# Is it possible to get an exponential capacitance curve from coded dip switch

I have a capacitor that controls frequency for a specialty IC with the relationship

$F \propto 1/C$

I want to be able to get a number of frequencies in evenly spaced intervals. Currently I have a string of series capacitors some of which I short out with regular dip switches in a binary code to give 16 steps. simulate this circuit – Schematic created using CircuitLab

This works as adding capacitors in series follows the relationship below giving linear steps.

$F \propto \frac{1}{C} = \frac{1}{C} + \frac{1}{2C} + \frac{1}{4C} +\frac{1}{8C}$

To make it more intuitive to set the frequency for people who are not used to binary code I would like to use a coded rotary dip switch such as this

This means the switches are not all separate and have a common pin. simulate this circuit

Because of this the relationship is exponential and it is not possible to get even steps based on this circuit

$F \propto \frac{1}{C} = \frac{1}{C + 2C + 3C+ 4C}$

TLDR

Is there a simple way to get evenly spaced frequency steps using a coded rotary dip switch, without needing too many more components.

• Ummm - use different value capacitors? Dec 3, 2015 at 2:34
• Different values will still have the same problem of changing step size Dec 3, 2015 at 2:59
• Does your IC also have a resistor that controls current ? Dec 3, 2015 at 3:56
• I have two pins that it expects me to apply capacitance across Dec 3, 2015 at 4:05
• Four relays? Connect each output of the rotary switch to the coil of an SPST relay. Dec 3, 2015 at 4:54

Edited:

The 4 relay idea from Tom C is one option.

Other then that, if using a group of relays is not practical, then an old fashion 16 position rotary switch could do the trick too.

You could create a similar capacitor string as in your first diagram using 16 equal value capacitors, (much easier then finding several specific values) then use the 16 position switch to connect to each of the 16 intersections within the string. That would give you the "division" format you seem to need.

Low cost 12 position switches can be had from surplus dealers.

• Yes they add together in a linear way giving linear capacitance steps, this gives non linear frequency steps Dec 4, 2015 at 6:44
• Where is the non-linear part? Your equation says "proportional to" doesn't it?
– Nedd
Dec 4, 2015 at 6:47
• Not really, it is proportionate to the inverse. Dec 4, 2015 at 6:49
• Made very clear tinyurl.com/n9og6tq Dec 4, 2015 at 6:56
• It could also be claimed that your Frequency intervals (per the list) are not "evenly" spaced, they are divisions (ratios), each step is a divide by n.
– Nedd
Dec 4, 2015 at 8:51