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I have a question about wiring up a trimmer in this circuit (NE555 - piano module) I have no background in electronics so please bear with me. One of the keys in the 'piano' sounds just off tune, so I would like to change the sound of that button by replacing a 2k resistor with a smaller resistor and a trimmer.

I have a 3296 trimmer of 1k

The only problem is that this thing has three pins, and I don't know exactly how to wire it properly. If I understand it correctly- I can wire it up like this?

Or is there no need to wire up pin three?

trimmer wiring

iridian
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  • Generally, when using a potentiometer as a rheostat, the best thing to do is to connect the wiper to one of the outer two leads. This minimizes contact noise and can prevent sparking while turning it. If this won't be a problem for you, you can also leave the third pin unconnected. – Hearth Mar 12 '19 at 14:06
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    Don't tie it to ground, leave floating. Lookup potentiometer diagrams to understand what's inside. – Eugene Sh. Mar 12 '19 at 14:06
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    If you change the value of R4 you will change the frequency for both switches S1 and S2, so you may need to do the same thing for R3 to bring S1 back into tune. – Elliot Alderson Mar 12 '19 at 14:16
  • Good thing I bought a ten-pack, thanks for the heads up Elliot! – iridian Mar 12 '19 at 15:07

2 Answers2

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A trimmer or potentiometer is like two resistors. If there are two different voltages on 1 and 3 any voltage between can be reached by twisting the knob. This is why there are three terminals.

However, you only want one resistor, so just use pins 1 and 2 (or 2 and 3) and you neededn't wire pin 3, that would be like connecting a resistor to ground.

enter image description here
Source: https://www.electronics-tutorials.ws/resistor/potentiometer.html

Voltage Spike
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  • I've learnt that there are many good reasons for why you should connect the "unused" pin to the wiper when using a potentiometer as a rheostat. I don't remember all the reasons but for example it maintains a resistance at all times if the wiper is "scratchy". – pipe Mar 12 '19 at 16:08
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Quite a lot of the notes are a little off with the chain you have there.

The equation for the frequency of a 555 astable is -

enter image description here

This spreadsheet calculates the frequencies of the semitones in an octave, and then I picked out the notes from a major scale, but adjusted the top note to give the frequency that the 555 will produce with the values given. Adjusting R5 will tune the entire set if you're picky about that.

The table shows the needed value for R1 (between the DISCH and TRIG pins, which is R5 plus the chain on your schematic) to generate the frequency for each semitone. Then for the scale notes, there's the incremental values for the chain given on the right. None are close to the chain values you have, so it might be worth using several of those trimmers if you want to get closer.

Since the voltage on the chain rises and falls between the set points in the 555 that turn the output high and low, don't connect the spare pin to ground, but to the wiper contact (the middle one)

Phil G
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  • Actually three of the notes sound off by ear (and those are S6, S7, S8) I just took S2 because I had room to adjust the picture there. But you are right that they don't sound as good as they could be. I think you made a lot of people happy with that chart you made! Thanks a lot, I think it will sound a whole lot better now. I also learned a few more things about resistors and trimmers from the answers. – iridian Mar 13 '19 at 10:01