How to increase the sensivity in MT8870

[Suraj143]

Hi guys I made this DTMF tester circuit & its working.

The problem is some times I have to press & hold the buttons in the keypad to make an output. Because steering pin (STD) won’t go HIGH sometimes it’s blinking.

It won’t detect any signal without the optional circuit marked in red (guard time adjustment) also it’s very poor to cell phone dialing tones.

Is there any way to increase the sensitivity in my circuit? Is my input area values are good?

Thanks.

 

[mvs sarma]

Hi Suraj,
you expect to receive audio tones in the frequency range 0.3 to 3.4 KHz- from the telephone line. as per your diagram you have coupled it using 100pF capacitors on each limb. and there after you have adopted two nos of 56K on each limb.
if you analyse the impedance of this for audio frequencies - you will appreciate that thhe circuit is under coupled. generallt we beleive that telephone impedance is 600 ohms. so anything above 10*600= 6K ohms is amounting high impedance. and the capacitors need to be around 1uF

more audiowill come to the 8870 and there is no need for maiking the circuit more sensitive.--
only you have to have guarding say two diodes in opposite parallel across the 8870 input, instead of 5.6 ZPYs after the isolation caps. this is beacuse, the audio level will generally be around -6 to -10 dbm. this will work out to less than few mV


hope this should serve you

Sarma

 

[audioguru]

The 100pF capacitors are for coupling RF, not DTMF audio tones.
Look at the datasheet of the ancient DTMF decoder.

 

[Suraj143]

Thanks Sarma.
Do I have to change only the 0.1uF/250V caps to 1uF/250V?

Do I have to replace the first two 56K resisters to below 6K resisters?

Then I can replace the two 5.6K zenners to normal diodes.

Thanks.

Audioguru I referred to the data sheet many times.

 

[audioguru]

You have 0.0001uF capacitors (100pF).
The circuit needs 1uF capacitors to pass DTMF audio tones. 10,000 times more.

Remove the zener diodes and connect two 1N4148 diodes back-to-back across the inputs of the IC.

 

[Suraj143]

Wow thanks a lot audioguru I'll replace the 100pF to 1uF/250V.
& also I'll replace the two zenners to 1N4148 back yo back.(connecting both anodes together & other two cathode sides to input lines.)

Many thanks.

 

[audioguru]

Sarma suggested reducing the values of all the resistors and using bigger capacitors.
The datasheet uses resistor values similar to yours and uses 0.01uF capacitors.
The datasheet doesn't have protection diodes.
I show the new capacitor value and back-to-back diodes here:

 

[Suraj143]

Wow Audioguru thanks a lot.I'll build your corrected diagram & tell the progress for sure this must well work with sensitivity.

Thank you very much Audioguru.

 

[Suraj143]

Hi I changed the value to your values & checked, it worked well than earlier.

Sometimes when I press the tel buttons continuously the MT 8870 getting stucked so again I have to place the receiver & give a call & lift the receiver.

Is this normal?

 

[audioguru]

It is not normal. I don't know what is wrong. I have never seen that old IC.

 

[Suraj143]

This mostly happens when a recording goes in the telephone receiver.Bcuz I'm dialing wrong numbers so its going a recording telling "the number you dialed not in use" so I continue pressing the numbers to detect the MT8870.

Not all the times, sometimes while pressing continuously its getting stucked the MT8870.So I place the receiver & lift again.After that ok.

Ok Thanks audioguru.

 

[Asif5955]

Hi audioguru
i have made a DTMF circuit withMT-8870 but having a problem:
I am using two cell phones (one as Transmitter & one as receiver) but when i call on Receiver-Mobile and press buttons from Transmitter-Mobile the tones are not captured by IC MT8870.What should i do.
Both Mobiles work fine when i connect to circuit one by one and press buttons
i am uploading the diagram please give me suggestions if capacitors or resistors values can be modified to get desired OUTPUT

 

[audioguru]

I do not know how much gain your circuit needs so here are some choices:

 

[Asif5955]

ok i will make these changes and tell you about my progress

 

[Asif5955]

Hi audioguru !!!
Your suggestions were very helpful....I made changes as you suggested and it worked.
Now my DTMF circuit is complete and WORKING..............Yee .......Haa......
Thanks alot
I am uploading it here as it is said "SHARING IS CARING !!!".Thanks again