Need Help On CM8870 IC ...

[Muhammad Abbas]

Hi .. I am working on CM8870 IC to make DTMF Decoder .. I had Made a Circuit Completely .. but when i Turn on the Circuit 2 LEDS are constantly ON (On Output) while three are Off .. and nothing happens when i press any button on Mobile Keypad .. I had attached the picture of Circuit i made .

 

[Martaine2005]

Hi
You might find this helpful
http://forum.allaboutcircuits.com/threads/troubleshooting-cm8870-dtmf-decoder-circuit.79443/

 

[Muhammad Abbas]

@Martaine2005 .. thanks But I've Tried this already ..

 

[Martaine2005]

I have no experience with this circuit at all.
But it appears a lot of people are having similar issues.

Have you tried the '0.1uf capacitor' across pins 18 and 9.?
Seems to work for some people.
Martin

EDIT; Have you double checked connections from phone?

 

[Arouse1973]

Looks like this device can only source 0.8 mA (800 uA). This probably won't be enough to light the LEDs. You could try and connect the LEDs from the positive supply rail and sink current through them instead, this will allow you 2.5 mA. This still might not be enough to light the LED to the required brightness, you may have to use a transistor to switch them to achieve a higher current. Another option is to use a non-inverting buffer IC with enough drive current for your particular LEDs
Adam

 

[Muhammad Abbas]

@Martaine2005 Thanks Bro ... It Worked ..
@Arouse1973 Thanks Too Bro For Sharing Info ..

 

[Martaine2005]

@Martaine2005 Thanks Bro ... It Worked ..
@Arouse1973 Thanks Too Bro For Sharing Info ..

Well done sir.
Martin

 

[asif5955]

Hi Muhammad Abbas !!!
i want to ask "Did you try two Mobiles for this DTMF circuit "
i have made a DTMF circuit (MT-8870 IC) but Mobiles are not communicating with each other
When i call from other Mobile and press buttons, the circuit is not recognizing tone .May be its TONE strength is very weak but
when i connect Mobiles one by one with circuit and press Buttons ,they work fine.
I am uploading Schematic...
Please give me suggestions