Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

DAA modules and DTMF decoders

Not open for further replies.


Member level 3
Jan 3, 2006
Reaction score
Trophy points
Activity points
mt8870 optocoupler

Hi everyone,
I am trying to build a circuit that interfaces with the telephone line. I could use CH1840 DAA for understanding the ring signal and control the hung up anf MT8870 or MT8888 for DTMF receiving and producing.

Unforunately I can't find these ic's in DIGIKEY,MOUSER etc. so if someone knows similar ic's from other companies(more well known) please help.

Thank you in advance

daa and dtmf

Is it only DTMF you want to do. No voice/data ?

daa modules

I also want to detect the Ring signal and Hook up the telephone line.

thuvu said:
I also want to detect the Ring signal and Huck up the telephone line.

Most case you need detect 25 - 110 Volt, 25 or 50 Hz AC for ring signal, and measure line current (or possible line voltage) to detect on hook state or not

in most case for example Modem application, using two optocoupler in full rectifier bridge for detect line current and direction (some contry shift polarity on line in time B-side lifting of hook).

Optocoupler can also using to measure if high voltage exists (30-50 Volt) in on hook state or low voltage (3-10 Volt) in of hook state. But you need some resistance and Zener diode to sure for more precise tripping voltage point - and opto couple is only allowed to draw very small amount of current in on hook state (< 1 mA or >= 1 MOhm load in most case)

in same case, you can also measure if ringing if output from optocouple have 25 or 50 Hz square wave if not using full bridge rectifier before and need in this case two optocoupler with opposite polarity to handle both polarity on line if you want measure both voltage and ringing detect on same optocoupler.

Thank you but is there any ic to do this detection?


Try **broken link removed**

I think that Mitel was bougth by Zarlink.

I hope this help.

Ok for the DTMF.
What about Ring and Hook signal?

this may help u
it contains the DTMF decoder and the ring detect
also the detection of the line status, off hook or on hook
hope that help u

So I use a optocoupler with the appropriate ratio Vin:Vout in order to transfor the high voltage Ring Signal to 5V?

What if I want to send a signal to the line?

Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to