Tuesday, 23 April 2013

Simple metal detector.

Hi everyone

This time I m gonna show you how a simple metal detector works.

I will be using the oscillator circuit and the coil transformer in the previous post, which I have modified it into a simple metal detector. The circuit is shown below:

The LED is made an indicator for the detector. The coil transformer is the detection coil. Object is put into the centre of the coil for detection.

I have tested this detector with different materials and got several results:

When I inserted the metallic object such as screwdriver or scissors, the LED dimmed.

However when I inserted a ferrite rod (which is not a metal), the LED brighten up.

Here is the video:


Note that the ferrite rod increases the LED brightness just a little, but if I connect the secondary winding of the coil transformer to the voltmeter instead of connecting it to the LED, the secondary voltage indeed increases but it is not sufficient to make increasing brightness of the LED apparent to the eyes.

The detector can be improved by using circular (disc) coils as the primary and secondary winding of the coil transformer instead of using the cylindrical coil transformer as I am using, and inserting the object to be scanned in between the discs.

The detector works by the principle of different core material for the transformer.

Any object inserted into the coil transformer will be automatically assigned as the core of the transformer.

If you insert a ferrite rod, it will act as the core. Since the ferrite rod has high magnetic permeability, the magnetic flux around the coil transformer finds a less reluctance (magnetic resistance) path, which is the ferrite rod, to flow through. Hence, the field lines reinforce together, reducing the lost magnetic flux and hence, making the LED shines brighter.

However if you insert a metallic object, it will create eddy current within it as a reaction to the changing magnetic field it resides within. This eddy current will in turn create other changing magnetic fields that oppose the magnetic field the metal resides within (which is the magnetic fields of the coil transformer), cancelling the magnetic fluxes together and hence generate less voltage across the LED, dimming it as a result.

This detector, although works, is not suitable for practical metal detection since you have to put the tested material/metal into the coil, which is not practical if you want to detect any hidden metal across a flat surface. Therefore, for that purpose, the coil needs to be flat as well to facilitate smooth scanning of the flat surface. Hence, flat coil transformer is frequently used in the actual metal detector design.

Flat coil is like a mosquito’s coil, and it is very hard to make by hand, and requires special tools and materials. That is why I resorted to using cylindrical design like the coil transformer.

I think that’s all for today. Thanks for reading.

Thursday, 11 April 2013

Lighting fluorescent bulb using 9V battery (Part II)

[To read Part I of this article, click]

As you have seen from Part I, I have used flyback transformer in the circuit to light up a fluorescent bulb using 9V battery.

Actually, I want to keep the whole circuit as simple and constructable as possible, where I want to remove some components that are rare, difficult to find such as flyback transformer and ferrite rods. As CRT tv and pc are obsolete nowadays, flyback transformer gets even rare and more expensive.

Therefore I set a rule when designing a new lighting strategy; no flyback transformer, and no ferrite rods !

I planned on using just the tissue roll former and copper wire and make them into a simple transformer.

Here how it looks like:
The red wire is primary while the yellow wire is secondary. Notice there is a centre-tapped wire (red)
at the middle

I wound the secondary first on the former, about 8 layers of winding and then two layers of primary on top of it, with centre tap wire. The copper wire I have used is SWG 27 (0.4mm thickness). The tissue former dimension is 3cm diameter and 5 cm length. The design is not too strict and you may decide by your own on how many number of layers/turns you want to make for secondary/primary. Its damn try-and-error thingy. When I made it I just do it as I like and no engineering calculation was done. 

The resulting secondary voltage from this transformer is not enough to light up the fluorescent bulb, though. It can only light up an LED. Therefore, I stepped up the voltage using a rechargeable battery charger transformer I have used before. Well, I am still havent break the rule cuz I am not using flyback transformer.

The rechargeable battery charger transformer, utilised as step up transformer to elevate the output voltage from the coil's secondary

The resultant voltage from the battery charger transformer can eventually light up the bulb.

Overall circuit, with the light bulb being light up involving both coil and transformer. The driving circuit is the same as used in Part I of this article, a Joule thief.

Close up.

By using this strategy, you can use items that is easily available such as copper wires and charger transformers and dont have to find old CRT tvs to salvage the flyback transformers from them, or spending dollars to buy ferrite rods. You also dont have to buy a new disposable camera just to get its flash transformer for this project.

You may also use other transformers other than battery charger transformer, such as those in electronic lamp ballast, and cellphone charger.

Thanks for reading. 

Tuesday, 9 April 2013

Lighting fluorescent bulb using 9V battery.

Dear everyone I am back!

Lately I ve been testing this circuit. Its called 'Joule thief' and I tried using it to power up a fluorescent bulb using just 9V battery.

Here is the schematic I ve been using:

Or if the above photo is not too clear...

I have used flyback transformer salvaged from old CRT television. The primary coil is centre-tapped with 8 turns for each coil. 2N3055 is used as switch due to its high power rating.

Choke 34uH) and electrolytic capacitor (100uF) are used to provide constant current and voltage to the primary coils.

Output voltage from the secondary winding of the transformer is capable of driving small fluorescent bulb.

The photos explain my project more:

My setup. Battery at the lower part, the circuit, flyback transformer (bulky black chunk) and the farthest stuff, the fluorescent bulb.

I have removed the circuit board below the bulb and just taking the bulb. I connected a wire to each of the terminal. Each is then connected to each terminal of the flyback transformer's secondary.

The video. 

 The bulb appears quite bright in the dark for 9V supply.

I tried adding a capacitor (1uF, or you may try any value) across or in parallel to the 500 ohm resistor and guess what, it made the bulb shines brighter! This time I ve changed the bulb to a longer one to spread the lights wider but I think this capacitor effect also works for the smaller bulb.

The longer fluorescent bulb

As usual I removed the adds on from the bottom (which comprises of a capacitor and a fuse-like stuff) and connect the bulb's terminals directly to the output of the flyback transformer.

 Before lighting up.

Before adding capacitor.

 After adding a capacitor.

Thanks for reading.

Update (11 Apr 2013): I tried using my homemade flyback transformer and the circuit works, with an additional capacitor  (around 0.1uF) attached between each end of the primary winding. Here is the photo:

The flyback transformer (homemade) that I have used:
The vertical ferrite rods are longer this time compare to the previous ones cuz those shorter ones are being used for other projects.