DIP switch and caps...

[mudmen]

Some time ago I've seen here a PCB of various caps with DIP switch to be used with wah pedals. I used 'search' but I couldn't find it. Anyone has it ?

[Peter Snowberg]

I think using multiple caps with DIP switches to vary an effect is one of the most under-rated "tuning" methods available.

I know the Teese Wahs have DIP switches inside, but his schematics are not available here. (Like several other small companies, he has asked it be that way and it is respected.)

You can add a DIP switch to ANY cap location for tuning very easily. You just have to figure out what range of values you want to be able to select. I'll give you a general purpose example here. Please remember that capacitors often have much wider tolerance than resistors so if you have a meter that measures them, try to select ones with values close to the rated (of better, the 'ideal') ones.

To get higher values, all you need to do is to wire the caps in parallel. Let's call the two wires going to the original cap A and B. Attach wire A to one side of all the switches. Attach wire B to one side of all the caps. Now attach the other side of the first switch to the other side of the first cap and repeat that for each switch/cap pair.

Here is an example for an 8 position switch and 8 caps that will give you a range of .001uF to .1uF in 255 steps.

switch position - - - - -	'Ideal' cap value - - - - -	part value
1	0.001	0.001
2	0.0012	0.001
3	0.0025	0.0022
4	0.005	0.0047
5	0.01	0.01
6	0.012	0.01
7	0.025	0.022
8	0.05	0.047

Say you want to get .068uF. If you add switches 7 and 8 together, you get 0.069uF which is just about dead on and almost too close for Rock & Roll. That is a valid real world value you can expect to find in a very good cap marked 0.068 on the outside.

You could also use switches 5, 6, and 8 to get 0.067uF.

If you add switches 1, 5, 6, and 8 together, you get exactly 0.068uF.

If you select your caps to be 0.0025, 0.005, 0.025, & 0.05 values, you will get a little better (more even) spacing between the selections.

By using binary math, you get get 255 different cap values out of the switch above. You can also add the switch/cap in parallel to another cap which will always be on to set the 'lowest' value available. That's handy if you want to adjust the range from 0.2 to 0.3 for instance. (this also gives you a 256th selection). 

An ultimate selection would come from using more switches and highly selected parts. Look at this list...

Switch - - - -	Cap Value
1	100pF
2	125pF
3	250pF
4	500pF
5	.00100uF
6	.00125uF
7	.00250uF
8	.005uF
9	.01uF
10	.0125uF
11	.025uF
12	.05uF
13	.1uF
14	.125uF
15	.25uF
16	.5uF

That list along with two 8 position dip switches will give you 65535 values between 100pF and .99uF.

Happy tuning.

[mudmen]

OMG, What an answer ! Thanks a lot Peter !!

[Connoisseur of Distortion]

ooooooooooooo

screw those rotary switches! why didn't I think of this!? 

nice little follow up, Peter!

[Peter Snowberg]

You're welcome.

I'll mention some other approaches....

One alternative is to use the same cap value in all positions. Say you use .01uF and you wire a 9th cap to be always on. That gives you 9 values between .01 and .09 depending on the number of switches turned on. You can also get 9 to 12 position switches for more range. It relieves anyone of having to think about switch combinations, but I would rather do the math and get 28 times the resolution.

Another option is to leave base 10 and use caps with the values like .001, .002, .004, .008, .016, .032, .064, & .128. Now your resolution is a constant .001uF between steps and you have a range of .001 to .256uF which is pretty nice. The first approach I posted above is far from perfect, but the values of the caps available dictate what you can do and you can't get caps in nice "powers of 2" values.

There are miniature hexadecimal rotary switches that you can panel mount to provide these controls to the player. A 4 bit rotary hex encoder will get you 16 values using 4 caps. Since you can't get exact values like .004uF or .008uF, you can use 15 caps, all of .001uF to get the exact values by paralleling a set of 8, a set of 4, and a pair. Add a .015uF which is always on and you get 16 values from .015 to .031 which is pretty nice for fine tuning stage coupling in many circuits. If you use .002uF caps and a .01uF always on cap, you'll get a range of .01uF to .04uF in 16 perfectly even steps. If you use a bunch of caps of the same value, you don't need to measure anything because manufacturing is VERY good for relative consistency with most film caps even if the actual values are not perfect.   

There are 10 kinds of people in the world, those who understand binary, and those who don't.