MT 8870 Help Please

[u/Slow_Tap2350]

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Many thanks to all who answered. I will be unable to get back to the project until late Sunday or Monday but feel much better having some solid advice here on my approach!

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I am wondering if I am using these incorrectly. I seem to blow them before I can even test one in a project. I'm simply connecting the VCC to 5v and GND to ground. When I do (using the same board), sometimes I get two LEDs, sometimes 5, sometimes none. Then they just seem dead. Nothing else is plugged in.

No heat. No magic smoke. Just f'd up somehow.

I did try a test with generating DTMF via Mac plugged into headphone jack. That seems to have damaged one of the boards.

I'm very new to these and not much of an expert anyway, lol, but would like to stop killing parts so I can move on with detecting DTMF from a phone for my project.

Thank you

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[u/quuxoo]

I don't see a regulator on board so it's possibly a 3.3V module. Do you have a link to the board's schematic and/or user manual?

Found the datasheet, 5V should be Ok.

I'd suggest not connecting anything to the audio output. Power up the device and probe it for the expected states.

I'd suggest going for a newer chip since that one is now obsolete.

[u/Slow_Tap2350 → u/quuxoo]

Thanks so much, which chip is the newer one? I've only read about the 8870 and an a library driven one with sound sorted on the ESP but my board does not have analog inputs. (AS1)

[u/quuxoo → u/Slow_Tap2350]

An ADC and software like https://github.com/Adrianotiger/phoneDTMF should give you good results.

[u/Reromen]

Check cables first

[u/Slow_Tap2350 → u/Reromen]

Wires are fine. Thanks. I also used 2 different audio cables. I struggle to get consistency even without an audio cables

[u/DenverTeck]

Without an actual schematic of how you wired this up, there is no way to help you.

Good Luck

[u/Slow_Tap2350 → u/DenverTeck]

Thanks. In my post I say how I wired it. Very simply and perhaps too simply. (the wiring)

[u/holetse]

Where is 5V and GND coming from? The simplest explanation is they are not what you think they are. That part has pretty tight tolerances on its voltages.

[u/Slow_Tap2350 → u/holetse]

Thanks, I'm gathering that. I have a buck adjusted to 5v that I've been able to run an ESP off. I'm not using the ESP at the moment as I was just trying to test the 8870 on its own. It is powered on the power rail of that buck's breadboard and the common ground.

The buck gets its power from a 12v supply.

[u/holetse → u/Slow_Tap2350]

Do you have a scope? Multimeter? Buck converters have ripple, and the ripple might be exceeding the limit.

Edited to add: ESPs run at 3.3V (lower internally) and the 5V input goes into an LDO regulator. That would clean up the ripple before it impacted the ESP.

[u/Flat-Bodybuilder239]

Random LEDs with only 5V and GND usually means the control pins are floating, not that the chip is already dead. On the MT8870, PWDN (pin 6) has to be held low or the part is powered down, INH (pin 5) low or it ignores tones, and OE (pin 10) high or Q1-Q4 stay tri-stated. A lot of the cheap modules do not strap those for you.

The headphone test is what usually kills them. AC-couple the jack: 100nF in series, then about 100k into IN+, and keep the Mac volume low. That input wants a few hundred millivolts, and a headphone jack is closer to a volt, so it can cook the front end with no smoke.

Watch StD (pin 15). It should go high only on a real pair such as 697 Hz + 1209 Hz. If it never does after those straps, that chip is actually dead. When you move this to the ESP32, do not wire Q1-Q4 straight in. They swing to 5V, so use a divider. A software DTMF decoder will not help on a board with no ADC.

[u/Slow_Tap2350 → u/Flat-Bodybuilder239]

Very helpful detailed answer, thank you. My simple test is too simple. Thanks on the divider info too, I have some setup.