I purchased a metal detector with the intent of having it "read" by an Uno. The detector is great and I am happy with it.
The cable to the double D coils at the foot has 5 wires: red, black, green , yellow and white. I tapped into all 5 with my 100MHz oscilloscope in every different combination while running the device and passing metal near it by hand. The onboard speaker confirms it is working just fine as I probe each two wire combon with my scope: but I see a clean sine wave (approx. 13KHz) no matter what two wired i tap into with my probe.
The picture on the oscilloscope does not change. I was hoping to see some sort of delta that the Uno could then detect, but the picture on the scope stays the same even though the speaker clearly indicates metal near by when I wave a wrench with my hand.
I guess you see small differences between the signals of the different wires when you detect metal. I wouldn't expect to get a signal detectable by the Arduino from these wires. What do you want to read by the UNO? Just the detection of metal. Then try to get a signal from the speaker output or maybe an LED or something similar.
A schematic of the metal detector would help a lot. The reason you did not see the signal change on your scope is it is measuring a phase shift not an amplitude change.
SMA to me is semi-precision coaxial RF connectors. SMD is surface mounted devices which is what I think you are talking about. If the audio is fixed you can use a tone detector chip to determine if the signal is on or off.
one coil generates a magnetic field. the other coil senses a magnetic field, and is wired 180 degrees - one half cycle - out of phase with the generated field
these are fed to a summing amp, which adds them together. if you put on the headphones you hear nothing, because the identical 180 degree out of phase coil voltages cancel each other. The Balance control is set to null out any differences.
if the coils pass over a metallic object, that object affects the transmitted magnetic field, and that affects the sensing coil. the received magnetic field is delayed, and you no longer have a balanced input to the summing amp. what you hear is the difference caused by that delay
different materials delay the received magnetic field for different time intervals, measured in degrees of phase variation. this is how the controller in the metal detector can discriminate between different metals
what you need is a 2 or more channel scope that can be triggered by the channel connected to the sending coil. if you trigger from the sending coil, the receiving coil will be 180 degrees out of phase with the sending coil in free air, and out of phase when in proximity to metal.
Yeh Gil, I stand corrected: SMD. I looked in eBay for a tone detector and did not come up with much that was practical / affordable with a Uno. Any suggestions? Thank you.