what size pot?

[fredy]

hello, i am building a mini leslie with 1 speaker. I will a motor which will spin a cardboard drum around a 3" speaker. This motor will be powered off a 9v battery but i will u se a 12v motor just in case. Anyhoo, i need t be able to change the speed of the motor. I was thinking about using a linear pot but what value should i get? I was thinking about getting the biggest value possible for most control, but what do you think the minimum value should be to get a decent controlable effect?

[fredy]

acctually, i think i will use a 1.5 - 6v motor which has 8000 rpm at 3v, i think that would be plenty. Not sure how it will perform with half a bisto box on it though.

[fredy]

if i use the 6v motor with a 9v battery i am going to need to lose 3v. I measure the current of a 9v battery to be 3mA so i would need to put a 100 ohm resistor before the motor on the negative side, yes?

[Thomeeque]

First build mechanical part of the leslie, than find voltages for lowest and highest rotation speed, measure current draw for these voltages, than decide how you will feed it. But IMO you can calmly forgot about 9V/3mA battery right now, it will need much stronger source

Btw. motor should be strong enough too of course, you will burn it if it's weak.

Thmq

[Fender56]

I think that motor speed controllers are usually created using PWM supply instead of varying the voltage with a voltage divider (via the pot).

If you are using a 6V motor, then pulse-modulate a 6 volt power supply. The duty-cycle will vary the speed of the motor (100% duty-cyle = full speed, 50% = half speed etc). I think you can use a LM555 to create a variable PWM (controlled via a pot) which will control a switching transistor (like a 2N2222, which can go up to 1 amp I believe, enough for your motor).

It is basically the same principle as a light dimmer, or a cheap chopper tremolo (but you are switching the motor supply instead of the guitar signal). Do a search on the net, I'm sure there are plenty of design for that application.

(And to reduce the power from a 9V cell down to 6V, I'd use a Zener diode, or a regulator like the LM7806).

[Fender56]

Here is a link that will help you:

http://www.dprg.org/tutorials/2005-11a/index.html

[fredy]

a zenor, thats a great idea! why didn't i think of that? thanks guys!

[Mick Bailey]

If you want to still use a pot you need to be looking at wirewound types. The value depends on your motor current draw, but the value I have used for similar applications is 30 ohm, 7w, which gives a good range of control.

You can also switch series diodes to give voltage drop intervals of about 0.6v, or use these in conjunction with a wirewound pot. The articles on PC motor fan control at http://www.cpemma.co.uk/index.html may give you some ideas. Look at the links at the top of the page. I've used many of these ideas and they work fine.

I've also used an LM317 3 terminal variable voltage regulator on small motors. Simple to construct; voltage reg, pot and a resistor. use a heatsink (metal case is fine).

Bear in mind that if you go the PWM way, these controllers can generate plenty of switching noise, as can the brushes on your motor (if it is the brush type) You may need to include some basic noise suppression across the motor terminals.

Also to consider is reducing the revs of your motor mechanically in the first place using a drive belt and couple of pulleys (salvage from video or tape deck). 8000 rpm is too high to begin with. This method will increase torque and reduce current draw. Think about the revs in conjunction with the desired effect. My Leslie cab got down to very low RPM (certainly single numbers).

[fredy]

i can only get my hands on carbon track pots and as for testing the current draw of the motor... well, i think i melted it with the soldering iron
Here is the choice i have for motors:
http://www.rshelectronics.co.uk/motors.htm
or
http://www.bowood-electronics.co.uk/motors-c-18.html

By the looks of things, this build is clearly not as simple as i first thought and there is alot to learn. When things should i be looking at/ thinking about etc.?

[Mick Bailey]

The first thing to consider is how fast you want the drum to rotate. The effect needs to be slow enough to be audible. Think of the range of a chorus pedal and how many times a second the effect cycles to give a nice effect. Too fast and you get into weirdness, too slow and you won't notice the change in modulation as it will be too gradual. So you probably need to be looking at something that rotates at between 50 and 400rpm, pretty close to a Leslie original.

I'm not sure that a rotating baffle will give you what you're looking for, as the Leslie effect is made up of two rotating elements - the upper rotating horns, and the lower rotating sub-woofer baffle. For guitar, the main perceived effect comes from the rotating horns which imparts a Doppler effect on the guitar waveform. If you replicate only the rotating baffle I don't think you will get the desired effect, though you will get tonal changes with the rotation.

I would suggest that at first you try a prototype that is rotated by hand before investing in components. It should be straightforward to get a couple of pulleys, an elastic band and a crank handle rigged up. If you're in the UK, see if Maplins still do their pulley kits. Meccano would actually be perfect for this setup.  For hand cranking you will need to gear up - say 5:1 to get some speed. You should then be able to crank the apparatus and either strum some open strings on your guitar, or play an audio source through it. This way you'll get a picture of what it will sound like before spending too much time and money on electronics. Or you could just spin the drum by hand.

If this looks promising, you can then progress with selecting the right motor and speed control.