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Project Notes

#526 Metal Detector Kit

An inductive metal detector kit powered by an 18650 with TP4333-based BMS charging circuit.


Here’s a quick demo..



I picked up a cheap Metal Detector Kit from an aliexpress seller for a few fairly prosaic reasons:

  • it comes with an 18650 battery pack and I was interested in taking a look at the BMS provided
  • it’s a nice black PCB;-)
  • .. and maybe I can use it as a stud finder/wiring detector in the house?

The Kit

The kit comes in two parts:

  • the metal detector circuit with USB connector. Working voltage: 3~5V
  • 18650 battery pack and TP4333-based BMS module. Various colours are available for the batter case. It works well as a standalone 5V USB power pack.

Parts for the metal detector:

Ref Name Qty
R1 200kΩ 1
R2 2kΩ 1
R3 470Ω 1
VR1 100Ω 1
C1,C4 100nF 2
C2,C3 2.2nF 2
C5 100μF 1
Q1 S9018 NPN 1
Q2,Q3 S9012 PNP 2
LED1 5mm LED 1
SW1 Switch 6P self-locking 1
SP1 buzzer HSD9*12 1
  USB male 1

kit_parts kit_pcb_front kit_pcb_rear


The following is my re-drawing of the core metal detector circuit. It is powered with 5V from the 18650/TP4333 battery module.



The circuit design is very similar to the Metal Detector Module available from protosupplies.

How the Detector Works

The detector comprises two inductor coils that are actually circular traces on opposite sides of the PCB.

  • L1 in series with C1 (2.2nF) connects to the base of Q1 in parallel to a high-value base resistor R1 200kΩ
  • L2 parallel with C2 (2.2nF) connects to the collector of Q1 and base of Q2
  • L1 and L2 are wound in opposing directions
  • VR1 potentiometer adjusts sensitivity by adjusting the resistance (voltage offset) at the Q1 emitter.

I think it basically works like this: When a metallic (ferrous?) object is near the coils, the inductance rises and the resonant frequencies of the L1/C1 and L2/C2 tuned circuits changes. Since L1 and L2 are in opposition, and oscillation can be sustained:

  • L1/C1 turning on Q2
  • Current flow through L2 will dampen L1 and throttle Q2

The TP4333 BMS

The TP4333 module comes pre-assembled and snaps cleanly into the battery case.

kit_bms_front kit_bms_rear

This is the first time I’ve seen the TP4333. It’s quite a capable little chip, combining charge control, protection and output boost converter:

  • Discharge output: 5V/1A
  • 1A output discharge efficiency up to 93%
  • Charging current: 0.8A
  • BAT discharge termination voltage: 2.85V
  • Intelligent temperature control and over-temperature protection
  • Integrated output overvoltage protection, short circuit protection, overload protection
  • Integrated over-charge and over-discharge protection
  • Supports trickle mode and zero voltage charging

The module appears to closely follow the typical application circuit from the datasheet. It doesn’t use the flashlight output though.


Low battery protection:

  • when the BAT voltage is greater than 3.2V on startup, the output boost circuit is enabled.
  • if the battery voltage falls bellow 3.0V during operation, LED2 will flash at a frequency of 1Hz
  • when the battery voltage falls bellow 2.85V, the discharge output is disabled and the TP4333 enters low-current standby mode

Load detection and low-power standby:

  • When the load is connected, it is detected by the TP4333 detect and boost circuit enabled.
  • When the load is disconnected, the boost circuit is disabled after an 18s delay, and the the TP4333 enters a low-current standby mode

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.