Controling LFO speed with a switch to ramp up/down Leslie style

[oliphaunt]

I would like to create a circuit that will replace the speed control of an LFO (I have no specific LFO in mind just yet).  I would like to be bale to control it like a Leslie speaker with a footswitch that ramps between fast and low speeds.  I would like to have control over the two speed settings and also the ramp time between them.

I have searched for this, and found this article: http://www.geofex.com/article_folders/lera/lera.htm

It uses a cap that is charged and then discharged to change the brightness of an LED and change the value of an LDR that replaces the LFO speed pot.  It makes sense to me but does not seem to offer control over the ramp time except perhaps if you switched out the 100u cap that is charging/discharging with a different value.  

Could this 100u cap be combined with another cap of a smaller value and blended to change the time?

Does anyone have suggestions for another ciruit?

Also, I would love for the speed control footswitch to be a momentary style so it easier to push, as the way I use a Leslie I may hit the switch dozens of times during a song.

[slacker]

Have a look at the these articles on http://www.geofex.com/ 

10/24/06  New - a Leslie style ramp/up down LFO adapted for the Phase 90
10/23/06    New - a Leslie style ramp/up down LFO, adapted for the Easyvibe.

They do pretty much exactly what you want, except for the momentary switch part.

[R.G.]

I would like to create a circuit that will replace the speed control of an LFO (I have no specific LFO in mind just yet).  I would like to be bale to control it like a Leslie speaker with a footswitch that ramps between fast and low speeds.  I would like to have control over the two speed settings and also the ramp time between them.

I have searched for this, and found this article: http://www.geofex.com/article_folders/lera/lera.htm

It uses a cap that is charged and then discharged to change the brightness of an LED and change the value of an LDR that replaces the LFO speed pot.  It makes sense to me but does not seem to offer control over the ramp time except perhaps if you switched out the 100u cap that is charging/discharging with a different value.  

Also, I would love for the speed control footswitch to be a momentary style so it easier to push, as the way I use a Leslie I may hit the switch dozens of times during a song.

You're either over- or under-thinking this.

Capacitors charge up at a speed determined by the voltage they're charged to and the resistor in series with that voltage. In LERA, there is a variable down speed resistor (4.7K and 20K pot) and a fixed up speed resistor of 22K. This is because an actual Leslie rotor always speeds up at the same rate. But it's easy enough to convert this to either (a) same speed up/down rate for both up and down or (b) independently adjustable up and down rates. The secret is that 20K/4.7K thingie. If you put this in series with the cap on the pole side of the switch and switch between 5V and ground, you get (a), same ramp speed up and down, because it's controlled by the same resistors both ways. Similarly, if you duplicate the 20K/4.7K adjustable setup in both throw sides, to both 5V and ground, you get independently adjustable speeds, case (b).

But the noted articles slacker mentioned are even better if you have no other circuit in mind yet.

[oliphaunt]

Thanks to you both.

That explanation makes perfect sense R.G., thanks for pointing it out.