the door effect stays on for a while after the board hears the tones.
Otherwise the outputs are just momentary while the tones are playing and shut off when the tone stops.


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## robp790 (Jan 8, 2008)

Okay so an output from my PC sound card will actuate this? Good. I was hoping to install a stereo phone jack and take the right output from the PC card and bring the tone track to the decode.

In reading the instructions it made it seem as though I was feeding the decoder a voltage from the phone as opposed to a tone.

The flip flop circuits I have found so far make it look like any other circuit for turning a relay on with a NPN transistor except using two in a polarizing fashion. does this check out?


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## HomeyDaClown (Oct 3, 2009)

Audio on a wire is voltage, AC voltage to be more specific.
Both a phone and your sound card produce the same signal over wires.

A single transistor will not do it unless it is a UJT type and even then you'll need to use another transistor to handle the load.

Here's a canned module to do it:
http://www.mcmelectronics.com/product/28-5115

Here's a trick using a 555 timer to do it:


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## robp790 (Jan 8, 2008)

Nice!
I appreciate the info and the links on that schematic, useful stuff!


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## skidrow (Feb 21, 2013)

I thought the asylum door prop came with a dtmf board...is that not true?


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## HomeyDaClown (Oct 3, 2009)

skidrow said:


> I thought the asylum door prop came with a dtmf board...is that not true?


Only if you buy the prop, not if you build it yourself.


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