Circuit to make beeps every time light is sensed?

[airman]

I'm making a project for one of my mechanical engineering classes where I have to design and recreate a theoretical problem in the book. The one I am building involves a rotating mass, and some cables. The question is to find the tension in a certain cable with a mass attached at a certain RPM.

 

I plan on using a circular disc attached to the rotating shaft with a small slit on the edge. On one side of the disc, a photo diode under the hole, and a light source on top. So - every time the mechanism completes one full rotation, the photo diode "sees" the light through the hole on the disc and beeps. The beeps would be counted over a certain period of time to calculate rotations per second. I'd like to build a circuit to accomplish this. I work at radioshack and I can get all the parts I need, I just don't kow which ones besides the diode and an LED.

 

I've found a schematic online, however I'm not really the best at reading them. (at least with the symbols etc - I know what a relay is, what a transistor is for, etc but I just don't have much experience in putting together stuff)

 

This is the one online, I could hook it to a beeper/buzzer:

http://www.circuitstoday.com/photo-relay-circuit

 

The only thing i'm confused about is the Q1 - we don't have that here and wikipedia says it is similar to the 2N3904, but with "different lead assignments". Can this be substituted?

 

Thanks for the help.

[ARandomOWL]

2N3904 will do fine. As you said, the pinouts are slightly different. With the flat side facing towards you the pinouts are as follows:

 

2N3904: E B C

BC548 : C B E

 

So you only need rotate the transistor's connections 90

[airman]

Thanks, that helps.

 

When you say rotate the transistor 90 degrees, howso? there are 3 leads on it...

 

Also, the schematic there is for when light is OFF, the relay is on. What needs to be reversed in order for the relay that controls the beeper to be ON only when it senses the light source through the rotating disc?

[paulktreg]

You would be better using something like this.

 

This particular one is just an example of the many available at Radio Spares in the UK. I can't find one at Radio Shack but I'm sure they will do them, i'll keep looking.

 

It has the photo diode and a photo transistor in same package. Your slotted disk runs in the gap and it will produce a signal everytime the slot goes through.

 

I can help you further if you need help, don't hesitate to ask.

 

Edit: Radio Shack used to do some pretty good electronic project books. They followed an hand written format on squared graph paper. Do they still do them? You might find something of use in them? By Forrest Mims?

 

Cheers

 

Paul

Edited July 19, 2009 by paulktreg

[airman]

Edit: Radio Shack used to do some pretty good electronic project books. They followed an hand written format on squared graph paper. Do they still do them? You might find something of use in them? By Forrest Mims?

 

yep, they do. I'm kinda wanting to further my knowledge by doing this project so i'm gonna go ahead and use this circuit diagram to build what i need. If we had those at RS though, I would probably have picked that up - but we don't. So since I can't get it there, I'm just gonna build it with what we have

 

I'm having trouble relating the diagram to the SPDT relay i picked up. Not sure how to read it.

Edited July 19, 2009 by airman

[paulktreg]

But you are furthering your knowledge by using "old" technology!

 

There must be, no are, suppliers in the US that will have the slotted opto detectors, they are used all over the place!

 

Integrated circuits are largly replacing transistors these days and if you want a modern design then that's the way to go. The detector I pointed out with something like a 741 op amp will do what you require. Why take a step back in time?

 

Cheers

 

Paul

[airman]

i'm really not trying to be difficult, but i'd like to make this work with what i already have - the RS i work at only has a few ICs and they are pretty much useless.

 

edit: ah, hell. I just realized I bought a double throw relay instead of a single throw. It was the only 9v one we had. Will it still work?

Edited July 20, 2009 by airman

[hardnrg]

Double throw just means the Normally Closed contacts are wired up, whereas Single Throw doesn't have (working) N/C contacts...

 

Both have Normally Open contacts though...

 

http://en.wikipedia.org/wiki/Relay#Pole_and_throw

 

 

Have you tried opening up a ball mouse to scavenge the photo diode/transistor pair?

 

What kind of RPM are you expecting? Is it going to be very low?

[airman]

Double throw just means the Normally Closed contacts are wired up, whereas Single Throw doesn't have (working) N/C contacts...

 

Both have Normally Open contacts though...

 

http://en.wikipedia.org/wiki/Relay#Pole_and_throw

 

 

Have you tried opening up a ball mouse to scavenge the photo diode/transistor pair?

 

What kind of RPM are you expecting? Is it going to be very low?

 

 

yep, very low. only a few per second.

 

why would a ball mouse have a photodiode/transistor in it? for the scroll wheel or something?

Edited July 20, 2009 by airman

[hardnrg]

You've never seen the X-Y rollers in a ball mouse?

[airman]

You've never seen the X-Y rollers in a ball mouse?

 

hundreds of times. but i didn't realize they were "read" by light...

[ARandomOWL]

Thanks, that helps.

 

When you say rotate the transistor 90 degrees, howso? there are 3 leads on it...

 

Also, the schematic there is for when light is OFF, the relay is on. What needs to be reversed in order for the relay that controls the beeper to be ON only when it senses the light source through the rotating disc?

 

I meant 180