B Why does my LC circuit not oscillate its energy between Electric & Magnetic fields?

[DaveE]

Every time I graze and brush the wire very closely to the positive terminal of the battery, the antenna picks up an interference louder than its normal signal.

This sounds more like a spark gap transmitter than a resonant circuit.

 

[James2018]

This sounds more like a spark gap transmitter than a resonant circuit.

True, remember the radio interference caused by the fluorescent tube starters? This is why they added a nanoFarad range capacitor inside the starter, a "radio interference suppression" capacitor.

Also when I move the wire away from the battery using my hand, there is a very short tap sound heard in the radio antenna. Even shorter than when I brush the wire against the battery terminal.

My guess is that a transient LC oscillation and a spark sound the same way, containing a wide range of frequencies because of the time-frequency uncertainty principle which links short durations of a wavepacket to a wide range of frequencies in the spectrum. The only way I would know if it's due to LC oscillation is if I sustain the oscillations with something that provides gain.

 

[James2018]

If the resonant frequency is ~100 MHz, it means the field direction reverses every gew nanoseconds. The compass needle won't move, and even if it did, your eye can't detect it.

Same with 500 KHz. Inertia prevents the needle from moving that fast. But my guess is that the charge stored in a 1 microfarad capacitor is not enough to discharge into the inductor and recharge a magnetic field strong enough to deflect the compass by 90 degrees.

 

[James2018]

This is what the simulation shows, a decaying oscillation when I disconnect the switch

 

[James2018]

Now, I connected the collector of a transistor in series with an inductor to the positive terminal of the battery. I connected in series the base of the transistor with a resistor and with the 1000 MHz quartz oscillator to the negative terminal of the battery. I connected the emitter of a transistor to the negative terminal of the battery also. And I connected a capacitor in parallel to the quartz oscillator. Can this circuit sustain LC oscillations?

 

[Baluncore]

Can this circuit sustain LC oscillations?

Probably not.
Without a circuit diagram, it is impossible to know.
Where did you find the 1000 MHz quartz oscillator? What part number?

You wanted to watch the compass change direction.
There appears to be no compass in the blurry picture.

 

[James2018]

Probably not.
Without a circuit diagram, it is impossible to know.
Where did you find the 1000 MHz quartz oscillator? What part number?

You wanted to watch the compass change direction.
There appears to be no compass in the blurry picture.

The radio antenna in AM makes a beep when I place it over the circuit. A 793 Hz beep can be heard on top of the static noise. And the beep disappears when I move the antenna away.

Sorry my mistake, it is a 4.194304 MHz quartz oscillator, so it resonates at 4 MHz.

No, I did not include the compass because I made the coil small enough to have the right inductance to resonate to 4 MHz

 

[James2018]

The idea is that the inductor is connected to the collector terminal of the transistor then to the positive terminal of the battery.

You connect a resistor to the base of the transistor, then the quartz crystal then the capacitor then connect to the negative terminal of the battery.

The emitter terminal of the transistor is directly connected to the negative terminal of the battery.

So you have two wires to the negative terminal of the battery (from emitter and base) and one wire (from the inductor which is connected to the collector) to the positive terminal of the battery.

 

[Baluncore]

The idea is that the capacitor is connected in series to the base terminal of the PNP transistor while the inductor is connected to the collector terminal of the transistor. And first you connect a resistor to the base of the transistor, then the quartz crystal then the capacitor. the emitter is directly connected to the negative terminal.

The idea is that you provide a circuit diagram.

 

[James2018]

The idea is that you provide a circuit diagram.

 

[James2018]

So the usual radio AM noise spectrum that sounds like white noise to my ears:

And its modification due to the presence of my circuit:

This is the second time I do the measurement and I find the same 793 Hz sound spike in the radio speaker. It sounds like a beep in my ears.

 

[Baluncore]

View attachment 351360

There is no base current, so the transistor will not turn on, and the circuit will not oscillate.

 

[DaveE]

That is a quartz crystal, not an oscillator. It won't do much by itself.

 

[James2018]

There is no base current, so the transistor will not turn on, and the circuit will not oscillate.

You are free to add to the circuit diagram if you want. I connected the base terminal and the emitter to the negative battery terminal and the collector to the positive one. I do not know what else to do.
My inspiration was the common-base Colpitts circuit. The inductor L and the series combination of C1 and C2 form the resonant tank circuit, which determines the frequency of the oscillator. The voltage across C2 is applied to the base-emitter junction of the transistor, as feedback to create oscillations.

https://en.m.wikipedia.org/wiki/Colpitts_oscillator

Maybe the beep on the radio antenna is from other circuits found in my room.

 

[Baluncore]

You got a diagram with a simplified bias circuit.
At the bottom of this section is one with the bias circuit.
https://en.wikipedia.org/wiki/Colpitts_oscillator#Theory

 

[James2018]

You got a diagram with a simplified bias circuit.
At the bottom of this section is one with the bias circuit.
https://en.wikipedia.org/wiki/Colpitts_oscillator#Theory

But the multimeter measures 2 Volts (sort of a varying value between 1.96 and 2.2 volts) at the terminals of the quartz crystal. And, there is a beep when I place the radio receiver antenna on top of the quartz crystal. I moved the circuit away from other circuits on my terrace and the beep is still heard on the radio when the antenna is on top of the crystal and not heard when the radio receiver antenna is moved away.

Remember, the base of the transistor and the emitter are connected using different wires to the negative terminal of the battery, while the collector is connected to the positive terminal.

Still, I am going to dissasemble it and build it differently to make sure it works.

 

[James2018]

I followed this Collpits oscillator diagram

which resulted in this circuit

Still it is unclear, which terminal of the battery is positive and which is negative in the diagram, I assumed the top terminal is positive.

 

[Vanadium 50]

This thread has gone all the way from "why doesn't my compass needle move?" to "what exactly am I hearing on the radio?" Multiple circuits - some of which appear not to be circuits - have been drawn.

It would be valuable of a) the OP were to explain what the question is NOW, so we can get past what the question WAS, and b) to tell us where this random walk is trying to get to.

 

[James2018]

Ok, how do I know if the circuit oscillates with LC oscillations or not? Do I need to set the radio receiver on a specific frequency on AM and FM? And inside the final diagram that has B1 as the 5 V battery with the top terminal positive and bottom terminal negative, there are two capacitors in the tank circuit connected in series, instead of one...

I did not use the values specified in the diagram... only the arrangement of circuit elements.

Instead I used 15 nanoFarads for the C2 capacitor and 220 nanoFarads for the C5 capacitor connected in series with 470 microHenries L1 inductor in the tank circuit... how do I calculate the resonant frequency with two capacitors connected in series to each other and to an inductor?

 

[Sagittarius A-Star]

how do I calculate the resonant frequency with two capacitors connected in series to each other and to an inductor?

Use the formula for 2 capacitors in series to get the resonant circuit capacitance.

Source (Tietze-Schenk):

 

[sophiecentaur]

I remember a similar experiment that I tried, which failed. Anyone who attempts an experiment like this, and questions the result, deserves every support. All the indications are, that they have the critical thinking skills needed, to make a great engineer or physicist one day.

To get a good result for this sort of 'obvious' phenomenon, you need the experience to make the right sort of guesses about what frequency to use, what inductor to use and how to display the result. That calls for a fair bit of experience or enthusiasm wanes before getting a decent result. (And a large source / choice of component values.) Plus you need the appropriate measuring equipment. A DVM makes things very difficult.

I (also) remember a similar play I had - that time with coupled LC oscillators which passed energy between them and back again (analogue of the two pendulums experiment). The main thing was to find suitable audio frequency inductors and appropriate coupling. The (successful) result was two strings of beads on two scope traces, which started with an impulse (switch off the DC supply) and gradually decayed. It lasted just long enough to satisfy me and impress colleagues who would also have done the same thing, given the time and inclination.

 

[Baluncore]

I did not use the values specified in the diagram... only the arrangement of circuit elements.

That circuit, with your changed values, will oscillate at about 63 kHz.
What frequency do you want ?

 

[Averagesupernova]

Ok, how do I know if the circuit oscillates with LC oscillations or not?

Do not try to build an oscillator that runs at radio frequencies. Start with audio which a cheap voltmeter is able to measure.

 

[James2018]

So C=Ca*Cb/(Ca+Cb) =(15*220) /(15+220) =14.0425 nF
The resonant frequency is f=1/(2*pi*sqrt{L*C}) = 61.951 KHz so approximately 62 KHz.
Thank you

 

[Sagittarius A-Star]

So C=Ca*Cb/(Ca+Cb) =(15*220) /(15+220) =14.0425 nF
The resonant frequency is f=1/(2*pi*sqrt{L*C}) = 61.951 KHz so approximately 62 KHz.
Thank you

That result is correct.

I propose that you use the next time for better readability LaTeX for formulas (see link to a LaTeX Guide below the input window and have esecially a look at chapter "Delimiting your LaTeX code").

In the middle term of your first formula, the unit $nF$ is missing.

 

[sophiecentaur]

Do not try to build an oscillator that runs at radio frequencies. Start with audio which a cheap voltmeter is able to measure.

Also bear in mind the old adage "Amplifiers oscillate and oscillators amplify"
Modern transistors (since 1955) can have an embarrassing amount of gain at high frequencies. They will 'see' signals, fed back from long bits of circuit wiring and wires laying close to each other and decide to oscillate many octaves above what you wanted. Short leads and a ground plane are not always available on a breadboard.

 

[Baluncore]

"Amplifiers oscillate and oscillators amplify"

There are many oscillators named after hopeful amplifier inventors, those who failed to tame positive feedback. Those oscillators are never simple, as they always have a subtle feedback mechanism. I would list some here, in a "rogues gallery", but that would not be fair, as I would probably forget to mention a dozen or more.

There are very few amplifiers named after their inventors. Can you name three?

 

[Sagittarius A-Star]

Video about a Colpitts oscillator with practical hints:

 

[James2018]

The multimeter detects the resulting output as being a voltage that oscillates between + 120 milliVolts and -120 milliVolts, far lower than the input 9 Volts, although the current from the battery reaches 7 milliAmperes. And I did use the capacitance values from the diagram, four 0,1 microfarads capacitors and one 1 microfarad capacitor.

The only thing I didn't follow was the resistance values, as I didn't find resistors for sale with these exact values: 2.2KΩ, 4.7KΩ, 10KΩ and 560 Ω.

Also I did use a 1.35 microhenries inductor to obtain a resonant frequency of 612,6 KHz for the tank circuit, given the effective capacitance of 50 nF.

 

[Sagittarius A-Star]

The multimeter detects the resulting output as being a voltage that oscillates between + 120 milliVolts and -120 milliVolts, far lower than the input 9 Volts
...to obtain a resonant frequency of 612,6 KHz

What is the AC frequency response of your multimeter?

See
https://electronics.stackexchange.c...-a-dmm-decrease-as-the-frequency-is-increased