# Electronicaly switching parallel LC circuit components

Let's say I have mentioned circuit and I want to be able enable/disable one of the 1uF capacitors to change the frequency of the LC circuit.

I would like to do this electronically with a micro-controller.

• I know a relay is ideal for this but is bulky and mechanical.
• Another option could be a an "analog switch ic" but they have a relative high internal resistance, and a low max current. The low max current is a problem for a parallel LC circuit because the current can go quite high.

Are there other solutions to do this?

EDIT

• bigger picture: This is just a picture from the interwebs, but what I would like to make is very close to this, the inductor will be a large solenoid for vehicle detection. If the inductance changes (because of the presence of a car) the resonant frequency will change. The comporator will convert this attenuated oscillation in a block wave, the microcontroller can than determine the frequency of the block wave and so detect any changes (traffic).
• Hi @dim I added a bigger picture at the bottom of my post, if you know a way how I could measure the inductance-"change" in the digital domain I'm interested. – Bruce Nov 27 '18 at 9:18
• What "Q" do you wish? Large MOSFET drivers may have 1 to 3 ohms resistance. 1uH and 1uF resonate at 160KHz, with Z of 1 ohm. If a 1ohm driver, your Q would be one. 100uH and 0.01uF also resonate at 160KHz, and 1 ohm provides Q=100. – analogsystemsrf Nov 27 '18 at 9:20
• @analogsystemsrf As far as I understand you can't use a mosfet to connect/disconnect another capacitor to GND. I might be wrong. – Bruce Nov 27 '18 at 9:37
• @dim right I'm trying to measure (a change in) inductance and would like to be able to do this at different frequencies. Any other suggestion on how to approach this with a microcontroller? Measuring the L/R time constant might be an option but with 20µH-2000µH this will be a very short time. – Bruce Nov 27 '18 at 9:40
• At about the frequencies you are using, you might find that the oscillation frequency doesn't change that much (rather the losses extracted from the magnetic field will possibly dominate and these might be a better "signal"). – Andy aka Nov 27 '18 at 12:10