This instructable is just another way to make one cheap and easy portable Metal Detector, using one of the 50 555's schemes that is available in the internet, in this book: http://www.talkingelectronics.com/projects/50%20-...
I've simply made my own portable metal detector with some little improvements, like a 3mm green LED for signalling when the device is searching and a potentiometer to adjust the volume of the searching task.
Step 1: Material
The list of components to make a metal detector like this is:
1 - NE555
1 - 1mH coil (I've reduced the value from 10mH to save some copper assembling my own coil, because less inductance => less wire spins)
2 - 2.2uF electrolyte Capacitors
1 - 10uF electrolyte Capacitor
1 - 47K resistor
1 - 8R speaker
1 - pressure button switch SPST NO
1 - 9V battery
1 - green LED
1 - 330R resistor
1 - 10K potentiometer
Step 2: To Construct the Coil
The bigger difficulty of the metal detectors, is the coil. I don't know if we can find some coils, with the specific value of mH that we need.
Therefore I've made my own coil with an empty round package of pringles, which has 7cm of diameter.
Considering the value of 1mH, I've just had to simulate the coil lenght and the number of spins, to start the construction of the coil.
For those simulations, I've used this Air cored inductor calculator: http://www.m0ukd.com/Calculators/air_core_inductor...
I've used a wire of 0,3mm and I've reached the result of 121 spins, with a lenght of 39mm for my coil.
I've cut off a piece with the 39mm, from the empty package of pringles and I've made the 121 spins of the 0,3mm copper wire, making the coil that we can see in the picture.
The calculator refers that the value is valid to values of lenght > 0,4 * diameter, and it has an accuracy of 1%. In that case, I've made a coil with 1mH, +/- 1%.
Step 3: Schematic
Like I've refered in the introduction, I've used the 555's scheme, making some changes, adding a signalling LED and a way to adjust the volume of sound in the speaker.
Well in this step we can see the small differences between the 2 schemes, in the 2 pictures.
Step 4: Assemble the Circuit in a Breadboard
The first assembly of the circuit must be done in a breadboard, to test if it works and to take the first conclusions about the range of detection and so...
I don't have any picture of my circuit assembled in the breadboard to share. :(
Step 5: Final Mounting
After be certain of the full functionality of the circuit in the breadboard, we're able to mount it in a PCB, or in pre-pierced board, like I did.
The image shows the device already mounted, inside one empty old toothpaste case, with the circuit hot-glued inside, the coil hot-glued on it's top and the cap of the potentiometer on.
Now, I have my own portable metal detector! I just need to push the button and start searching... :)