Dec 23rd 2005
Adjustable capacitors are usually limited to the 1-50pF range, are very sensitive to changes in temperature, and when you use a screwdriver to adjust them the screwdriver's pressure screws everything up. Variable inductors with moving ferrite slugs aren't much better. I've hated every adjustable passive component I've ever used. Digital potentiometers are nice and useful, but they aren't cheap and there are only so many places that you can use them. 

Comatose and Afroman present: Digital capacitors
(and sometimes digital inductors too)

Overview
By manipulating the input/output states on a microcontroller, you can effectively switch capacitors in and out of circuit, as long as the circuit requires one end of the capacitor to be connected to ground when it is in use. When you set a PIC's tristate buffer bit to be 1, it makes the pin an input and makes it high impedance as if it is an open circuit. When you set a PIC's tristate buffer bit to be 0 and set the data bit to 0, it makes the pin an effective ground. With this system you can adjust capacitance to whatever you want, with range being dictated by your choice of capacitors and the resolution being dictated by the number of microcontroller pins you have available. This system is suitable for a large number of low power circuits. Here are some crappy pictures explaining it:







Using CCS PCWH, setting the tristate buffer's value is as easy as set_tris_c(capacitancevalue);

Applications
Radio transmission
An extremely cheap and simple FSK data transmitter can be made using a PIC12F675 and a 555 timer. If you were to output the carrier frequency directly from the PIC, you would have to use a lot of resources (timers etc) to generate the carrier, and even then you would be limited to a frequency of a few dozen kHz since a PIC doesn't have much processing power. If you use a digital capacitor system to modulate the frequency of a wave generated by a 555, you could have an FSK transmitter operating in the legal ~1Mhz AM band easily. The code would require minimal resources since all you are doing is changing the state of 1 microcontroller pin. This leaves you with plenty of processing power free to do other important things like preparing the packet data and calculating CRCs.

In this example circuit the 555 is set to oscillate at (theoretically) 1069kHz using R1, R2 and C3. The capacitor C4 connected to the PIC's pin is just floating around doing nothing for now. When the PIC's pin changes from high Z input to a digital 0V output, the 33pF capacitor becomes part of the 555 oscillator, and the frequency changes to 952kHz. To change the frequency back, change the pin's tristate value to be an input.



Use this page to help you calculate the components you need, and remember that all 555 chips are different and many are so shitty that they can't reach above 100kHz. Also most 555 guides fail to mention how important caps C1 and C2 are for stable operation, so always use them.

Radio reception
The typical front end of any radio receiver is an LC circuit approximately tuned to the band you are trying to listen into. To tune the radio to a different band, most hobbyists use mechanical rotary switches that physically switch components in/out of the circuit, which creates the problem of finding a switch with multiple positions, and makes frequency scanning a pain in the ass.

For example here is the front of a classic crystal AM radio receiver, tuned to 530Khz. If you want to tune it to 1.5Mhz, you either need to mess around with adjustable passive components or build a heterodyne receiver (not easy for beginners). If you wanted to tune into 40Mhz, you would have to physically switch out the components entirely. Doing this with a PIC can be done in microseconds, and requires only basic knowledge of radio receiver and microcontroller design. 



Here is an 8 bit PIC controlled AM radio receiver front end circuit using a digital capacitor system



I haven't actually made this, but in theory you should be able to digitally control entire LC filters, and not just capacitors. 


Using a digitally controlled capacitor and/or LC filter selector, you could design a radio scanner with a very wide input range, and never have to mess around with heterodyne receiver design. The front end LC filters can have very high Q and good rejection of unwanted frequencies. Since the whole thing is controlled by a microcontroller, you could make a wide range radio scanner controlled by your PC's serial port very easily.


Digital equalizers and effects sans DSP
To change the bass/midrange/treble in audio circuits, you either have to do it with potentiometers in a purely analog solution, or use a high powered DSP to do all sorts of crazy shit to the audio data before it even hits the DAC. With a digital capacitor there is some middle ground, and you can have better control over how your music sounds without knowing anything about DSP. What you would do is create a digital notch filter to attenuate the frequencies of your choice, then just amplify everything. In most cases a potentiometer will work great, but if you want to control things electrically instead of by hand, then a digital pot or digital cap might be helpful. The audio will sound pretty weird, but that may be exactly what you want.


Music synthesis & VFOs
Similar to the FSK idea, you can use a PIC plus a 555 to make a ghetto synth (variable frequency oscillator). A PIC16F627A with 8 capacitors on port B would give you an 8 bit synth: 256 possible oscillation frequencies. You could make a stereo synth with a PIC16F72, 16 capacitors, and two 555s - use Port B for the left channel and Port C for the right channel. Here is a simple 4 bit mono example using a PIC16F627A. Here is the video


The source code is as simple as it gets:
void main()
{
   int i;
   output_b(0);
   while(TRUE)
   {
         for(i=0;i<255;i++)
         {
            set_tris_b(i);
            delay_ms(50);
         }
   }
}


I have no idea if this stuff is patented or not or if it is being used in any commercial designs, but I haven't found any webpage mentioning it despite a lot of searching. In any case I am writing this article because it is a seriously useful trick for hobbyists to use in their projects and more people should try it.

Meg