# LC tank Circuit powered w/magnet.

#### sidwelle

Joined Oct 2, 2009
8
Hello,
I Need to design an LC circuit with just 2 components L & C. (L having an Iron core)
What will happen is the Circuit will be mounted on an arm that moves and when it passes by a magnet will energize the coil of the LC and transmit a brief spike on a specific frequency. Allowing another receiver to receive the transmission spike and detect the location.
There are alot of optical sensors that could easily do this, but I was trying to go very low tech and be weather resistant.

If this too far off ??!!
Any help is appreciated.
Thank you.

#### MisterBill2

Joined Jan 23, 2018
15,653
That description is similar to the magnet circuit on a small gas engine. So yes, it could work but mechanically it is not that simple. And a whole lot depends on the desired frequency. So more details are needed.

#### Alec_t

Joined Sep 17, 2013
13,784
Welcome to AAC!
transmit a brief spike on a specific frequency
A quick simulation in LTspice suggests that the dominant frequency generated in the tuned circuit, regardless of the tuning, is governed by the speed of movement of the arm/coil past the magnet pole. That movement induces a single-cycle wave (depending on the pole geometry) of a speed-dependent frequency (not a specific one). That frequency and its harmonics seem to be the coil signal. The coil signal could, however, be used to gate an oscillator of a specific frequency.

#### sidwelle

Joined Oct 2, 2009
8
The movement of the arm would energize the coil, and you are stating that it would dictate the frequency ?
My mental picture shows that the initial wave would energize the coil and then after the magnet passed, the circuit would continue to oscillate for a short period of time dictated by the value of the coil and capacitor. You are stating that the frequency would be dictated by the speed of the magnet approaching the coil ? Yes but the frequency of the tank circuit would be much higher and even oscillate as the magnet approached the coil and as it departed.
Values: Frequency and cap and inductor sizes that are practical. Something about the size of nickel or quarter ?
Somewhere in the toy frequency, 49 mHz ( I think )

Joined Feb 20, 2016
4,314
I doubt you could get it to work.
The speed of the magnet to inject a sharp pulse into the coil to get it to ring would be very fast.
And just one pulse probably will not cause it to ring for more than a few cycles if you manage to get it to even do that.
Some more real information on your project may help. So far, it is very vague and that does not help us to help you.

#### shortbus

Joined Sep 30, 2009
9,942
Sounds like a job for a hall sensor, or a magnetic pickup, not a LC circuit.

#### MisterBill2

Joined Jan 23, 2018
15,653
Actually it might be sort of possible by adding a really fast diode and having a very high "Q" resonant circuit. But it seems that the TS is looking for a beacon circuit of some sort, and there are quite a few of those available. Any mechanical arrangement will be much larger. The transmitter part could be similar to one of those product ID tags that respond with the product code when interrogated by a transmitter.
So really, the technology for that sort of system is fairly mature and available for purchase.

#### Alec_t

Joined Sep 17, 2013
13,784
You are stating that the frequency would be dictated by the speed of the magnet approaching the coil ?
The dominant frequency, yes.
the frequency of the tank circuit would be much higher and even oscillate as the magnet approached the coil and as it departed.
Simulation shows that doesn't happen.
How many rpm is the arm doing? What radius? How big is the magnet? What are the coil's inductance and the cap value?

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#### MisterBill2

Joined Jan 23, 2018
15,653
There are two frequencies involved, the pulse rate and the resonant frequency of the tuned circuit. It is not likely that anything in the 49 MHZ range would work well enough to be worth the effort.
And now my big question is: "What is the intended purpose of this system?"
At best the amount of power would be quite small, and the stability quite poor, so what is the goal to be reached by generating such signal bursts? There may be a much more effective means of reaching it.

#### nsaspook

Joined Aug 27, 2009
11,768
To generate detectable high frequency RF in a LC tank system using simple magnetic induction typically relies on negative-resistance (to transfer energy from electric to magnetic and back in the tank circuits at the required tuned frequency) in the form of an arc (plasma). Simply pulsing a magnetic field in the tank at some low frequency gets you just about nothing as the transferred energy will be dissipated by circuit resistance and maybe core/dielectric losses at the higher end.

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#### MisterBill2

Joined Jan 23, 2018
15,653
In addition, the core material needed to concentrate the magnetic field needs to have low losses at the resonant frequency. An iron or steel core will not work at 49 MHz

#### sidwelle

Joined Oct 2, 2009
8
Purpose: To record the angular location of a flywheel.
Q: If the core of the inductor would not sufficiently provide energy to the tank circuit, Maybe the inductor could just be used to power a separate small tank circuit (or equivalent, crystal radio circuit ?) ?

#### DickCappels

Joined Aug 21, 2008
10,067
Oooohhh, @sidwelle just anticipated my post by less than a minute. It might be better to not have an tank in the front end, but to harvest the energy from the coil and use it to power something simple like a tunnel diode oscillator.

#### sidwelle

Joined Oct 2, 2009
8
When we were kids (70s), our neighbor had a remote control TV that had a self powered remote control.
It worked by dragging a spring loaded iron core out of the coil and then releasing it with a snap and the spring would drag the core back into the center of the coil, energizing the coil and transmitting a brief RF signal.
The channel would shift up one channel for each push or the button, remote had 3 other buttons for Ch down and vol up/dn.
It was crude and bulky, but it worked. Just thought it would be easy to replicate.

#### nsaspook

Joined Aug 27, 2009
11,768
When we were kids (70s), our neighbor had a remote control TV that had a self powered remote control.
It worked by dragging a spring loaded iron core out of the coil and then releasing it with a snap and the spring would drag the core back into the center of the coil, energizing the coil and transmitting a brief RF signal.
The channel would shift up one channel for each push or the button, remote had 3 other buttons for Ch down and vol up/dn.
It was crude and bulky, but it worked. Just thought it would be easy to replicate.
If it was an old 'Zenith Space Command' type then it was ultrasonic not RF.

#### sidwelle

Joined Oct 2, 2009
8
That's it, didn't know if was ultrasonic. Neighbors never would let me look inside ?!

#### MisterBill2

Joined Jan 23, 2018
15,653
If it was an old 'Zenith Space Command' type then it was ultrasonic not RF.
EXACTLY!! I still have a couple of those "Space Command" receivers. They are classics.
Reading the position of a flywheel based on one pulse? That sounds rather uncertain. Really it should have one pickup for a reference position and then count gear teeth to know where it is. OR, use a PLL to generate a frequency 360 times the reference point frequency, and then latch the count at the event.
But Wait! If this is a Hartridge engineer asking then the company needs to pay me a whole lot for an answer. Yes, it is a personal grudge based on the management behavior. I recall that last day very well.

#### shortbus

Joined Sep 30, 2009
9,942
Purpose: To record the angular location of a flywheel.
That is done in cars all of the time. But not with to my knowledge an oscillating signal. It's done with ether a magnetic or Hall sensor counting teeth in a gear attached to the crankshaft, the gear has a tooth or two missing to decide where the zero position is. Also used to tell where the camshaft position is.
https://en.wikipedia.org/wiki/Crankshaft_position_sensor