HC-8 Remote Control by Telephone

[bsnut]

I am looking to control the HC-8 outputs by decoding the DTMF tones generated by all phone keypads, with the basic goal to turn on and off the outputs. I also know that I need to use the option header I/O pins to do this and I also know that I will need to protect these I/O pins.

I found this spin code demo in the Parallax Obex website that will decode the DTMF tones based on the standard Propeller sigma-delta ADC.
http://obex.parallax.com/object/746

I also found this circuit on this website
http://www.siongboon.com/projects/2005-07-18_dtmf_circuits/
which shows the same connections to the decoder chip for the phone circuit that found on another website.

My goal is to use the option header pins, because I want to leave TTL pins for manual control over the HC-8 outputs. The DMX address dip switches will be used to set the board address (password) for turning the outputs on and off.
   

[JonnyMac]

I think you'll need to go with the hardware decoder; the Option pins have pull-ups and I don't think that will work with the software DMTF decoder which needs a very specify configuration to form a sigma-delta ADC.

Be careful; the Option pins go straight to the Propeller -- 5v on these and the Propeller could die (or at least break a pin). What I suggest is that you put transistor buffers on the output bits of the DTMF decoder. Use NPN so that an active output will pull the connected Option line low. Flipping the bits of this port is trivial in software.

  dtmfcode := ina[OPT_BR..OPT_SM] ^ %1111

[bsnut]

I think you'll need to go with the hardware decoder; the Option pins have pull-ups and I don't think that will work with the software DMTF decoder which needs a very specify configuration to form a sigma-delta ADC.

Yes, you are right. The are DTMF-tones are sinewaves.

This is what I was thinking and that was using a 600:600 ohm isolation transformer to provide some protection to the Propeller pins. But, do you think this would work with the pull-ups that are on the I/O pins?

[JonnyMac]

If you use transistor buffers, yes. The transistor circuit will pull the pin low when the output of the DMTF decoder goes high. I don't think the direct pin (with current limiters) approach will work unless you can guarantee that the outputs from the decoder are driven low when not on.