reading electrical schematics help needed

[bfitz]

I am having trouble learning how to read schematics. I got interested in it after reading the thread on building your own piezo pick ups and seeing the basic preamp that was on here (I can't remember who designed it but I know I have seen him post here a few times). I have looked around the internet for different effects and devices that i might want to build and have found a very simple (or at least i think so) Theremin to try and build but i cant get it to work. this is also the first time that i am using a breadboard and i don't quite get how they work. i posted the picture on here and hopefully it shows up. all of the parts are from radio shack and i got the ones that were listed on the website. i have some books from when my girlfriend was in college for electronics but there all a little over my head, i think that they are all from her second or third year. does anyone know of a good, SIMPLE, website or book to start with to explain how to read schematics, not so much the design or math involved in developing them? eventually i do want to learn all of that, but for know its just some free time projects. i understand the basic things, what the resistor, diodes, pots, things like that look like. what i have a really problem with is following what is connected with what, like in this schematic the photo resistor, capacitor, and the transistor connect to for what looks like a T, and then the capacitor goes of to the speaker. would i just solder all three ends together and the other end of the capacitor to the wire from the speaker? when i get this working i plan on buying a cheap acoustic guitar from rondo music and putting a piezo pick up, this Theremin, and maybe a switch to turn off and on a distortion/fuzz effect that i found that is small and simple, just to have something to show the friends when they come around. thanks for any help and sorry about the long post.


Brian

[mje]

I would strongly recommend Craig Anderton's book, "Electronic Projects for Musicians" as a volume that will teach the basics of wiring, layout, how to prototype, how to handle componants and so forth. It's an excellent introduction.

There's an inconsistency in the way that schematic is drawn- that "T" is indeed a junction, where the wires are all joined. So is the point where the speaker is connectied to the transistor and there's a dot. Where one line curves as it crosses another there's no connection.

Usually you use dot/no dot to indicated connection/no connection, or you use the curve to indicate no connection, and uncurved, crossed lines to indicate a connection. I can figure out what the person who drew the diagram intends, though.

The circuit isn't a Theremin... it's a simple oscillator whose frequency is dependant on how much light falls on the photoresistor. As the light increases, the pitch increases. I'm guessing this is from an old book, as today no one would use a photoresistor and a darlington pair of transistors. They'd use a phototransistor and an op-amp or a 555 timer.

A quick Google found this interesting page:

http://www.oldtemecula.com/theremin/

Originally Posted by mje

I would strongly recommend Craig Anderton's book, "Electronic Projects for Musicians" as a volume that will teach the basics of wiring, layout, how to prototype, how to handle componants and so forth. It's an excellent introduction.

There's an inconsistency in the way that schematic is drawn- that "T" is indeed a junction, where the wires are all joined. So is the point where the speaker is connectied to the transistor and there's a dot. Where one line curves as it crosses another there's no connection.

Usually you use dot/no dot to indicated connection/no connection, or you use the curve to indicate no connection, and uncurved, crossed lines to indicate a connection. I can figure out what the person who drew the diagram intends, though.

The circuit isn't a Theremin... it's a simple oscillator whose frequency is dependant on how much light falls on the photoresistor. As the light increases, the pitch increases. I'm guessing this is from an old book, as today no one would use a photoresistor and a darlington pair of transistors. They'd use a phototransistor and an op-amp or a 555 timer.

A quick Google found this interesting page:

http://www.oldtemecula.com/theremin/

Yes, Anderton's book is pretty much the standard book to start out with and I also highly recommend getting a copy of it. In case you want to be completely cheap, check these out:

http://arts.ucsc.edu/EMS/Music/tech_...ReadSchem.html
http://www.freeinfosociety.com/site.php?postnum=13
http://www.elixant.com/~stompbox/smf....php?board=6.0
http://www.geofex.com/

[theshadow2001]

I might be mistaken but from what I remember don't darlington pairs use two of the same kind of transistor either two NPN or two PNP not one of each. I might be wrong though it's a couple of years since I've seen one.

As for the breadboard. From the ones I've used, if you look at it with the longest side going left to right. There are two rows at the top and two rows at the bottom. Each dot on the rows going left to right (or right to left its all the same) are all connected together. In between these two rows there are columns where each dot on the column is connected from the bottom to top (or top to bottom)

->->->->->->->->->->->->->->
->->->->->->->->->->->->->->

^^^^^^^^^^^^^^^^^^^^
|||||||||||||||||||||||||||||||

^^^^^^^^^^^^^^^^^^^^
|||||||||||||||||||||||||||||||

->->->->->->->->->->->->->->
->->->->->->->->->->->->->->

I don't know if what I illustrated above makes it any clearer but the arrows point in which way the dots on the breadboard are connected. The schematic doesn't give enough information like the voltage of the power supply the capcitance of the capcitor or what sort of photo resistor to use. Assembling anything from this schematic is unlikely to work unless you have some experience at electronics.

If this has explained nothing just say the word

[mje]

Originally Posted by theshadow2001

I might be mistaken but from what I remember don't darlington pairs use two of the same kind of transistor either two NPN or two PNP not one of each. I might be wrong though it's a couple of years since I've seen one.....

You're absolutely right- I just glanced at the circuit, and thought "darlington" even though it's nothing of the sort. It's actually a simple bipolar oscillator circuit of the sort that used to be very common, used for code practice oscillators and such.

[theshadow2001]

What exactly happens in that circuit? The capicitor charges until it reaches the voltage that opens up the first transistor. Which opens the second one turning the second one on powering the speaker. The capictor discharges and the whole thing repeats. The frequency changes with the amount of light recieved by the photo resistor. Is this someway right? I'm not the best on figuring out electronics

[mje]

Originally Posted by theshadow2001

What exactly happens in that circuit? The capicitor charges until it reaches the voltage that opens up the first transistor. Which opens the second one turning the second one on powering the speaker. The capictor discharges and the whole thing repeats. The frequency changes with the amount of light recieved by the photo resistor. Is this someway right? I'm not the best on figuring out electronics

Sounds right to me. The photoresistor changes the RC time constant- i.e., how long it takes the capacitor to charge through the photoresistor and the speaker.

The output waveform should be something like a ramp wave- gradual voltage rise as C1 charges through the photoresistor and the speaker, and then an abrupt falloff as Q2 turns on and C1 discharges through the speaker alone. That sound right?

[bfitz]

Thanks for all the info everyone. I didn't get it to work on the breadboard until recently due to the fact that I couldn't get it in my head how the current was supposed to go. I did finally got fed up with that and just grabbed a piece of perf board (I think that is what it was called, the cardboardish piece with the holes punched in it) and just soldered it together and it worked, but it is very quite. I haven’t made one with an output jack yet to see how it would sound through a small guitar amp, but I think that will be done shortly. my next thing that I have been thinking of is to put together a series of switches with different resistors on them to make a really, really simple type of keyboard. I tried one resistor already, just one I had laying around from something else, to replace the photo resistor and it worked, just one continuous low note. am I thinking right that the lower the resistance, the higher the note?

[mje]

Originally Posted by bfitz

... am I thinking right that the lower the resistance, the higher the note?

Yes. The lower the resistance, the faster the capacitor charges.

[Johnny L]

Originally Posted by bfitz

...like in this schematic the photo resistor, capacitor, and the transistor connect to for what looks like a T, and then the capacitor goes off to the speaker. would i just solder all three ends together and the other end of the capacitor to the wire from the speaker?

Yes, and don't forget the thingamajig the cable plugs into as well as the "E" side of Q2 when you connect the speaker to the capacitor. The cap is polarity sensitive so it matters which side you connect to what (the straight line is + and the curved side is -).

Electrically, the drawing will show you what needs to be connected where. If you're mounting everything on a board and you have room, I'd recommend placing the components and making connections exactly as is shown on the schematic where possible so that you run into as little confusion as possible. Also capacitors, the collector and emitter of transistors are often placed in parallel with the power supply.

Notice also the transistors, how they are daisy chained...how one controls the other's response...affecting the speaker's response as well as whatever you plug into.

Anyway when you get more comfortable, you can hug all the components together on your later projects and make it all look "prettier".

PS oh it looks like you got lots of help already. Man I want to recall darling pairs but I've forgotten so much