SparkFun Forums

[pavanvd]

Hello All,

I'm a newbie to electronics, so pardon my possible naive questions. I am doing a project for fun and have successfully interfaced a SIM900 based GSM modem with a Pi. Using specific AT commands I can send/receive SMSes and also initiate and receive calls. But I am now trying to get audio in and out of the modem. While a simple electret microphone seems to be working ok, I'm having hard time figuring out how to get a decent audio out.

FWIW this is the specific modem I am using http://www.rhydolabz.com/wiki/?p=10450 for the purpose.

I tried to connect a 8 ohm, 0.5w speaker to the GND/SPK pins, but the audio out is extremely low. I even tried connecting a headset to the pins, but still no luck with the audio level. FWIW AT+CLVL is set to 100 i.e. max value.

Can someone guide me if I must use an audio AMP to get any decent audio out? If so, which one? Or am I using a wrong speaker? Does this need 32 ohm speaker?

My goal is create a speaker phone out of it. So the audio should be audible just like a regular speaker phone. Also, if you could suggest the correct microphone to use, that will be great.

Much appreciate the help. And apologies if the topic is covered elsewhere. My search did not yield any results.

Thanks

[Valen]

It's a TTL UART modem. You'll never get any (voice) audio out of it. Not even old fashioned dial-up modem sounds. *beep*-*beep*-*low hum*-*scratchy noise* You'll need a sort of voltage-to-frequency modulator for that.

At the most the speaker will make click sounds when the TX level changes and moves the membrane. It's not suited for that.

If you need audible output then maybe a buzzer with integrated oscilator and digital on-off input might work. I don't have an example at hand though.

[pavanvd]

Thanks for the reply. The TTL UART part is to interface with the micro controller, right? For example, I have connected Pi's TX/RX pins to the respective RX/TX pins of the modem and sending AT commands using a python serial terminal. But there are separate pins to get audio out and mic in. Otherwise how do one even make calls using such a device?

Also, FWIW I do get the audio. If I push the speaker close to my ear, I can hear what the caller is saying at the other end of the call. Just that the audio level is extremely low.

[Valen]

Ok, I couldn't find an explanation of the pins on the board. Or a datasheet for it. If it does have analog audio out then it is probably only line-out level (1 volt peak-peak I think) You'll need an amplifier for that. This should suffice to get enough volume out of that speaker:

https://www.sparkfun.com/products/11044

[EDIT: Yes TTL UART is to interface with a microcontroller, like the pi, digitally.]

[EDIT2: Hooking the output up to your computer's audio line-in should also make it audible through it's speakers. Then it only costs a 3.5 mm jack-plug.

[pavanvd]

Ok, I couldn't find an explanation of the pins on the board. Or a datasheet for it. If it does have analog audio out then it is probably only line-out level (1 volt peak-peak I think) You'll need an amplifier for that. This should suffice to get enough volume out of that speaker:

https://www.sparkfun.com/products/11044

Thanks for the pointer. Would something like PAM8302 or LM386 do the trick as well? I think I can source them more easily here.

[EDIT2: Hooking the output up to your computer's audio line-in should also make it audible through it's speakers. Then it only costs a 3.5 mm jack-plug.

Ah ok. I can try that too. While it seemed that am AMP is required, I am trying to ensure if I am making other mistakes. Especially because the manufacturer wiki does not say anything about AMP and actually shows a pic where the output is connected to a headset 3.5 mm jack directly.

Thanks

[Valen]

Ok, I couldn't find an explanation of the pins on the board. Or a datasheet for it. If it does have analog audio out then it is probably only line-out level (1 volt peak-peak I think) You'll need an amplifier for that. This should suffice to get enough volume out of that speaker:

https://www.sparkfun.com/products/11044

Thanks for the pointer. Would something like PAM8302 or LM386 do the trick as well? I think I can source them more easily here.

The PAM8302 chip seems functionally equivalent as the Sparkfun amplifier has. So yes, I guess so. Adafruit sells a similar amplifier board based on that chip. I don't know about the LM386 or what is required for them to be honest. Never played around with those although they seem to be pretty popular.

[EDIT2: Hooking the output up to your computer's audio line-in should also make it audible through it's speakers. Then it only costs a 3.5 mm jack-plug.

Ah ok. I can try that too. While it seemed that am AMP is required, I am trying to ensure if I am making other mistakes. Especially because the manufacturer wiki does not say anything about AMP and actually shows a pic where the output is connected to a headset 3.5 mm jack directly.

Thanks

It being connected to a 3.5mm jackplug on the image doesn't imply anything about amplification. Needed or not. What is on the other end of the cable would or could, but they didn't show.

[edit]That manual does state on page 14 "Thereceiver outputs only can directly drive 32 Ohm."
So, your speaker are too big of a load for it. I don't know what impedance earplug headphones have but they might work better.

[pavanvd]

The PAM8302 chip seems functionally equivalent as the Sparkfun amplifier has. So yes, I guess so. Adafruit sells a similar amplifier board based on that chip. I don't know about the LM386 or what is required for them to be honest. Never played around with those although they seem to be pretty popular.

Thanks. So I hooked up the 8 ohm, 0.5w speaker to PAM8403. Its a two channel amp, but that's the one I was able to procure immediately. The audio is now much much better, but still not loud enough to serve as a speaker phone. Its also noisy, which could be either because of poor wiring or the amp. I will wait for other two amplifiers and also another 1W 4 ohm speaker that I've ordered to see if the voice quality and decibel improves.

[edit]That manual does state on page 14 "Thereceiver outputs only can directly drive 32 Ohm."
So, your speaker are too big of a load for it. I don't know what impedance earplug headphones have but they might work better.

Hmm.. interesting. So lower the impedance, higher the load? I always thought its the other way round. The earplug impedance as I measured was much higher (150 ohm or so). Its a Motorola earplug that comes with Moto G. But the audio wasn't any good (I haven't tried with the amp though).