# making a 555 'organ' monophonic

Discussion in 'The Projects Forum' started by ivan., Nov 13, 2008.

1. ### ivan. Thread Starter New Member

Nov 13, 2008
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basically, im trying to create a circuit that uses a 555 oscillating circuit that will be able to give a range of frequencies - by using different resistor values in the potential divider to change the input at pin 7, each triggered by a switch of some sort.. but im stuck at how to only allow one resistors value at a time. btw, this is for an audio type project.
i presume their will be a way of using logic for this, but we havent really been taught enough for me to work it out
any advice would be appreciated, even if its 'your doing everything wrong'!.

ivan.

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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The "real" way to do this is to put all the resistors in a string, and short out more or less of them. You can do this with a mosfet between each resistor junction and ground.

eric

Apr 5, 2008
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Last edited: Apr 11, 2009
4. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,178
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Ooops...I meant short them to Vcc...not ground. !

eric

5. ### SgtWookie Expert

Jul 17, 2007
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Well, for a basic "toy" monophonic organ, you could use something like this:
http://www.design-technology.info/KS3/Y8/page9.htm

If you used pushbuttons, limiting it to a single resistor at a time is going to be a trick.

You might have to resort to using something like a series of CMOS 4066 analog switches, coupled with a decoder/multiplexer.

[eta]
I like the schematic Bertus posted better than the one I found; the 1st schematic avoids the problem of more than one button being pressed at a time. However, you'd get better results if you used potentiometers wired as rheostats for the multiple 1k resistors between the switches.
It will be quite a chore tuning the thing.

Last edited: Nov 13, 2008
6. ### ivan. Thread Starter New Member

Nov 13, 2008
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0
cheers people, the Experimentalists post was alot of help.
il try out some stuff tmorra, and i shall look up the CMOS stuff, see what is achievable with high school resources!
ivan.

7. ### mik3 Senior Member

Feb 4, 2008
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You can take resistor with values of about 1K, 2K, 4K, 8K, 16K and so on (or whatever values you want). Connect the one end of each one to pin 7 of the 555 and leave the other end of each one unconnected. Then use a switch for each one of the resistors to connect/disconnect them from pin 6 of the 555. By closing and opening the switches you can make different combinations and change the total resistance. With only 6 resistor you can make 63 tones.