DMTF Testing

[dramrattan]

Hi,

I can't connect to a telephone line (i.e. I don't have access to the TIP and RING lines in the school LAB) to test the attached circuit which is suppose to test the MT8870 DTMF decoder so i am looking for an alternative to test the chip.

Could i use 48V from a power supply across where TIP and RING lines suppose to be from the attached circuit?...and use another power supply to produce 5V for the MT8870?...

I ask this because when the phone is on-hook there is approximately 48V across the lines but when the phone goes off-hook the voltage across the lines drop between 3-10V. Would placing 48V from a power supply achieve this change when the phone goes off-hook?(My circuit for off-hook detection is same as the attached document "Caller ID using Microcontroller")...I don't think so, but i am certainly not experienced in this field so would like your help.

Also, I saw in some articles, that a 7805 voltage regulator is used to produce 5V for all ICs that require 5V for operation but when i looked at the datasheet of the 7805 i'm seeing that the max input voltage it could handle is 40V...so how is this possible since there is 48V across TIP and RING when the phone is on-hook and up to 90V RMS when the phone is ringing? ...It is shown to be used in the attached document "Caller ID using Microcontroller.pdf"...

Could i use a voltage regulator like the LM2936 which have a max input voltage of 60V across the TIP and RING lines and then feed the output to all the ICs that required 5V in my circuit which is similar to that of the "Caller ID using Microcontrollers" document?...But still this doesn't account for the 90V ringing voltage.

 

[Super_voip]

if you have a VOIP box eg Linksys SPA3000(I know it's old) they can generate a callerid signal. All you would need is 2 free voip accounts, make calls from your PC to the box, the box takes care of the ring signals and line voltages.

 

[Super_voip]

**broken link removed** is a good place to find a ready made one, just don't expect to receive it for 3 weeks.
I have 2, work well

 

[audioguru]

To use the DTMF tester circuit you don't need 90VAC ringing and you don't need 48VDC.
Instead use 12VDC and connect a 560 ohm resistor from it to the phone and the phone should be able to dial touch-tones.

The 12VDC can feed a 7805 voltage regulator for the IC.

Change the values of the 10M and other resistor to detect off-hook.

 

[Externet]

Or, connect a microphone to your DTMF decoder and bring it next to the handset earphone or speakerphone; then dial something.
Depending on the type of telephone, could be connected electrically to the handset earphone too.
Miguel