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> My Next Project: The Organ, need...more...beepy sounds...
AdamO
Posted: January 11, 2008 07:04 am
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The next project I would like to attempt is the organ. This is its schematic: http://www.aaroncake.net/circuits/torgan.gif

It looks like a nice and simple circuit that does something too much fun for me to pass up. I am only missing the UJT 2N4891. I did a search for substitutes for that UJT and found like one or two but I had neither of those either.

I have PCBs from a CD changer and some TV sets (one of which is pre-remote control) and these are where I get all my components. Just pull one out when I need it. Nowhere on these things did I find this UJT or any of its equivalents. Does anyone know what kind of electronic device uses this UJT, that I may dumpster-dive for it and retrieve the coveted UJT? Or else, does anyone know where I can get one of these? (Radioshack is not an acceptable answer. I'm a cheap bastard).


Thanks,
Adam O.
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Sch3mat1c
Posted: January 11, 2008 11:47 am
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Straight out of Getting Started In Electronics, IIRC.

Try a PUT (Programmable UJT). You can make one with two transistors (it's like an SCR). You'll probably need to change the circuit a little to accommodate.

Tim


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MacFromOK
Posted: January 11, 2008 04:37 pm
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AdamO
Posted: January 11, 2008 10:06 pm
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Ah, nice one, Mac. I imagine BC108 will be much easier to come by? Or its equivalents for that matter. Great site in general. Thanks!
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MacFromOK
Posted: January 11, 2008 10:17 pm
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Any small signal NPN transistor should work, such as a 2N2222, 2N3904, etc. wink.gif

Mac


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BlueVanACD2005
Posted: January 11, 2008 10:20 pm
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I remember making one of these once.... It was pretty neat for what it was. Could make some neat little sounds with it.
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tekwiz
Posted: January 12, 2008 03:24 am
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QUOTE (MacFromOK @ January 11, 2008 01:17 pm)
Any small signal NPN transistor should work, such as a 2N2222, 2N3904, etc. wink.gif

Mac

So will most any salvaged small NPN transistors. The transistor requirements for that organ circuit are suprizingly non-critical. cool.gif


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AdamO
Posted: January 12, 2008 09:10 pm
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Ok, I built my organ. I used the schematic that Mac gave since it did not require a UJT. I didn't have enough room on my breadboard so I just soldered a bunch of little push-button switches to their corresponding resistors and then soldered them all together in parallel onto two copper rods which act like buses. Now I have two more questions, unrelated to each other.

ONE: most of the PCBs from which I harvest components indicate E, B, and C for their transistors so I just use my own little color code with droplets of acrylic paint to mark the transistors once I pull them out. However, I still don't know how to tell if the transistor is NPN or PNP. How can I determine this?

TWO: the resistors I used to create the various tones are: 10k, 15k, 22k, 33k, 47k, 68k, 100k, and 150k. These were values suggested by "Getting Started in Electronics." However, they result in a rather awkward sounding octave. Forrest M. Mims III knows his electronics well, but not his music theory. Here is what I want to do: I want to look up the audio frequencies for an octave of musical notes going from Middle C up an octave in the key of C. Then I want to use those audio frequency figures to calibrate my little "keyboard" by choosing the appropriate resistors. I'd like to use fixed resistors as it is less wasteful than using rheostats. My question is this: how do I calculate the resistance I need for each switch that will produce the audio frequency that I need?
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MacFromOK
Posted: January 12, 2008 09:50 pm
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user posted image

From this page:
http://www.design-technology.info/KS3/Y8/page9.htm

You can use a pot for tuning each note, then measure the resistance at that setting and use an appropriate resistor.

However... resistors are made within certain tolerances (normally 5-10%, but higher precision should be available somewhere?), so you may have a hard time getting it precise without trimpots on each note (or at least some notes).

Mac


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AdamO
Posted: January 12, 2008 11:03 pm
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Awesome! Thanks a lot Mac!
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MacFromOK
Posted: January 12, 2008 11:42 pm
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No problemo, glad to assist. thumbsup.gif

Mac


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Village Idiot
Posted: January 13, 2008 02:41 am
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Adding to the information Mac provided, you can calculate the frequency of other notes on the scale:
To get the next higher semitone, multiply the frequency by the 12th root of 2, or 1.059463094.
To get the next lower semitone, divide by 1.059463094.

So, A# = 440 * 1.059463094 = 466.164 Hz

Full tones are different by the 6th root of 2 or 1.22462048

Remember that the difference between A and B is a full tone, and between B and C is a semitone, etc.

Edit:
Quite a few years ago, I saw plans for a neat little toy organ that had a piano style keyboard layout etched onto a PCB. You played it using a test probe from a voltmeter, touching the note you wanted to play. Otherwise the circuit was pretty much the same, a single UJT relaxation oscillator.
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