# Society of Robots - Robot Forum

## Electronics => Electronics => Topic started by: airman00 on July 22, 2008, 12:13:00 PM

Title: DTMF 8870 Circuit Explanation
Post by: airman00 on July 22, 2008, 12:13:00 PM
I want to use the 8870 DTMF decoder chip in one of my projects and I am having a bit of trouble understanding the circuit.

Why are the two capacitors placed there and how were they calculated?
How were the resistor values for R , 100K , and 330K  calculated? Is it just V = IR or is it something else correlating to the decoding?

(http://www.qsl.net/oe3mzc/mt8870.gif)

A site explaining decoding using the 8870 chip , but it doesn't show how to calculate the resistor values
http://www.siongboon.com/projects/2005-07-18_dtmf_circuits/

That site also has a schematic, but that schematic has slightly different circuit

Thanks,
Eric
Title: Re: DTMF 8870 Circuit Explanation
Post by: vidam on July 22, 2008, 12:50:39 PM
On the left side of the 8870DE the 2 resistors are chosen to set up the gain of the op-amp. I don't have the equation for solving for the gain of the op-amp. But you can google it. The capacitor for the op-amp is for filtering noise.  see the block diagram on page 1 of the datasheet and match it to the pins under pin configuration. Then look up the equation for the op-amp and solve for the 2 resistors and capacitors.

On the right hand side the resistor and capacitor are chosen to set up the time (ms) to detect phone rings. So the larger the values of the capacitor and resistor the longer the phone ring it will detect. You can read about it in the datasheet you linked over under the section "Steering Circuit". See the equation for the Basic Steering Circuit on page 3 and for gaurd time adjustment on figure 5 of the datasheet.
Title: Re: DTMF 8870 Circuit Explanation
Post by: airman00 on July 22, 2008, 01:15:33 PM
ahhh I get it now !

Thank you very much Melanie!   ;D
Title: Re: DTMF 8870 Circuit Explanation
Post by: airman00 on December 26, 2008, 08:39:55 AM
*Reviving Topic*

I have some time on my hands , would anyone be interested in a short tutorial on using 8870 DTMF Decoder ?
Title: Re: DTMF 8870 Circuit Explanation
Post by: Admin on January 09, 2009, 07:22:27 AM
Actually, I see people asking about DTMF decoders occasionally. I can see it as useful.
Title: Re: DTMF 8870 Circuit Explanation
Post by: angad on August 23, 2011, 12:48:15 AM
i want to use 8870 in my project
my project requires an output voltage of 5v from 8870
but output which i am getting from 8870 is 1.70v.
i have 741 ic amplifier  but dont know how to use it.
can you please help me in finding the reason for low output voltage?
Title: Re: DTMF 8870 Circuit Explanation
Post by: Soeren on August 24, 2011, 09:51:18 PM
Hi,

i want to use 8870 in my project
my project requires an output voltage of 5v from 8870
but output which i am getting from 8870 is 1.70v.
i have 741 ic amplifier  but dont know how to use it.
can you please help me in finding the reason for low output voltage?
How should we be able to, when you post absolutely no info on what you've done, no schematic with component values and no nothing at all?

The 8870 gives (close to) 5V out when driven from +5V, so either the 8870 is dead, you didn't use 5V or you messed up something else, but that's as close as we can get without any info.

If you don't understand a single op-amp, then how do you dare hoping to understand a circuit that contains so much more than a couple of op-amps?

Op-amps are fairly simple. their entire purpose in life is to keep both their inputs at the same potential and it will change its output to achieve that if possible (or hit the supply levels trying).
You can find lots of tutorials on op-amp circuits on the net.
Title: Re: DTMF 8870 Circuit Explanation
Post by: Admin on August 25, 2011, 04:42:57 PM
No flaming please . . .

angad, the issue is you asked us to debug your circuit, but you didn't post your circuit. Also, see the datasheet for wiring of your amplifier. Do your best to wire it, and if you're still confused, post your schematic and we'll double check it for you.
Title: Re: DTMF 8870 Circuit Explanation
Post by: angad on September 12, 2011, 03:31:22 AM
Hi,

i want to use 8870 in my project
my project requires an output voltage of 5v from 8870
but output which i am getting from 8870 is 1.70v.
i have 741 ic amplifier  but dont know how to use it.
can you please help me in finding the reason for low output voltage?
How should we be able to, when you post absolutely no info on what you've done, no schematic with component values and no nothing at all?

The 8870 gives (close to) 5V out when driven from +5V, so either the 8870 is dead, you didn't use 5V or you messed up something else, but that's as close as we can get without any info.

If you don't understand a single op-amp, then how do you dare hoping to understand a circuit that contains so much more than a couple of op-amps?

Op-amps are fairly simple. their entire purpose in life is to keep both their inputs at the same potential and it will change its output to achieve that if possible (or hit the supply levels trying).
You can find lots of tutorials on op-amp circuits on the net.

*Reviving Topic*

I have some time on my hands , would anyone be interested in a short tutorial on using 8870 DTMF Decoder ?
i want to operate an 4 wheeler robo using mobile phone's dtm frequency i have checked my 8870de as well as ckt  connections i m getting correct output but it is low in voltage example  on pressing 2 from mobile phone i m getting output at correct pin but its voltage is low and if i use op-amp it make robo less mobile as it require an input votage of 12v which i have from adapter only this would make me use 4 adapter i will post ckt dia soon as i ve met with an accident last need 2 week 2 recover i ve plaster on my right hand. need urgent help minor project need to be submitted next month .
Title: Re: DTMF 8870 Circuit Explanation
Post by: Soeren on September 12, 2011, 03:41:13 PM
Hi,

i want to operate an 4 wheeler robo using mobile phone's dtm frequency i have checked my 8870de as well as ckt  connections i m getting correct output but it is low in voltage example  on pressing 2 from mobile phone i m getting output at correct pin but its voltage is low
Sounds like you forgot to pull pin 10 high?

and if i use op-amp it make robo less mobile as it require an input votage of 12v which i have from adapter only this would make me use 4 adapter
I doubt that you'd need an op-amp, but even if, you can get op-amps working down to under 1V supply voltage and transistors is yet an option.
But do connect pin 10 to logic high to enable the internal pull-ups.

i will post ckt dia soon as i ve met with an accident last need 2 week 2 recover i ve plaster on my right hand. need urgent help minor project need to be submitted next month .
Test this and post back if it still acts up and drive (more) carefully  ;D
Title: Re: DTMF 8870 Circuit Explanation
Post by: angad on September 12, 2011, 10:32:06 PM
i ll post my ckt tomorow
Title: Re: DTMF 8870 Circuit Explanation
Post by: angad on September 12, 2011, 10:38:52 PM
Hi,

i want to operate an 4 wheeler robo using mobile phone's dtm frequency i have checked my 8870de as well as ckt  connections i m getting correct output but it is low in voltage example  on pressing 2 from mobile phone i m getting output at correct pin but its voltage is low
Sounds like you forgot to pull pin 10 high?

and if i use op-amp it make robo less mobile as it require an input votage of 12v which i have from adapter only this would make me use 4 adapter
I doubt that you'd need an op-amp, but even if, you can get op-amps working down to under 1V supply voltage and transistors is yet an option.
But do connect pin 10 to logic high to enable the internal pull-ups.

i will post ckt dia soon as i ve met with an accident last need 2 week 2 recover i ve plaster on my right hand. need urgent help minor project need to be submitted next month .
Test this and post back if it still acts up and drive (more) carefully  ;D

i will recheck my pin 10 connections again can u suggest me some good project ideas for my minor project for b.tech final year student.
Title: Re: DTMF 8870 Circuit Explanation
Post by: gursimransingh01 on January 15, 2012, 08:20:05 AM
Does anyone knows how to reset the output pins?? please help as this is the big problem i am facing.

for example, the output pins should remain high till the button is pressed, but actually output pins remain high even after button is released.

Title: Re: DTMF 8870 Circuit Explanation
Post by: Soeren on January 15, 2012, 11:43:21 AM
Does anyone knows how to reset the output pins?? please help as this is the big problem i am facing.

for example, the output pins should remain high till the button is pressed, but actually output pins remain high even after button is released.
Just connect the pull down resistors as in the schematic in the first post in this thread.
Title: Re: DTMF 8870 Circuit Explanation
Post by: gursimransingh01 on January 16, 2012, 05:31:40 AM

Just connect the pull down resistors as in the schematic in the first post in this thread.
[/quote]

I have connected the pull down resistor as you told, but still it is not working.

I have connected the circuit as shown in the attachment. I am using CM8870 DTMF IC...

Title: Re: DTMF 8870 Circuit Explanation
Post by: Soeren on January 16, 2012, 03:40:18 PM
Hi,

I have connected the pull down resistor as you told, but still it is not working.

I have connected the circuit as shown in the attachment. I am using CM8870 DTMF IC...
Your attachment just is the datasheet and none of the schematics shows it with the pull downs?

That particular chip would probably be best off with higher value pull downs, but that's not the immediate issue.

If you actually read the datasheet, you'll see that you need to pull TOE (Three-state Output Enable, erroneously called TOW in fig. 2) low to make the outputs high impedance (so that the pull downs can pull the outputs low).
Title: Re: DTMF 8870 Circuit Explanation
Post by: gursimransingh01 on January 18, 2012, 05:23:32 AM

[/quote]
Your attachment just is the datasheet and none of the schematics shows it with the pull downs?

That particular chip would probably be best off with higher value pull downs, but that's not the immediate issue.

If you actually read the datasheet, you'll see that you need to pull TOE (Three-state Output Enable, erroneously called TOW in fig. 2) low to make the outputs high impedance (so that the pull downs can pull the outputs low).
[/quote]

Hi !

Thank you so much. Now it is working properly. It worked by connecting TOE with StD, so that TOE will remain high only when StD is high. I had also connected pull down resistors with the output pins.

Thank you once again.
Title: Re: DTMF 8870 Circuit Explanation
Post by: Soeren on January 18, 2012, 01:09:23 PM
Hi,

Great!

And thanks for posting back what worked, it will possibly help others having similar issues :)
Title: Re: DTMF 8870 Circuit Explanation
Post by: robotprince on April 29, 2016, 05:47:50 AM
Hi,

After reading this forum, I got some interesting info about 8870. But I came to know that when we press a button in the mobile phone, it splits into two frequencies (i.e. high and low frequency) which will produce a single tone. This tone is sent to 8870 IC through headphone jack and they are separated by bandpass filter. Then they are decoded using digital counting techniques to receive the digital output.

Nothing much, just wanted to share my knowledge.