New Project, need some advice (electronics and such)

Afternoon all. As usual, I have a ton of 3000 and 4000 level homework which I am avoiding doing, so I have a project in mind that I've some questions about.

Here's the basic idea: A pendulum suspended above a grid of halved ping pong balls. Attached to the pendulum bob are a few lasers, aimed so that the two, maybe three, beams converge on the ping pong ball halves. Inclosed in the ping pong balls are photoresistors (the pingpong ball halves diffuse the laser beam to get a larger trigger area) attached to a small circuit wave generator so that as the lasers hit the balls, the resistors trigger a tone generator which makes a sound. I'd like each of the balls to have a different tone, perhaps a diatonic scale (no wrong notes). As the laser tracks its path (a Lissajous figure), which can be chaotic if set up correctly, it triggers sounds in a random pattern, which will be different every time the pendulum is released.

Assume that it will be operated in a dark room so that ambient light is of no concern and that I'd be using red lasers pointers.

now on to the design:

Option 1:
I'm thinking a 555 Timer square wave generator might work, but I'm unsure how much frequency control I'll have and what the resistance range on photoresistors typically is.

here's the schematic for the 555 timer circuit:



From the description on the page it came from, varying the 6.2k resistor will change the tone, but I'm not sure what kind of frequency range will be viable. Also, how high will the resistance have to be in order to make no tone at all? I've never worked with a 555 timer.

The circuit looks the same as a Drawdio , which uses the varying resistance of a graphite pencil line to vary the tone.

so basically here's the questions:
1. will the photoresitor's resistance drop quickly to (approximately) zero when hit with a laser pointer's beam or will it slowly drop down to zero producing a tone which starts high and slowly lowers. (basically will it operate like an on/off switch or a dimmer)
2: can I wire up several photoresistor/resistor combinations in series hooked up to only 1 555 Timer circuit? i.e. only one will be operating at a time, so in series the non-lit ones will be like an open circuit (assuming the resistance of the photoresistor is enough to keep the timer from operating).

Option 2:

Photoresistors hooked up to an arduino board, which is in turn hooked up to a midi out leading to a controler. This one is a lot more expensive, since I have neither an arduino or a midi box, but I'm sure the results would be better. I dont want to do this one, because I'm broke and I have a bunch of HW to do.

First question - do you WANT the soound to change tone? If so, then photoresistors will work in the manner you describe - ie the resistance will linearally increase and decrease in resistance depending on the ammount of light striking it. Thus pitch will change accordingly.

If NOT, and you want the sound to keep a perfect tone, then you will need to use phototransistors instead. These will turn on or off, rather than change value. You will need to use fixed value resistors on each output of the phototransistors, to get the change in resistance for the differeing frequencies produced by the circuit.

Any reason for using a laser (other than 'cool' factor)? You may find that IR LEDs will provide a much better option as they can prevent problems from ambient light levels.

SS

thanks, that answered my question about the resistors. still need to know about series wiring thing, i guess if phototransistors operate like switches, it should work.

I was wanting to use lasers to keep the beam precise, so that only one ball is being hit at a time.

more to come.

VonHart wrote:
I'm thinking a 555 Timer square wave generator might work

Interesting project! You really don't want a square wave generator though - it will sound pretty terrible. About 25 years ago I made a sort of a musical instrument this way, and really didn't like the noise it made.

Many years later, playing with computers and voice pitch shifting software I discovered another potential issue - all those high frequency components can blow the tweeters in expensive loudspeakers if you're not careful to low-pass filter it first.

Anyway: the moral - you don't want a square wave generator.

Reading all this, why don't you find something like an old kiddies Casio keyboard and connect the optical switches to the key contacts?

That way, it's in tune, and it has a range of preprogrammed sounds to play with.