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Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Is there an oscillator circuit that will change output frequency with a change in a capacitor? The capacitance will vary on the order of tens of pF. I would rather it be a resistor capacitor type instead of a inductor capacitor type. Thanks!
EDIT: Looks like if nothing else I might try a Hartley Oscillator. It uses an inductor but that might have to work.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
What is the purpose of the oscillator? What frequency do want to achieve? Sine or square or what? Need more info here, but generally speaking if you replace the cap in any osacillator circuit with a variable cap, you achieve your goal.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Sorry for leaving out info. I'm trying to set up a capacitive way to sense the position of a projectile in a coilgun. Basically, the projectile will be the moving plate in a variable capacitor. I had a set up where the amplitude of a wave changed in relation to the projectile's position. I'm opting away from this because the time required to read the voltage of the signal on my uC. It would be faster to count the frequency of the signal.
I'm looking for any kind of wave really, I can figure out how to condition it. Frequency should anywhere between 500kHz and 100kHz with a swing of at least 50kHz.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Look up a relaxation oscillator, , though the simplest and easiest to implement is an RC round an inverting schmidt trigger like an HC14 device.
The amount of swing you'll get between projectile absent and present is all down to the ratio of C you can acheive between those two states. Generally this means striving for very low C to start with, so that you don't need too much extra capacitance from the projectile. This means putting the active device right on top of the barrel rather than conecting it by a foot of cable.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
There are many ways (RC, Relaxation, Hartley etc.) The problem to me is the signal that will be picked up from the coil which would overwhelm a small-signal oscillator. If the sensor is just to set the start position then maybe a simple mechanical arrangement would do?
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
I had wondered about the signal from the coils firing. That is why I wanted to stay away from an oscillator with an inductor. Will the em pulse damage the oscillator if the board is mounted physically on the coil? Would tvs diodes and shielding be enough to protect the circuit? Would these measures alter the performance of the oscillator?
The reason I wanted to try a capacitive sensor instead of optical was mainly to avoid punching holes in the barrel. It will also provide more than a simple break beam system that will only have two states; blocked or unblocked.
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
Servo positioning systems with capacitive feedback normally drive the capacitor with the output of an oscillator ranging from a few hundred kHz to several MHz. the output is rectified by some diodes and you get a DC voltage proportional to position. I've seen those little canned TTL oscillators used with success.
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Capacitance controled oscillator
