Simple circuit using audio to light LED
Ravenkallen
Posts: 1,057
Hey guys. Sorry i am posting this in the Prop forum, but i seem to get the best answers here....I had a audio out cord on my laptop hooked up to a multimeter. Even at the highest volume level it only outputs about 0.20 volts. This is not even enough to turn on a average transistor. So how does somebody boost the voltage of a audio line to a higher voltage? Do you need to convert it to a AC signal, boost the voltage and then turn it back into DC? Or is it already AC? AND is there some kind of simple circuit that uses a couple of easily found components?
Comments
See the schematic at the top of the ADC spin file, but for AC/Audio inputs substitute a small cap ( .01, .1 etc) for the 150k R. Connect an LED and watch it blink when the program sees audio.
You will likely want to use a 10k or so pot as voltage divider on the front end to drop the signal.
The test code below works very well, and could be tweaked very easily to suit your own needs.
BTW, the beauty of the Prop is that you don't need much else to do a whole lot of stuff.
Humanoido
Since your not driving any audio, hence the amplification quality does not need to be that great, you can use a couple NPN transistors and a few resistors to get the job done.
The trick in amplifying signals that low, is to bias the transistor so it operates in it's linear region or that it's very close to being turned 'on' with a DC voltage usually set by a voltage reference or a voltage divider. Then your audio signal can influence the transistor very easily.
I could have probably done this with one transistor, but it was less of a balancing act to use two.
I think what PJ Allen is commenting on here is that this topic probably should have been put into the Sandbox but since T_Chap made a reference to the Propeller, this thread (bacon) gets saved from the Sandbox (fire).
PJ, at the risk of inciting more riotous moribund off-topictude, I must say I like your new avatar a lot.
@Beau.... I kinda like your idea. I never thought of that. Just add an external voltage to the transistor until it is almost turned on, then the tiny audio signal should have no problem turning it on seeing as it is almost on anyway. I will try your circuit first.....You said that you could get away with using only one transistor?
@T. Chap... I will try your idea if the transistor one does not work. It sounds pretty easy to setup....
@all... Thanks for the help.
"Thanks for not steering this on-topic, Beau"
- Well, honestly there are just some times you can have way too much overkill. Yes, throwing an Op amp, or an Audio Amp, or even the Sigma Delta ADC capabilities of the Propeller at it will most likely work in all of the suggested circumstances. - Even here a Sigma delta ADC would be too much.
- The suggestion that I posted, is actually closer in principle to the Propellers Sigma Delta ADC than you might think, just an analog version. In fact with a similar approach of just biasing the Propeller IO pin with a DC offset voltage you'd have a very similar effect as my analog circuit I previously posted.
Below is an example of using a single Propeller I/O. simply look at the IO with ina or inb and direct the internal program logic to do whatever you want it to do.
Please do not start threads in a forum "because I seem to get the best answers here". It's against the forum rules and it's a disservice to others looking in a particular forum for ideas on that subject (and either finding something unrelated or not finding what they're looking for).
@PJ Allen,
It sounds like you're very frustrated. Maybe you need to take a few deep breaths or take a walk outside or something like that. In the past, some of your sarcasm has been positively brilliant. Don't waste it.
http://forums.parallax.com/showthread.php?t=124943
I wired up a simple one with an LM386 which seems to be working well. I like the simple examples that Beau just added and will have to try that out.
Robert
Yes, PJ can be very sarcastic at times. He also can be very helpful. That's the nature of a community or a family.
http://wolfstone.halloweenhost.com/ColorOrgans/clofix_DebuggingColorOrgans.html#OptocouplerBasedColorOrgans
I suppose it should have a cap on the input but I built the first half of it on the breadboard and it worked ok.
Robert
I guess i will try to make plans to use the LM386...The transistor idea should have worked(even though i didn't use the exact values specified)
-Phil
@Phil....Uh, no. I am guessing that is the problem? If is a AC signal i will need to convert it to DC right?
3 - .01 caps
1 - 100k resistor.
If the LED comes on with no audio, just swap the + and - leads on the LM339 comparator. This behavior is dependent on the comparator's input offset voltage and can vary from part to part. For higher input levels than those from a microphone, you can bias the positive lead further away from the threshold with a pullup resistor to +5V. A value between 1M and 10M should do the trick.
The open collector output of the LM339 is sufficient to drive the LED, but you may want to add the 47uF cap shown to smooth out the audio peaks, so the LED response appears more proportional to the audio level.
-Phil
@T chap...sometimes i don't want to find a "easy" way to do something. I want to find a hardware solution. But by no means am i saying that your idea is not good, i just want a slight bit of a challenge. I want to know why it works, just not how it works.
@Phil. I don't think i even have any comparators. I need to go through my drawer sometime and take an inventory.