LFO's

[iqoruvuc]

I recently built an LFO to control the pitch on a keyboard.  When there is no sound being played you can hear the pulse of the LFO very faintly on the speaker.  It is not very loud but I wondered if there was away to stop this?  I tried adding a resistor in series and that had no effect.  Any ideas?

[gmeredith]

I made an LFO for vibrato on my Casio SK8. I used a 7555 chip, which is a VERY low power drain version of the 555. Don't know what you're using, perhaps the power drain from your chip has something to do with the interference. Mine gives absolutely NO interference whatsoever. Mine is also connected to my circuit using a vactrol style photoresistor, ie. the LFO circuit is not directly connected to the circuit. You can check it out here in Yahoo groups:

http://launch.groups.yahoo.com/group/CasioSK/

Go to the FILES section on the page, to a folder called "LFO circuit"
(You will need to join the group in order to access the files section, though)

Hope that helps,

Cheers, Graham

[iqoruvuc]

Ahh I didn't do that.  I just connected it straight to the pitch resistor.  I have never made a vactrol but have heard about them - an led glued to a Light Dependant Resistor in simple terms.  Of course there would be no current flowing though it to cause interference, I totally forgot about Vactrols.

Does anyone know a good light dependant resistor to use to make a vactrol.  What is a good value to use, anything else I need to know.  With the LED does it matter what colour I use, and how bright should it be?  Is 5000 MCD okay/too little/too much?

Also.....I wonder if it would be possible to achieve the same effect using a transistor, putting the current from the 555 into the base of the transistor to control the pitch?  I know nothing about transistors though, and wouldn't even know where to begin to start?

[andy_wheels]

here's a link to a vactrol diagram:
http://www.carrionsound.com/gallery/schems/vactrol.htm

no need to join up on the sk yahoo group. why oh why do you get asked to fill in info as to the reasons for joining the group!? completely over the top stupidness.... it's a small discussion group about a couple of small synths!

sorry to rant... 

so are lfo's fairly easy to make...? hmmm, might have a go.

[iqoruvuc]

Hi - the type I made makes a kind of squarewave LFO, using a 555 timer IC which charges a capacitor and the rate it is charged/discharged is controlled by a resistor.  The other resistor controls the duration of the signal.  At first it sounds like you could make pulse width modulation but each resistor alters the effect of the other.  So far it seems that to get a fast LFO you end up with wave where the duartion of the signal (that increases the pitch) is very short, and to get a balanced square wave (or approximation of) the rate has to be slow.  A 555 can be configured in different ways so I will see what else I can come up with.  So far I got good results using a 2.2uf capacitor, with a 470K pot to alter rate and a 100K pot to alter the duration.....but in short yeah they are easy to make    

I used a diagram in an electronics book to make a flashing LED as the basis of mine.  I will post more information when I have tried other configurations (if I understand the datasheet!)

[Signal:Noise]

I found this.

http://www.mikmo.dk/cblfo.html

I've just succeeded on making one on my second attempt.

[gmeredith]

Hi iqoruvuc,
Here is the photoresistor I used:

http://www1.jaycar.com.au/productView.asp?ID=RD3485&CATID=33&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=788

its Light resistance is 2.8K ohm min 8.4K ohm max.

The lower minimum resistance, generally the better, depending on what you're doing. Here is a higher minimum resistance one:

http://www1.jaycar.com.au/productView.asp?ID=RD3480&CATID=33&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=788

Try them and see which is best.

As for the LED's  - the brighter the better - 5000 MCD is ok, 10000 is better. White is possibly the better colour, but any should work.



Hi Signal:noise,

The LFO on the mikmo site link you gave is the one I'm using. It's a good, simple design. Works well, hey?

Cheers, Graham

[iqoruvuc]

Hi I have tried the LFO as described above and yes it works really, really well.

Many toy keyboards have a LED that flashes with the tempo so I though that I might try disconnecting it and replacing it with a brighter LED and then using that LED to power a Vactrol for tempo controlled effects.  Although the effect should (theoretically) increase/decrease with the changes in tempo the LED would only flash every beat, and only work when the rhythm or accompaniment were playing. 

Still I will give it a go, though I wonder if anyone here has tried this or even knows how to get the LED to flash 4 times every beat etc?  It must be possible but I have absolutely no idea how to do this?

[iqoruvuc]

Following on from my ramblings above I thought that maybe i could make a delay circuit and then connect what was originally intended to go the LED of the toy to the audio input, and the connect the audio output to the LED of the vactrol.  It might not work, but then if it does it opens up all kind of possibilities....I'll keep you posted, remembering I have yet to disconnect the LED from the toy to see if it will even power a super bright LED...but the idea is there

[iqoruvuc]

I have been bending a Casio PT-100 and a couple of the bends add distortion to either the chords or normal keys.  I built some LFO's with 555's using the DIY vactrol approach, a method I have previously not had any problems with.  However this time when the vactrol is on it is injecting a pop into the audio every time the LED flashes, even when the keys aren't pressed.  I don't know if anyone knows why this might be?

I am using very bright LED's so I don't know if this is to blame, and also I was using power supply rather batteries.  I am going to try again with a lower output led for the vactrol and powered via batteries but if anyone knows what might be causing this it will be much appreciated!