I got a cheap DIY Metal Measure Kit Electronic DIY Soldering Exercise Board with the following specifications:
Working voltage: DC3-5V
Standby current: ≤ 5mA
Working current: ≤40mA
Measure depth: ≤ 60 mm
Measure alarm mode: sound, light
PCB size: 86 x 61mm
There was a just the circuit board and some components on the bag.
The circuit board looks quite nice. It has the search coil implemented with this circuit board traces.
The kit was easy to solder with instructions provided on-line only. Did not take too long ot build.
The circuit works. The trimmer controls the sensitivity.
The next part is to figure out how it works. I did testing by holding a coil connected to oscilloscope near this circuit and figured out that the circuit works at around 300 kHz frequency.
Let’s take a closer look at the circuit diagram provided at the instructions manual:
The circuit is an oscillator built around Q1. It normally oscillates and keeps Q2 turned on.
When metal object becomes near, it reduces the oscillation (can even stop it), which turns Q2 off.
When Q2 is off, Q3 start to conduct and give power to the buzzer and LED.
The trimmer controls the oscillator amplification. Less amplification, more easily the oscillator is disturbed.
I finally converted this circuit to a three wire Arduino sensor just by soldering signal wire to buzzer + pin and connecting +5V and GND to the screw connection block: