Adafruit CLUE Metal Detector User Manual

CLUE Metal Detector in CircuitPython

Created by Kevin Walters

Last updated on 2021-02-08 06:48:17 PM EST

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Guide Contents

Guide Contents Overview

Parts

CLUE version Circuit Playground Bluefruit with TFT Gizmo version Circuit Playground Bluefruit only version Common

Design

Inductance and Permeability Metal Detection

Beat Frequency From An Oscillator Pair RLC Filters Charging a Capacitor with RLD Continuous Charge/Discharge with RLD Two Coil Systems

CLUE Metal Detector

Microcontrollers vs Inductors

Microcontrollers vs Inductors GPIO Protection Larger Coil Currents

Circuit

Components Coil Construction Circuit Construction

CLUE board Circuit Playground Bluefruit with TFT Gizmo Circuit Playground Bluefruit board only

CircuitPython on CLUE

Set up CircuitPython Quick Start!

CircuitPython on Circuit Playground Bluefruit Install or Update CircuitPython CircuitPython

Libraries

Libraries for Metal Detector for CLUE Libraries for Metal Detector for Circuit Playground Bluefruit with TFT Gizmo

Development Testing

Metal Detector

Example Video Troubleshooting Operation Code Code Discussion

Voltage from ADC Values Using Global Variables in Python Positional Arguments Practical Issues with displayio Graphics Filters with and without ulab Library Magnetometer Baseline and Code Reviews

ADC Analysis

Voltage across Capacitor in the Metal Detector

Going Further

Ideas for Areas to Explore Related Projects Further Reading

Overview

This project creates a metal detector using an Adafruit CLUE with a few common components and an

easy-to-make coil.

The program is written in CircuitPython for version 5.1.0 or later. The code also runs on the Circuit

Playground Bluefruit (CPB) with the TFT Gizmo screen. The program can be used without a screen on the

CPB in audio/light mode only.

Alligator clips to male jumpers can be used with or without the Adafruit Dragontail to connect the CLUE

and the coil to the breadboard. The pictures feature alternate products.

This project was inspired by an old Ray Marston book featuring a metal detector project and the

Detectorists (https://adafru.it/L7d) BBC TV series.

Parts

CLUE version

1 x Adafruit CLUE

Adafruit CLUE - nRF52840 Express with Bluetooth LE

1 x Adafruit DragonTail

Adafruit DragonTail for micro:bit - Fully Assembled (or use 3 Alligator Clip to Male Jumper Wires)

1 x 1k Resistor

Through-Hole Resistors - 1.0K ohm 5% 1/4W - Pack of 25 (1 needed)

Circuit Playground Bluefruit with TFT Gizmo version

Add to Cart

Out of

Stock

Out of

Stock

1 x Circuit Playground Bluefruit (CPB)

Circuit Playground Bluefruit - Bluetooth Low Energy

1 x Circuit Playground TFT Gizmo (LCD Screen)

Circuit Playground TFT Gizmo - Bolt-on Display + Audio Amplifier

2 x STEMMA 3-Pin to Male Header Cable

STEMMA JST PH 3-Pin to Male Header Cable - 200mm

Circuit Playground Bluefruit only version

1 x Circuit Playground Bluefruit (CPB)

Circuit Playground Bluefruit - Bluetooth Low Energy

1 x 1k Resistor

Through-Hole Resistors - 1.0K ohm 5% 1/4W - Pack of 25 (1 needed)

Common

1 x USB 1m cable - A to Micro-B

USB cable - USB A to Micro-B - 3 foot long

1 x Half-size breadboard

Half-size breadboard

1 x Alligator Clip to Male Jumper Wires

Small Alligator Clip to Male Jumper Wire Bundle - 6 Pieces (2 needed for coil)

1 x Male/Male Jumper Wires

Premium Male/Male Jumper Wires - 40 x 3" (75mm)

1 x Signal Diode

1N4148 Signal Diode - 10 pack (1 needed)

1 x 0.1uF Capacitor

0.1uF ceramic capacitors - 10 pack (1 needed)

1 x 36ft (11m) Wire

Enameled Copper Magnet Wire – 11 meters / 0.1mm diameter (5-8m of most insulated wire will work fine)

Add to Cart

Out of

Stock

Add to Cart

Out of

Stock

Add to Cart

Out of

Stock

Add to Cart

1 x 3 x AAA Switched Battery Holder

3 x AAA Battery Holder with On/Off Switch and 2-Pin JST (if you want to be mobile!)

1 x AAA Batteries

Pack of 3

Add to Cart

Design

Inductance is a key part of many technologies in daily life, for example:

charging - electric toothbrushes, the latest smartphones and some wireless, in-ear headphones;

heating - induction cooking (https://adafru.it/L7e) with metal cookware;

communication - contactless smartcards using NFC (https://adafru.it/L7f), RFID (https://adafru.it/L7A)

tags and traditional tuning circuits for radios;

power supplies - transformers (https://adafru.it/L7B) reduce the mains AC voltage to a more practical

level;

metal detection - airport security, automatic car park exit gates, pipe/cable finders and hunting for

treasure.

Leon Theremin's The Thing (https://adafru.it/L7C) is an interesting, minimalist example of a resonant cavity

microphone, the equivalent of using an inductor for L (https://adafru.it/L7D)C (https://adafru.it/L7E) tuning,

an application of band-pass filtering (https://adafru.it/L7F).

Inductance and Permeability

A current flowing produces a magnetic field around it. Inductors are electrical components designed to

store energy in that magnetic field. These are typically coils and often wrapped around a core. The

magnetic field can be affected by:

the material it passes through, this property is referred to as magnetic

permeability; (https://adafru.it/L8a)

the presence of a conductor nearby changing the effective inductance of the coil from the induced

eddy currents (https://adafru.it/L8b) in that conductor creating their own magnetic field;

other magnetic fields.

These first two properties make the inductor useful for detecting conductive objects.

MAKE Presents: The Inductor (https://adafru.it/L8c) is an excellent video introduction to inductors by Collin

Cunningham (https://adafru.it/L8d).

Metal Detection

The effect of nearby conductors on an inductor makes them a useful component for detecting metal. A

classic implementation of this in electronics uses heterodyning (https://adafru.it/L8e) where the beat

frequency from mixing an inductor-based search oscillator with a reference oscillator is output to

headphones.

Beat Frequency From An Oscillator Pair

The schematic on the left from R.M. Marston's

20 Solid State

Projects For The Home (1969)

shows a transitor-based

detector with two colpitts oscillators (https://adafru.it/L8f). One

oscillator uses the search coil and the other a tuneable

reference coil which the users adjust to reduce the beat

frequency audio output to near 0Hz away from the target

material.

RLC Filters

Filters can easily be created with a resistor (R), an inductor (L) and a capacitor (C). There are a variety of

configurations of RLC filters (https://adafru.it/L8A) and many of them could be used to filter the square

wave output from a microcontroller which could then be sampled to check the attenuation of the filter

which would vary with the inductance.

An initial test of this approach with an Adafruit CLUE and a low-pass filter didn't yield promising results.

The plots below show theoretical plots for a band-stop (notch) filter made with a resistor and a parallel LC

circuit which might be worth exploring.

The lower resistor values might not be practical as they put a higher current demand on the GPIO port.

The annotated linear plot below is better for seeing how this attenuation could potentially be used to

detect small variations in inductance.

This would require sampling the 989Hz signal to determine the attenuation by the filter. A high

inductance is attractive here as it will lower the frequency making the determination of the attenuation

more accurate.

A frequency sweeping approach is an alternative for finding the frequency of the filter. This is likely to be

slower but it would be less ambiguous. A simple measurement approach at one frequency, say 2.741V,

corresponds to

two

frequencies and therefore two different inductance values.

Charging a Capacitor with RLD

An Arduino-based project on Instructables (https://adafru.it/L8B) uses an RL circuit (https://adafru.it/L8C)

with the output rectified with a diode which then charges a capacitor. The steps in the measurement of

the inductance are:

1. A few pulses are output through the circuit to charge the capacitor. A higher inductance will result in

a higher final voltage across the capacitor.

2. An analogue input then measures the capacitor's voltage with over-sampling aiming to improve the

accuracy.

3. The analogue input is changed momentarily to

output mode

to empty (sink (https://adafru.it/L8D)) the

charge from the capacitor.

A C++ program (sketch) on the Arduino Uno offers precise timing. This is essential for this approach to

give accurate results for the inductance.

In CircuitPython, the pulseio library (https://adafru.it/L8E) can be used for creating PWM signals and pulse

trains with microsecond precision. In general, as an interpreted language with garbage collection, it does

not offer precise timing. The unpredictable delay between step 1 and step 2 is likely to affect the final

accuracy of the measurement causing sporadic, spurious indications.

Continuous Charge/Discharge with RLD

The previous approach can be used in a continuous fashion where a constant series of pulses flow

through the RLD. A circuit diagram from the Falstad Circuit Simulator (https://adafru.it/L8F) is shown below.

This design could be considered as an RL filter with an envelope detector (https://adafru.it/L9a).

The value of the capacitor affects how quickly it discharges. A

tiny capacitance will cause a rapid discharge causing a ripple

which may reduce the accuracy or complicate the voltage

measurement. A large capacitor value will take time to charge

and discharge and this could make the sensing unresponsive.

A value of 0.1uF (which can be written as 100nF) was chosen

from experimental testing. For comparison, a simulation with

100pF (top left) shows a very undesirable 197mV of ripple

whereas 0.1uF only has ~1mV ripple.

A small amount of steady voltage drop around 1mV is actually useful here to ensure over-sampling is an

effective technique to improve the resolution. In the (unlikely) absence of noise or variation, a

theoretically

perfect

analogue to digital converter (ADC) (https://adafru.it/eYp) would output the same value repeatedly

for a constant voltage. The ADC Analysis (https://adafru.it/L9b) page takes a closer look at this.

Two Coil Systems

Modern metal detectors using the induction balanced approach use two, often partially overlapping

search coils. One is used for transmitting and one for receiving. A relatively small overlap will create a

section with increased sensitivity. These detectors can discriminate to some extent between metals by

reporting on the phase difference (https://adafru.it/L9c) between the transmitted and received signal. This

is typically presented to the user as a numerical value with different ranges giving an approximate

identification. Garrett's chart for their AT Pro metal detector is shown below.

CLUE Metal Detector

Some initial testing of the Continuous Charge/Discharge with RLD approach worked well so this was

selected for the project.

The CLUE has an onboard LIS3MDL, a triple-axis magnetometer. This is a useful addition for finding

magnets and magnetised items.

Microcontrollers vs Inductors

Inductors can generate high voltages which may exceed the desired levels in a circuit. The video above

shows a single-cell battery connected to an inductor (top right) in series with three white LEDs. The white

LEDs require over 9V to illuminate but a mere 1.5V battery is able to briefly illuminate them due to the

inductor's effect.

In this case the red wire is being used to briefly short across the non-conducting LEDs to allow current to

flow from the battery through the inductor. The inductor is storing energy in its magnetic field and this field

products the momentary higher voltage as the red wire is removed from the circuit. This demonstration of

voltage spikes suggests care is required when using inductors in circuits to keep voltage levels at normal

levels to avoid damaging sensitive components.

TDK (https://adafru.it/Lb1), a company founded on the invention of ferrite (https://adafru.it/Lb2), offers an

explanation of this below with a parallel version of the circuit lighting a 70V neon

lamp (https://adafru.it/Lb3) from a 4.5V battery. This is from TDK's The Wonders of Electromagnetism:

Power Inductors in Mobile Phones (https://adafru.it/Lb4).

GPIO Protection

The general-purpose input/output (GPIO) (https://adafru.it/Lb5) pins on microcontrollers typically have

some limited protection built-in for adverse voltages often to deal with static electricity

(ESD (https://adafru.it/Lb6)). The CLUE board uses an nRF52 series chip and this has two internal

diodes (https://adafru.it/Lb7) on each GPIO pin. The partial schematic below shows an example of how

these these two diodes are used for one pin.

The schematic shows the CLUE board's 1 Megaohm resistor. There's one resistor per large pad used for

the capacitive touch (https://adafru.it/Lb8) implementation. The schematic also shows an

external

resistor.

This is another precaution that's typically used to limit output current but it will also reduce any current

flowing through these

very small

, protective diodes in the microcontroller.

The metal detector circuit on the next page uses a resistor

primarily to limit the current from the P1

output

but it will also

reduce any adverse currents from under or over voltages

caused by the inductor.

The square wave (3.3V pk-pk (https://adafru.it/oDb), 84% duty

cycle (https://adafru.it/Lb9)) can be seen with and without the

inductor in the circuit here. The inductor does cause a small

negative

voltage which briefly "peaks" at -0.6V on the P1

pin/pad. The magnitude and brevity of this spike and the

current protection from the external 1k resistor mean the

microcontroller is not at risk.

Larger Coil Currents

If more current was being used through the coil then an external protection diode capable of handling this

higher current would be a wise precaution. The CLUE's nRF52840 can only supply low currents, higher

currents would need a separate power supply and switching with a transistor. This could aid isolation of

the GPIO from the maleffects of the voltage spikes.

Diodes are commonly found across motors (https://adafru.it/CkQ), relays and solonoids protecting against

back EMF (https://adafru.it/Lba) and are sometimes referred to as "flyback" diodes (https://adafru.it/Lbb).

Circuit

This page describes how the components are used on the breadboard to make the circuit for the metal

detector. It also describes how to make and connect the coil.

Components

The components in the circuit are:

R1 - 1k resistor.

D2 - 1N4148 signal diode (there is no D1).

C1 - 0.1uF (100nF) ceramic capacitor. These small capacitors are often labelled "104".

L1 - home-made coil.

The prototype was made with a 1N4004 rectifier diode and also tested with a germanium diode from a

crystal radio set, both worked well and could be used as alternatives to the 1N4148 diode.

Coil Construction

A coil with about 4-8m (13-26ft) is a good starting point to avoid using too much wire. Insulated wire will

work but "enamelled" copper wire (https://adafru.it/Lbc) allows a more compact coil. The enamel is a

misnomer, the coating will be something like polyurethane varnish. This insulation must be

scraped

or

burnt off

with a soldering iron at the ends to expose the copper to connect it to the circuit.

The coil shown at the top of the page is enamelled 0.56mm wire wrapped around an 84mm tube (3.3in). It

has 12 coils then 9 more coils over those then 7 coil more coils over those totalling 28. Placing the coils

close to the edge improves the effective search range but care needs to be taken to ensure the coil does

not fall off! A tiny ridge has been made on end of the tube with masking tape to reduce that risk.

The coil either needs to be very tight or held in place as movement of the wire in the coil will subtly affect

the inductance and parasitic capacitance (https://adafru.it/Lbd) of the coil.

A prototype coil was also made (not shown) with 20 turns around a core of a roll of masking tape with

diameter 116mm (4.6in). This worked well too.

Circuit Construction

The diagrams and pictures below show how the circuit can be implemented on a breadboard for the three

different configurations.

CLUE board

Adafruit CLUE Metal Detector User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual