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very new, need help on resistors


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hello. i am still very new to any kind of electronic projects. i am to the point where i can read simple schematics and put them together and have them work on the first couple of tries but i am trying to do something here kind of of the cuff. i found this schematic for a photo theremin (which i was later told is not any kind of theremin but some kind of transistor, but any way). i have built one now and got it to work and know i want to take the photo resistor out and put other things in to controle the pitch. what i am planning on know is a series of swithches connected to resistors to controle the pitch, puch one and its this sound, push another and its this pitch, basicly trying to make one of those cheap keyboards i used to get when i was a kid that had candy in one compartment. i originaly thought, for some reason, that i could take two resistors and hook them together and it would combine there resistance, then i put it on a breadboard and it dawned on me that it is only going to be the resistence of the lowest resistor (i think).

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thanks for the reply, mike. i have to say that i am very, very new at all of this and i think i get what you are saying, but lets see. i think the schematic you put up is in series. when you flip one switch it uses the value of just the one resistor but if you flip two or more, then you would use the equation to figure out the value? i'm sorry because i know this is all probably all very easy but i just can't seem to get my head around it.

thanks again
brian

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The circuit is in series.
If you flip one switch you will get the resistance of all the resistors between points A and B.  Just imagine the lines (Wires) as pipes.  With only one valve closed at at time the 'water' must flow from A to B through a series of resistors.

The photo-resistor you had in your original circuit can be replaced with the circuit I provided.  I am not sure what the range of the device you were using is so I will guess that it is a 10,000 ohm resistor when dark and a 1,000 ohm resistor in bright light.

To approximate the same range you could use ten 1,000 ohm (Brown - Black - Red) resistors.  By closing only 1 switch the circuit will see 1,000 ohms (Or 1k).
As you close successive switches the resistance can be changed all the way up to 10k.

Unfortunately this design only allows 1 resistor to be connected at a time. Pressing two switches will effectively SHORT out a resistor thus removing it from the stack. 

In musical terms your instrument is monophonic, or single-voiced.  Flutes, Tubas, and the human voice are all monophonic.

If you want to close two or more switches and get polyphonic (Multi-voiced) sound such as in guitars, Harps, and pianos, you will need more oscillators.

You could try etching a series of circuit boards that all have a micro switch on them.
Each one would be equipped with a circuit similar to yours and a variable resistor.
By tuning each circuit to a musical tone you could mount all the boards together and create a keyboard.  Many of the early electronic instruments used this method.

I have included a simple drawing of 3 resistors in series and parallel.
The same values are used in both drawings.

-Mike



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