Science Buddies: "Ask an Expert"

hello,

Currently i'm working on a theremin circuit using arduino. Precisely, this circuit: http://interface.khm.de/index.php/lab/i ... ng-device/

The sound output however is not high enough for me. So i want to use an amplifier to increase my sound. However, i have no idea how to do this. I want to buy this amplifier: http://abra-electronics.com/robotics-em ... 2-ada.html Will it suffice. What else should i buy (what potentiometer to control volume, what speakers, etc). How do i set up this circuit. What goes where? I'm very confused about wiring this circuit.

I'm trying to build this to be able to perform demos at my science fair. I would appreciate a detailed answer and any guidance provided. Thank you!

The PWM output from the Arduino is a 5V signal, so you can't just put it into an external amp. You could feed the signal into one of the fixed pins on a potentiometer with the other fixed pin to ground, then adjust the pot so the tap (center pin) is giving you a 1V signal. That should be fine into an external amp of your choice - but be careful with the volume. the square wave output from the Arduino might be a little tough on the speakers.

Alternatively you coud build a little amp to add to your circuit. http://rcarduino.blogspot.com/2012/08/a ... jects.html has some easy-to-follow directions. http://www.adafruit.com/product/987 (it might be the same card you mentioned above??) is another little card you can build that would work fine as an amp.

If you want to get a little fancier, there's an Arduino shield intended to make music players. http://www.adafruit.com/products/94 has a DAC, speaker and headphone output and an SD slot. They expect you to put music on the SD card and make a player out of your Arduino but you could simply send waveforms out the DAC - sending sine waves would make it sound more like a real theremin if that matters.

The adafruit link that you sent seems to be the one that i want to go with.
So i don't need the Pot for this??

That card also has a tutorial with it which is great. But i have one question. I don't know how my arduino output would go into this amp though. On this picture, https://learn.adafruit.com/assets/3730
you can see that there's a right and left component of the input. My input however is just one pin and a ground. So how do i wire this up?


The wave shield might be a cool idea too. But that's for later. I'm just needing a concept proof right now.

That Adafruit amp board wants line level in, so you will still need the pot. It's a stereo amp, so you'll just use either L or R for both the input and the speaker.

Howard

I see. So let me just make sure I have this right.

Arduino output to fixed pin of potentiometer. Other pin to ground, middle pin to Right component of amplifier. The R- is grounded. left component is left alone with nothing.

So is a 50k pot a good size . Also, why 1 volt. How did you know that I need to decrease voltage to 1. And how do I measure that I'm getting 1 volt. Can I just put a voltmeter across the tap and the ground?

That sounds right, and a 50K pot ought to be fine. Have your Arduino spit out a waveform and adjust the pot until you see an output with a peak of approximately 1V. That will give you a place to start. (if you don't have an oscilloscope it would be a good chance to find someone who has one. If not, just turn the pot all the way down and turn it up *slowly* while playing with the amp until it's loud but not distorted. - And I suppose you could preset the pot by connected it between 5V and ground and adjusting the output to 1V on a meter - yes.)

These amps like audio line level input. Wikipedia has a nice summary: http://en.wikipedia.org/wiki/Line_level . Ideally the waveform should go between +1 and -1V. You could do this by taking a 2V output and mucking about with it but I think you'll be fine with just dropping the 5V output to a safe level and amplifying it.

Thank you so much Howard. I'll search around my school to find our old scope. Hopefully i can find it. If not i'll resort to the multimeter or just my ears. I guess the potentiometer can also protect the arduino in case of a short.

Thanks again, i'll come back if i run into any problems.