How to make a “Butterfly” metal detector using just 2 transistors

Who among us has not dreamed of taking a metal detector with us and going to ancient ruins to look for antiques. All this is very interesting and entertaining. It’s a pity that modern Chinese metal detectors are quite expensive, despite their low sensitivity.
You can make such a device yourself in no time. You don't need expensive chips or parts for this. Just 2 transistors and a few more parts that won’t be difficult to find.

Details

• Two transistors KT940 (2SC1569) -
  
  
  
  
• Two capacitors 4700 pF and 1000 pF.
  
  
  
  
• Two 100 kOhm resistors.
  
  
  
  
• Dynamic head 8 Ohm and 0.5 W.
  
• Roll of wire 0.3 mm.

Diagram and principle of operation of the metal detector

The circuit consists of two identical generators that consume current through the dynamic head. They operate at the same frequency and mutually exclusive pulsation in the dynamics is not audible.
As soon as metal appears in the coil of one of the generators, the frequency of the generator changes sharply, the frequency difference between the generators is reproduced in the dynamic head in the form of a whistle. The scheme has been tested for decades.

Making a metal detector

We solder the collectors of the transistors and solder the capacitors to the bases.

We solder the 4700 pF capacitors and connect them with a common wire.

We solder the resistors and power wires.

Solder the coil. It contains 30 turns of 0.3 mm wire wound on a diameter of 160 mm with a tap from the 10th turn (see diagram). You need to make two such coils.

Solder the second coil and the metal detector is ready.

Testing the device

We supply 9 Volt power.

We take turns bringing the copper and aluminum pieces to one of the coils. Everything works, a clear squeak is heard in the dynamic head.

The same thing happens with ferrous metals. The device reacts to any metal without exception.
The circuit does not require any configuration and starts working immediately.
If you want to configure the device for maximum sensitivity, then instead of any resistor, solder a variable one with a nominal value of 150 kOhm. Then they can clearly set the response limit.

Watch the video