dtmfout problem

[lerameur]

I dont know yet, but I remember some chip requires +15 and 15- supply , I only have about 6 or 7v supply.
I tried a single stage RC, with 0.2uF and 400ohms, that did not do anything..

[skimask]

I dont know yet, but I remember some chip requires +15 and 15- supply , I only have about 6 or 7v supply.
I tried a single stage RC, with 0.2uF and 400ohms, that did not do anything..

.2uF & 400 ohm gives you an RC time constant of 12.5Khz. (.2/1000000 * 400 = .00008, 1/.00008 = 12,500)
It might work a bit better if you had a bit more cap's and a bit less resistor, say 40 ohms instead of 400, which would give you an RC time constant of 1.25Khz. And since you're trying a single stage RC filter, the roll-off would be fairly shallow, so 1.25Khz may work out fairly decent.
My suggestion...get a copy of 'The Art of Electronics'. It's one of my 'Good Books', take it with me on every trip I take, page thru it all the time. A few good sections on filters, op-amps, etc.etc.etc.

[lerameur]

ok look at this pdf:
AN6_DTMF_gen.pdf
search for it on google, last page shows the calculation i did.

aslo, when I used my 0.2uF cap. I decided to switch to a variable 2k pot, went through all the resistance values pretty quicky. The only thing this did is to increase or decrease the overall sound level, kind of a volume control.

[Jumper]

Hi,

The things you have seen is just what I have seen too. RC filters are not that successful in making Low Pass filters since they dont this nice sudden cutoff FQ.

these links will get you some help designing an active filter.

http://www.microchip.com/stellent/id...cName=en010007

http://www.national.com/appinfo/amps...h_filters.html

There are many Op-amps that can be used with a single 5V supply. This way you get an active filter with very sharp cutoff FQ.

/me

[lerameur]

that is pretty cool software from microchip.
One thing though, they show all parts excepts for the op amp, can I use 741 op amd or any similar amp with a 5v supply?

[lerameur]

I finished my low pass filter but I realized one other thing wrong with my circuit.
I do not read good frequencies with my scope, maybe it is because of DTMF, this is the first time working with this this. I posted a screen capture of my scope while the dtmfout id dialing 5 constantly.
http://www3.sympatico.ca/lerameur/LPF/
The scope shows under 10hz... is this a good output for dtmf ?
is this good for dtmf, I do not get any better result with my low pass filter added..

[Jumper]

Try to connect a small speaker to the pin... I usually use normal external PC-speakers with built in amplifier and just connect GND straight and a capacitor in serie with the pin for the signal wire. Then you can compare 5 5 5 5 5 5 5 5 5 5 from the PIC with the beginning of Homer Simpson's phone number 555- dialed on your normal phone.

Some signals is really hard to measure with a scope.... plan B which I also tend to use alot is to take a normal windows audio sampling program and use the PC microfon to sample it. Most softwares allows the wave to be shown and you can also zoom in and out... here you can try to call Homer again to get a reference.

Who will go crazy first? You or your neighbour?

Don't call Moe......


/me

[skimask]

I do not read good frequencies with my scope, maybe it is because of DTMF, this is the first time working with this this. I posted a screen capture of my scope while the dtmfout id dialing 5 constantly.

Are you set for AC or DC coupling on the 'scope?