Driving multiple optoisolators in parallel

Thread Starter

jb8256

Joined Dec 7, 2013
18
Hello all,

I am somewhat of a novice, but not a total newbie as far as electronics goes.I am working on a project in which an audio signal such as a drum beat is fed into a peak detector. The output of the peak detector goes into an optoisolator, which is connected to two points in an electronic toy that triggers a sound. This part works perfectly; the sound is triggered perfectly in rhythm with the drum beat.

A more complicated implementation of this involves a 49 key Casio keyboard with 10 voice polyphony. I will use 49 optoisolators, one per key. I want to be able to send the output of the peak detector (follower?) to as many as 10 optoisolators at a time, thereby triggering as many as 10 keys on the keyboard simultaneously from this one signal from the peak detector. On the breadboard, however, whether I wire the optoisolators in series or parallel, the triggering diminishes a little as I add each one, and before I can get to 10 optoisolators connected, it doesn't trigger at all. What I need to happen is have the signal from the peak detector remain the same (voltage AND current?) no matter what the change in the load is (1 optoisolator being triggered or 10). I don't know if this problem is a function of a drop in voltage or current. The optoisolators must be in parallel because I need to trigger any of the 49 keys at a given moment: Imagine a string of 49 LEDs - I want to send this pulsing signal to any 1 to 10 of them at a time. I have read it's not a good idea to wire LEDs in parallel (essentially this is what I'm doing because the optos have an LED and phototransistor). I hope I'm not pipe dreaming and that there is a way to accomplish this.

I thank you in advance for any advice you might have for me!
 

MikeML

Joined Oct 2, 2009
5,444
One way to do this is by putting a current-limiting resistor in-series with each opto-isolator LED, just as you would do with parallel LEDs. You might have to crank-up the output of the peak detector a little to overcome the voltage drop across the current-limiting resistors.
 

MikeML

Joined Oct 2, 2009
5,444
Mike,

I forgot to mention I had current resistors in series. I though about maybe boosting the signal.
Then you need to post a schematic of the peak detector. You probably need 5 to 10mA per opto, and whatever you are using just cant deliver that kind of total current.
 

#12

Joined Nov 30, 2010
18,210
Then again, maybe I got the 2.7K resistor on the wrong side of the capacitor. I was pretty high on paint fumes at 4 this morning.:eek:

Can I assume you arrived at 1 meg and 1 uf because you tried it and it works? Can I asssume you know there are a couple of gain stages that can be inverted or altered if you want to? Don't take my stuff as gospel when in fact, I don't quite understand your stuff.
 

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Thread Starter

jb8256

Joined Dec 7, 2013
18
Thanks again #12. Hopefully today I can try your schematic. I will keep you posted on my progress. I really appreciate it.
 

Thread Starter

jb8256

Joined Dec 7, 2013
18
Hey #12...I breadboarded your modifications and it worked PERFECTLY. I can't thank you enough! I had my follower circuit driving 5 notes on the keyboard flawlessly without any missed triggers. Awesome!
 

#12

Joined Nov 30, 2010
18,210
It should drive 10 optos (as mentioned in post #1). I just didn't draw them because the principle has been demonstrated. Hang a few more on the output line if you want to.
 
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