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Please login or register. Did you miss your activation email? VegaObscura Robot Overlord Posts: I am attempting to make a circuit with the MT chip.

What I want to do is have a plug going from a cell phone, into the circuit board in a way so that the MT will decode the button presses and light up LED’s to indicate in binary which button was pressed. Later on the circuit will be more complicated, but I need to make it work in this simple setup first.

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This has been done many times by many people, but typically it is done with an old Nokia phone. I need to do this with a modern smartphone. The phone I am using is a Samsung Galaxy S4. First I set up the circuit on a breadboard in its normal fashion, as can be seen in this diagram: This circuit did not show any response to button presses. After a lot of googling, an obscure forum post in an indian forum stated that the k resistor between tip and pin 2 works for old Nokias, but is too strong for modern phones.

He said that his circuit worked fine after replacing that k resistor decdoer a 4. So I tried using less resistance between tip and pin 2. I didn’t want to go quite as low as 4.

With the 10k resistor in place, as soon as I fired up the circuit and started hitting buttons on my dhmf the LEDs started reacting exactly as they were supposed to.

I thought I had won. But then another problem arose. After a few seconds of playing with the buttons, the whole thing suddenly stopped responding. The lights didn’t all go off, they just stayed in the position they were in and would not react to the keypresses.

I tried unplugging the power and plugging it back in. This reset all the lights set them all to off but they still wouldn’t dmf to keypresses. After some playing around aimlessly, it started working again. But, again, after a few seconds it stopped responding.

Unplugging the circuit and plugging it back in does nothing. It seems as if unplugging the phone from the circuit works sometimes. I tried reducing the resistance again. Both of these behaved the same way as the 10k resistor. As it stands, the circuit will work for a few seconds, and then lock up.

If, while it is working, I keep pressing buttons on the ctmf constantly, it will not lock up.


California Micro Devices Corp – datasheet pdf

But if I stop pressing anything for even five seconds, it locks up every time. I suspect that when I changed the resistor from k to 10k, I would have needed to change something else as well. Possibly the resistor between pin 2 and pin 3?

Possibly the capacitor between tip and the now 10k resistor? Here is the circuit as it is now, the only difference from the diagram above is the resistor at the top left: The 5V I am using comes from a 5V regulator connected to a 9V battery. Using the multimeter, all voltages and continuities on the board are appearing correct.

I understand most of the circuit.

I understand the bottom half completely. I understand that the 0. I understand that the resistors at the top left of the diagram are for controlling the op-amp, but I don’t understand exactly how they do that.

I don’t understand the 0. Here is a link to the datasheet for the MT http: Any assistance in helping me to stop it from locking up is appreciated. Do you have an oscilloscope or logic analyzer that you can connect to diagnose what is going on when it’s locking up? If not, then I would suggest getting one of those is the first thing you should do. I don’t think I can afford one of those.

Billy Robot Overlord Posts: VegaObscura on April 30, Yes, the cellphone is connected to the circuit by the audio jack. I have tried using two k resistors in parallel for a resistance of 50k. This seemed to cause a slight “lag” in the response time, but it still locked up the same. I will try using a potentiometer to see what is the highest resistance that will still let the chip recognize the signal and see if that stops it from locking up. It never occured to me that the phone might stop putting out the signal.

I will try plugging in my computer instead and see if that changes anything, but the sound card on the computer might do the same thing as the phone and stop putting out signal. Billy on May 01, It’s not late until its two, then it’s too late. I tried adding a k resistor between tip and ring. I then tried adding a second k resistor in parallel with the first, for a combined resistance of 50k. This still did nothing.

The circuit still locks up the same. I don’t think the phone is sensing that there are no headphones connected, because it has a headphone icon in the corner the entire time that it is plugged in. I will try getting a headphone jack splitter and plugging in a set of headphones with the phone and cecoder board, so I can listen and make sure the tdmf is putting out the audio signal. May 02, VegaObscura on May 02, Billy on May 02, I picked up a sound jack splitter so I could plug both the circuit board and a headset into the phone at the same time.


I can now confirm that the phone is not the problem. The phone is putting out sound the entire time. The problem is something with the circuit. My next test will be to put a potentiometer in place of the resistor between tip and pin 2 so I can see if setting the resistance close to the “cutoff point” where the circuit reads the signal helps anything. I would get a audio amp kit from Velleman or sparkfunto deal with the audio. The DTMF frequencies are from to Hz, so you should not have any frequency roll off on the audio amp.

It does take a certain amount of time for the chip to identify the tone; no less than mc8870pi, I think. You can take pictures of your waveform, to analyze later, or pass to dexoder techs for their advice. July 27, Get Your Ad Here. April 30, Logged jwatte Supreme Robot Posts: May 01, Logged Billy Robot Overlord Posts: To make sure I understand. Quote After a few seconds of playing with the buttons, the whole thing suddenly stopped responding. Dscoder I had to guess, I would say the audio driver in the phone is shutting down, and not your circuit.

I’ve seen modern electronics sense if a connector is unplugged or shorted and shut off. The phone probably senses something is not quite correct and shuts vtmf the driver for protection. Try using a K resistor and see if that clears it up. Quote I understand that the 0. It’s a minor point, but the way the cap is in the circuit, it is c8870pi filtering noise, it is filtering DC signal, so if the audio from the phone has a DC offset, the DC is blocked and only the AC audio gets through.

Is it too late to modify my answer? When you are not pressing buttons, the output will be DC and the capacitor will look like an open circuit, the phone will think there are no ear phones connected, so it is shutting off the output a few seconds recoder you stop pushing buttons.

Again because when you’re not pushing buttons it decides there are no longer ear phones connected. You need to make the phone think there are ear phones connected all the time. To do that you need to provide a low impedance path to DC signals. To do drmf, install a 50K – K resistor between the tip and ring lines. This high side driver has a good explanation of how it senses a disconnected load.

You could also confuse this high side driver with a xecoder in series with the output, just as you’re doing to the phone. Well, it was worth a shot. Without a cmm8870pi or other way to get data, you have dtkf form theories and test them open loop.

Last modified: July 2, 2020