In this tutorial we will learn how to Decode the telephone audio tones into 4-bit binary codes. In short you can use your mobile phone to send instructions to your microcontroller. Here we will use the mt8870 IC.
Schematic:
Parts List:
| Prt.No | Description | Spec. | Qty (Nos.) |
|---|---|---|---|
| 1 | MicroController Reset Circuit | - | - |
| 2 | mt8870 | - | 1 |
| 3 | Resistor-1 | 100k | 2 |
| 4 | Resistor-2 | 330k | 1 |
| 5 | Capacitor | Disc 0.1 uf | 2 |
| 6 | Piezo-electric Crystal | 3.57 MHz | 1 |
| 7 | capacitor | 22pf | 2 |
| 8 | Connecting Wires | - | - |
Step 1:
In this tutorial we will use the decoder to detect and decode all 16 DTMF tone-pairs into a 4-bit code.
We dont have to build the basic Reset circuit as we are only decoding the telephone tones .
The MT8870 IC is placed on the breadboard first.
So What’s dtmf???
| 1209 Hz | 1336 Hz | 1477 Hz | 1633 Hz | |
|---|---|---|---|---|
| 697 Hz | 1 | 2 | 3 | A |
| 770 Hz | 4 | 5 | 6 | B |
| 852 Hz | 7 | 8 | 9 | C |
| 941 Hz | * | 0 | # | D |
Dual-tone multi-frequency signaling (DTMF) is used for telecommunication signaling over analog telephone lines in the voice-frequency band between telephone handsets and other communications devices and the switching center.
The tones are divided into two groups with rows representing a low frequency and coloumns a high frequency
Step 2:
Now take a 100k resistor and connect it across pins 2 and 3. Connect pins 16 and 17 with a 330k resistor. Then Connect pins 1 and 4 together.
Step 3:
Now connect pin 9 to ground and connect pins 10 and 18 to +5v. Then connect pin 17 to +5v via a 0.1uF capacitor.
Step 4:
Now connect a crystal oscillator (3.57 MHz) across pins 7 and 8. Then connect two 22pf capacitors with one end of each to the terminals of the oscillator and the other end is connected to the ground.
Step 5:
Now the basic connections being complete we can connect LEDs to the pins 11 to 15 to test the output.
Step 6:
To send telephone codes we need to connect a telephone or a mobile phone to the IC. We can do this by connecting a handsfree to the mobile and then cut one earbud. Then there will be two wires connecting the earbud , we connect one wire to the ground and the other wire to pin 2 of the IC (via a 100k resistor if needed).
Thats it!!
Thank you for following this tutorial, I hope you have followed all the parts of this tutorial. If you did’nt you can feel free to post your doubts in the comments section.
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This reminds me of what i had learnt during my first year of college
. I would really recommend you get yourself a youtube channel.
Good Work !
That’s cleared my thoughts. Thanks for contriibutng.
Sure…thanx for the advice
i am in 12th grade,and i found it way out of my reach…but i could understand bits and pieces…real great work…keep the good work going:D
we are working on similar paths ..
This reminds me of what i had learnt during my first year of college
. I would really recommend you get yourself a youtube channel.
Good Work !
hi this is intresting i hav tried to board it up but i failed with assembling the mobile jack as my input i hav the jack of my mobile nokia 2626 .the jack has two wires and in which one is red and the other is green i know that the red is ring and green is tip i hav grounded the red and connected the other end to input is there any wrong with these connection plz reply me
just check if the hands free is detected in ur mobile(i.e when the jack is connected no tones must be heard outside). if it still dosen’t work make sure ur circuit connections are right. Another way of testing is to use ur landline phone. just remove the wire connecting the receiver and identify the gnd and input wires and connect them to the circuit. You can then press the buttons on the base to test the circuit. Good Luck!!
thanx for ur post…hey m using cm8870 instead of mt8870.in datasheet pin 5 & 6 are grounded…so do i need to gnd or nt…coz m nt gettin output whether i gnd or nt instead on giving supply, led’s on pin 11,12 &15 goes high & e1 nothin happens on givin dtmf input…so wer do i go wrng…pls help me out…is it compulsory to connect 22pf capacitors across oscilllator?
Nice dispatch and this enter helped me alot in my college assignement. Thanks you seeking your information.
Hello Sir! My name is Franck. I am trying to test that DTMF circuit but i am not getting any output. I am using the MT8870 IC and i think my problem is the connection between the Phone and the IC, but i am not sure. Please help me Sir. Thank you!
hey hii i m working on the same project and basically trying to decode dtmf tones from my cell phone samsung star s5233.i m not pretty sure that ic mt8870 will serve my purpose or not kindly help me with it .. also i wanted to know that why hav u used the resistor and capacitor at the dtmf input of the ic in the above project and how to decide their values..thanks.
Hello sir!, i just followed the work that you gave about the nokia dtmf decoder. My headphone connection is perfect. But still i couldn’t get the decoded output or even i found my ciruit dead(no reaction at all).
What could be the reason????, sir
Nice project sir. I am having a problem that when I connect the battery to the circuit the leds light up without sending any tone to the circuit. What might be the problem? Plz help!