Metal Detector Circuit

Thread Starter

ro169

Joined Oct 10, 2014
62
Hi all.

Been working on a metal detector project. I've come across quite a few schematics online.
I've tried to replicate the MKII by Talking Electronics: http://www.talkingelectronics.com/projects/MetalDetectorMkII/MetalDetectorMkII.html
Here is the Schematic:
MetalDetectorMkIICct.jpg
However I have changed a few things due to availability and the circuit doesn't perform as expected.
I have used 1.25mm gauge fluxed wire forming 100mm diameter coils (According to the site the size of wire or coil isn't important)
I have replaced one of the BC547 transistors with a BC237
In place of "390R" surface mount I have used a 330R
When switched on I have observed the speaker voltage to be a constant 2.6V despite the position of the coils relative to each other or the position of the dial on the potentiometer - i.e no tone is produced on the speaker.

Any input will be appreciated :)
 

bertus

Joined Apr 5, 2008
22,270
Hello,

Did you read the "setting up" part?
SETTING-UP
Keep the two coils slightly apart and set the 500R pot to mid-position. The output will produce a tone.
Slide the coils slightly closer together and turn the pot to stop the tone. If the tone does not stop, turn one of the coils over.
If the pot has no range, remove one turn of the 50 turn coil. Keep doing this until the two coils are touching and the pot has mid-range. Tape the two coils together.
Adjust the pot until you get a very low-frequency clicking noise. This is the most sensitivity setting.
Bertus
 

Thread Starter

ro169

Joined Oct 10, 2014
62
Yes I have.
When switched on I have observed the speaker voltage to be a constant 2.6V despite the position of the coils relative to each other or the position of the dial on the potentiometer - i.e no tone is produced on the speaker.
 

AlbertHall

Joined Jun 4, 2014
12,345
Use the two BC547 transistors that you have as the two leftmost transistors in the schematic as they are the most critical. Use the BC237 as the third transistor in the schematic.
 

Thread Starter

ro169

Joined Oct 10, 2014
62
Use the two BC547 transistors that you have as the two leftmost transistors in the schematic as they are the most critical. Use the BC237 as the third transistor in the schematic.
Yes this was my thinking as well! I did do this :). Do you think that using a BC237 could be the problem? I will try using a BC547 as soon as I get my hands on another.

One of my concerns is the 390R surface mount.
390R.jpg
I'm not sure exactly how the resistor "adjusts for the type of wire and coil-size".
However I've assumed that this is a standard resistor and not something unique and used the 330R in place of it as the site states that its purpose is to help you customize and build your own projects using the blocks in this 1.
 

Sensacell

Joined Jun 19, 2012
3,432
This circuit looks very simple, making it attractive to a beginner, but it's actually very tricky to get it working.
More complex circuits are actually easier to troubleshoot.
 
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