Coil, led and magnet - Page 1
Forum section: electronics / beginners

author: myf
author_rank: Regular Contributor
timestamp: February 14, 2025, 11:48:31 am
content: Hello,I'm trying to create a flash of light using a coil, an LED and a magnet. I'm using a neodyne magnet 15mm in diameter and 3mm thick (from aliexpress.com). The coil has about 15 turns (of 0.3mm diameter enamelled copper wire, with a resistance of 0.4ohm) with a diameter of about 20mm. The red LED has a voltage of 1.9 V (mounted without a resistor). The air gap is about 1mm.The LED does not flash when I slide the magnet quickly over the top.Can I measure the induced voltage with a voltmeter, or do I need to use an oscilloscope?How many turns do I need to do this experiment ? if possible.F. form France and translated with DeepL.com (free version)

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2025, 11:58:32 am
content: 15 turns is nowhere near enough.A volt meter won't be fast enough. You'll need an oscilloscope to measure the voltage. The voltage is proportional to the number of turns. Aim for at least 2V.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 14, 2025, 12:00:43 pm
content: You can measure the voltage with a voltmeter (set on AC volts), but the voltage will be very small. 15 turns is not enough. You will need some hundreds of turns.Look how many turns these magnet-shaking flashlights have:https://duckduckgo.com/?t=ffab&q=shaking+flashlight+magnet&iax=images&ia=images

author: PGPG
author_rank: Frequent Contributor
timestamp: February 14, 2025, 12:04:48 pm
content: You rather need oscilloscope.You have little chance to try to measure it with voltmeter when doing peak detector before it.But standard solution (diode+capacitor) will give you nothing until voltage will be higher then 0.7V.Using germanium diode you can notice pulses may be from 0.3V.There are 'ideal diode' circuits using OpAms... but simpler nowadays will be to buy oscilloscope. If you plan more electric/electronic experiments you will have to have it in any case.

author: radiolistener
author_rank: Super Contributor
timestamp: February 14, 2025, 01:10:21 pm
content: you can move magnet fast with electric motor, and 15 turns is too small, try to increase...

author: calzap
author_rank: Frequent Contributor
timestamp: February 14, 2025, 04:23:29 pm
content: If you use a regular LED and standard geometry coil (spooled in one direction around an axis), you can get a flash when moving the magnet in one direction but not the other. The reason is the LED is a diode, and the voltage produced from passing in one direction is DC. In one direction, current can flow through the LED; it’s blocked for the other. Choice of diode, magnet, coil, geometry and speed of movement will dictate whether you get enough power to light the LED regardless of direction of movement.Mike

author: Grandchuck
author_rank: Super Contributor
timestamp: February 14, 2025, 04:26:31 pm
content: What Mike said.Also, two LEDs in inverse parallel will allow both directions/polarities to work if other conditions are good.

author: ledtester
author_rank: Super Contributor
timestamp: February 14, 2025, 04:31:25 pm
content: Google "diy shake flashlight" for some guidance, e.g.:https://www.instructables.com/Shake-Flashlight-1/Quote

author: janoc
author_rank: Super Contributor
timestamp: February 14, 2025, 04:34:08 pm
content: You will need at least couple of hundred turns for this to work. 500-1000 would be a good start.

author: TimFox
author_rank: Super Contributor
timestamp: February 14, 2025, 05:45:40 pm
content: If you move a coil in a magnetic field, the flux linked by the coil is proportional to the number of turns.Faraday’s Law states that the EMF (voltage) induced in a coil is the time derivative of that flux, where magnetic flux is the product of field times area.

author: PGPG
author_rank: Frequent Contributor
timestamp: February 15, 2025, 12:02:34 pm
content: Quote from: janoc on February 14, 2025, 04:34:08 pmWhen I was 9 and my father explained to me how a transformer works I decided to wound one at about 30 W core I had. I wound 7 turns of primary and 1 turn of secondary and was very surprised to not see 30V at secondary when I connected primary to AC220.

author: myf
author_rank: Regular Contributor
timestamp: February 16, 2025, 11:57:07 am
content: Thanks a lot for all your answers ! Each one teaches me something.I'm going to borrow an oscilloscope to measure this voltage, and I need to make a coil with a few hundred turns. But it's going to be a pain to wind (about 50m of wire to wind and thread).Can I use ready-made cd127 coils like this 12x12x7mm one? These are the biggest I've found on aliexpress.com. They are shielded, can the shielding be removed?What inductance should I use (in µH or mH) to get around 300 turns? Or are there better references?https://fr.aliexpress.com/item/10000000674853.htmlCapture d’écran du 2025-02-16 12-25-45.png

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2025, 06:40:38 pm
content: Those inductors are shielded. They have a screen to block external fields, such as those generated by your magnet.

author: xvr
author_rank: Frequent Contributor
timestamp: February 16, 2025, 09:09:32 pm
content: You can disassemble relay (power relay or signal relay), remove contacts and extract coil. It will suit you.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 16, 2025, 09:13:20 pm
content: Quote from: PGPG on February 15, 2025, 12:02:34 pmSo you got smoke instead of volts, right?

author: Analog Kid
author_rank: Super Contributor
timestamp: February 16, 2025, 09:17:13 pm
content: To the OP: you could try winding your own coil. Not difficult.You'll need some kind of core material (ferrite is good), which can be harvested by busting open salvaged transformers if you have no other source, and magnet wire, which you can buy just about anywhere.This postingshows one way to wind it using a hand drill. Even without that you can easily do it by hand, although it might not look as nice.

author: calzap
author_rank: Frequent Contributor
timestamp: February 17, 2025, 04:38:24 pm
content: Or OP could just buy a spool of small gauge wire. When done using it as an inductor, use it for projects and repair. Recommend a plastic, not metal, spool holder. Flanges on end of metal spool holder could cause strange effects if using the spool as an inductor.Mike