Pulse Induction Metal Detectors

Improved Pulse Metal Detector

Improved Pulse Metal Detector

Like metal detectors of other types, Pulse Induction (PI) metal detectors are continuously being improved. The application of new circuitry solutions has led to even higher sensitivity in these devices. For example, sensitivity of the metal object detector described in the previous section can be enhanced.

In the author’s opinion, the design of the proposed device, similar to the metal detector discussed in the previous section, is sufficiently complex for beginners in amateur radio to replicate. Additionally, certain difficulties may arise during the adjustment of this device. It is crucial to note that errors in assembly and incorrect adjustment of the device can lead to the failure of expensive components.

The device considered in this section uses a microprocessor with corresponding software. Unfortunately, at the time of publishing this book, it was not possible to provide a 100% functional version of the firmware. Therefore, interested and prepared readers have the opportunity to test their skills in creating firmware for the microcontroller.

Schematic Diagram

The schematic diagram of the improved pulse induction metal detector can be conventionally divided into two parts, namely: the transmitter block and the receiver block. Unfortunately, the limited scope of this book does not allow for a detailed discussion of all the features of the circuit solutions used in creating this device. Therefore, the basics of the operation of only the most important nodes and cascades will be considered further.

As mentioned earlier, this metal detector is an enhanced version of the device discussed in the previous section of this chapter. Certain changes have been made to the pulse formation and synchronization module, transmitter, and voltage converter. The schematic of the receiver block has undergone more significant changes (Fig. 3.18).

Fig. 3.18. Schematic diagram of the transmitter block of the improved pulse induction metal detector

The transmitter block consists of the pulse formation and synchronization module, the transmitter itself, and the voltage converter.

The main component of the entire structure is the pulse formation and synchronization module, implemented on the AT89S2051 microprocessor by ATMEL. This module is responsible for generating pulses for the transmitter and signals that control the operation of all other blocks. The operating frequency of the microcontroller IC1 is stabilized by a quartz resonator (6 MHz). At the specified frequency, the microprocessor generates a periodic sequence of control pulses for various cascades of the metal detector.

Initially, a control pulse for transistor T6 is generated at the IC1/14 microprocessor output, and after its completion, a similar pulse is generated at the IC1/15 output for transistor T7. This process is then repeated. As a result, the voltage converter is initiated.

Next, pulses to start the transmitter are sequentially generated at the IC1/8, IC1/7, IC1/6, IC1/17, IC1/16, and IC1/18 outputs. These pulses have the same duration, but each subsequent pulse is delayed relative to the previous one by several clock cycles. The beginning of the first pulse generated at the IC1/8 output coincides with the middle of the second pulse at the IC1/15 output. Using the P1 switch, you can select the delay time of the transmitter start pulse relative to the starting pulse.

A short strobe pulse for the amplifier-analyzer is formed at the IC1/2 output a few clock cycles after the end of the pulse at the IC1/18 output. In contrast to the previously considered scheme, in this device, a second strobe pulse is generated on the same microcontroller output a few clock cycles later.

In addition, control signals for transistors T31 and T32 of the receiver block are generated at the IC1/12 and IC1/13 outputs of the microprocessor. The middle of the control pulse for transistor T31 coincides with the middle of the first strobe pulse at the IC1/2 output, but the duration of the pulse at the IC1/12 output is almost twice as long. This pulse has a negative polarity. The beginning of the control pulse signal at the IC1/13 output almost coincides with the middle of the second pulse at the IC1/14 microprocessor output, ending a few clock cycles after the end of the second strobe pulse formed at the IC1/2 output. Then, a control signal for transistor T35 of the acoustic signaling circuit of the receiver block is generated at the IC1/11 output.

After a short pause, the sequence of control pulses on the corresponding outputs of the microcontroller is generated again.

A +5 V power supply, previously stabilized by the IC2 chip, is supplied to the IC1/20 microcontroller output.

The voltage converter, implemented with transistors T6-T8 and the IC3 stabilizer, provides the formation of a +5 V power supply necessary for powering the receiver section. Control signals for transistors T7 and T8 are generated at the corresponding outputs of the microcontroller IC1, with the signal to transistor T8 passing through a level converter assembled on transistor T6. The generated power supply voltage is then stabilized by the IC3 chip, from which the +5 V voltage is supplied to the receiver cascades.

The output cascades of the transmitter are built on powerful transistors T1, T2, and T3, operating on a common load, represented by coil L1, shunted by a chain of resistors R1-R6. The operation of the output cascade transistors is controlled by transistor T4. The control signal to the base of transistor T4 is supplied from the corresponding output of the IC1 processor through transistor T5.

Similar to the metal detector discussed in the previous section, the pulse generated by the IC1 microprocessor, according to the program stored in its memory, is applied to the input of transistor T5 via a switch, and then, through transistor T4, to the output cascades of the transmitter built on transistors T1-T3, and then – to the receiving-transmitting coil L1. When a metallic object enters the detection zone of coil L1, its surface is excited by the external electromagnetic field initiated by the transmitter’s pulse, inducing eddy currents. The duration of these currents depends on the duration of the pulse emitted by coil L1.

Eddy currents serve as the source of a secondary pulse signal, which is received by coil L1, amplified, and fed into the analysis circuit. Due to the phenomenon of self-induction, the duration of the secondary signal will be greater than the duration of the pulse emitted by transmitting coil L1. The shape of the secondary pulse signal depends on the properties of the material from which the detected metal object is made. Processing information about the differences in the parameters of the pulses emitted and received by coil L1 provides data for the indication block about the presence of a metal object.

The receiver block (Fig. 3.19) includes a two-stage input signal amplifier, sample signal amplifiers, an amplifier-analyzer, an active narrow-band filter, a low-frequency filter, a bias voltage generation scheme, switching schemes, and a sound indication scheme.

Fig. 3.19. Schematic diagram of the receiver block of the improved pulse induction metal detector

The signal from the metal object is received by coil L1 and, through the protection circuit consisting of diodes D1 and D2, is fed to the input two-stage amplifier with capacitive feedback, implemented on operational amplifiers IC31 and IC32. The amplified pulse signal from the IC32 chip (pin IC32/6) is fed to the amplifier-analyzer, implemented on the IC33 chip.

During the operation of the device, amplifier IC33 is constantly off, and power is supplied to it only upon the arrival of the corresponding strobe pulses at its input (pin IC33/8). After the power supply voltage is cut off, the received signal level, fixed during the action of the strobe pulses, is maintained at the amplifier output (pin IC33/5) for several seconds. The duration of signal retention depends on the capacitance of capacitor C65. Thus, the received pulse signal is applied to one input of the amplifier (pin IC33/3), and the second input (pin IC33/8) receives the corresponding strobe pulse from the pulse formation and synchronization module (pin IC1/2) through capacitors C64.

Next, the selected signal passes through an active filter implemented on the IC34a element, tuned to a frequency of 6 MHz. To achieve the specified parameters on the schematic for individual elements of this filter, it is recommended to use parallel connection of resistors and capacitors. For example, the specified capacitance value of capacitor C67 (0.044 µF) is achieved by parallel connection of two capacitors with a capacitance of 0.022 µF each. It should be noted that when using a quartz element Q1 with a working frequency different from 6 MHz, the values of individual filter elements should be recalculated.

The signal from the filter output is fed to a synchronous detector, where an inverting amplifier with a gain factor of 1, implemented on the IC34b element, is installed at the input. The closure of the corresponding pairs of contacts of the IC37 chip (pins IC37/1,2 and IC37/3,4) is used to switch the negative signal supplied to the integrating circuit with capacitor C71. Control signals for the IC37 chip are generated by cascades implemented on transistors T31-T33.

The pulse signal from the integrating circuit is then applied to the input of an amplification cascade, implemented on the IC35 chip, which simultaneously functions as a low-frequency filter. The voltage drop at the output of the operational amplifier (pin IC35/6) opens transistor T34 and connects it to the common wire of the headphones BF1. When a corresponding signal is applied from the IC1 microcontroller output (pin IC1/11) to transistor T35, a control signal will be heard in the headphones at the sound frequency. Resistor R77 limits the current flowing through the headphones BF1, and its adjustment can regulate the volume of the acoustic signal.

The signal from the IC35/6 output is also applied to the input of another operational amplifier (pin IC36/2), whose task is to zero the output signal. Its use is explained by the fact that the changing output signal over time at the IC33 chip will be generated even in the absence of metallic objects in the detection zone, so the amplitude of the resulting signal will be non-zero. With the help of resistor R86, the bias voltage is applied to the input of the second amplification cascade (pin IC32/2) at the moment of the first strobe pulse. The required level of bias voltage depends on the level of the output signal at the IC35/6 pin and is generated using the integrating circuit C73, R78-R80, and the amplification cascade on the IC36 chip.

The bias voltage generation circuit operates only during the closure of the corresponding contacts of the IC37 chip (pins IC37/9,8). The duration of this time segment is three clock cycles. Control signals for the IC37 chip are supplied from cascades implemented on transistors T31-T33. Thus, the levels of signals formed at the moments of the first and second strobe pulses are aligned. By pressing the S2 button, the duration of the zeroing process can be significantly reduced.

Details and Construction

All components of the device under consideration (excluding the search coil L1, switch P1, switch S1, and button S2) are located on a printed circuit board (Fig. 3.20) with dimensions of 95×65 mm, made of double-sided foil-clad fiberglass or textolite.

Fig. 3.20. Printed circuit board of the improved pulse induction metal detector

There are no special requirements for the components used in this device. It is recommended to use any compact capacitors and resistors that can be easily placed on the printed circuit board. It should be noted that to achieve the specified parameters for individual elements on the schematic, it is advisable to use parallel connection of resistors and capacitors (Fig. 3.21). The printed circuit board provides additional space for placing such elements.

The LF356 type chips (IC31, IC32) can be replaced with LM318 or NE5534, but such a replacement may lead to adjustment problems. For the IC35 amplifier, in addition to the IL071 chip specified on the schematic, you can use CA3140, OP27, or OP37 chips. The RO61 type chip (IC36) can be easily replaced with CA3140.

  • Transistors T1-T3, in addition to those specified in the schematic, can be replaced with transistors such as BU2508, BU2515, or ST2408.
  • The operating frequency of the quartz resonator should be 6 MHz. You can use any other quartz element with a resonance frequency from 2 to 6 MHz, but in this case, you will need to recalculate the parameters of the filter elements, performed on the IC34a component.
  • For the installation of the microprocessor IC1, a special panel should be used. The microcontroller is installed on the board only after all assembly work is completed. This condition must be observed during adjustment work related to soldering when selecting individual element values.
  • Special attention should be paid to the manufacture of coil L1, the inductance of which should be 500 µH. The construction of this coil is almost identical to the construction of search coil L1 used in the metal detector discussed in the previous section. It is made in the form of a ring with a diameter of 250 mm and contains 30 turns of wire with a diameter not exceeding 0.5 mm. Using a larger diameter wire will increase the current in the coil, but the values of parasitic eddy currents will also increase, leading to a deterioration of the sensitivity of the device.
  • It is recommended not to use lacquered wire for winding coil L1 because the potential difference between adjacent turns during pulse emission can reach 20 V. If conductors, such as the first and fifth turns, end up close during winding, insulation breakdown is almost guaranteed. This can lead to the failure of the transmitter transistors and other elements. Therefore, the wire used in making coil L1 should be at least in polyvinyl chloride insulation. The finished coil is also recommended to be well insulated, for which epoxy resin or various foam fillers can be used.
  • Coil L1 should be connected to the board using a two-core well-insulated wire, and the diameter of each core should be no less than the diameter of the wire from which the coil itself is made. It is not recommended to use coaxial cable due to its significant inherent capacitance.
  • Headphones with a resistance of 8 to 32 ohms or a compact loudspeaker with a similar coil resistance can serve as a source of audio signals.
  • It is recommended to use a rechargeable battery with a capacity of about 2 Ah as the power source (B1) since the current consumed by this metal detector exceeds 200 mA.
  • The printed circuit board with the components and the power source can be placed in any suitable case. The switch P1, connectors for connecting headphones BF1 and coil L1, as well as the switch S1 and button S2, are installed on the case lid.

This device should be calibrated in conditions where any metallic objects are removed from the search coil L1 at a distance of at least 1.5 meters.

The peculiarity of adjusting and tuning the considered metal detector lies in the fact that its individual blocks and cascades are connected gradually. At the same time, each connection operation (soldering) is performed with the power source turned off.

First, it is necessary to check the presence and voltage level on the corresponding contacts of the IC1 microchip panel in the absence of the microcontroller. If this voltage is normal, then it is necessary to install the microprocessor on the board and use a frequency meter or oscilloscope to check the signal on IC1/4 and IC1/5 outputs. The frequency of the pilot signal on these outputs should correspond to the operating frequency of the quartz resonator used.

After connecting the voltage converter transistors (without load), the current consumption should increase by approximately 50 mA. The voltage on capacitor C10 without load should not exceed 20 V.

Then, connect the transmitter cascades. The operating modes of transistors T1-T4 should be the same and are set by adjusting the resistor values R13-R16.

The resistance of coil L1, shunted by resistors R1-R3, should be approximately 500 ohms. The coil and resistor connections must be well soldered since a contact failure in this circuit can lead to the failure of the transmitter output transistors.

To check the functionality of the transmitter cascades, hold coil L1 near your ear and turn on the metal detector. After about half a second (after the microcontroller is reset), you should hear a low-tone signal, the occurrence of which is due to the microvibration of individual turns of the coil. At the collectors of transistors T1-T3, an unmodulated sharp pulse with a duration of about 10–20 µs will be formed, the shape of which can be monitored with an oscilloscope. Increasing the resistance of resistors R1-R3 leads to an increase in the amplitude of the output pulse with a decrease in its duration. To adjust the resistance of the shunt across coil L1, it is not recommended to use a variable resistor, as even a momentary contact disruption between the engine and the conductive track can damage the output transistors of the transmitter. Therefore, it is advisable to gradually change the shunt resistance with a step of 50 ohms. Before replacing components, be sure to turn off the power to the device.

Next, you can proceed to adjust the receiving part. If all the components are in working order, and the assembly is error-free, then after turning on the metal detector (approximately 20 µs after the end of the start pulse), an exponentially increasing signal can be observed at the output of the IC31 microchip (IC31/6) using an oscilloscope, transitioning to a constant level signal. Distortions of the front of this signal are eliminated by adjusting the resistors R1, R2, and R3, shunting coil L1.

After that, check the shape and amplitude of the signal at the output of the IC32 microchip (IC32/6). The maximum amplitude of this signal is set by adjusting the resistor R64. During adjustment, the bias voltage at IC32/2 output can be supplied from a separate voltage divider, for which you can use a variable resistor with a nominal value of 5-50 kΩ, connected, for example, between IC32/4,7 outputs. The potentiometer knob is connected to resistor R86.

At the output of the IC33 microchip (IC33/5), you can observe a square wave signal, the amplitude of which is adjusted by a temporarily connected potentiometer. Then it is necessary to control the signals at the outputs of the elements IC34a and IC34b. At the outputs IC34/6,7, correct sine waves should be present. As a result, a constant voltage is formed on capacitor C71, which is fed to the input of the IC35 microchip.

During adjustment, you can observe the device’s reaction to changes in the position of the temporarily connected potentiometer, after which it should be replaced with the R84, R85 divider.

The operating procedure for the metal detector does not have significant differences from the use of the metal detector discussed in the previous section.

Before practical use of this metal detector, set the P1 switch to the minimum pulse delay. If a metallic object is found within the operating range of search coil L1 during operation, an acoustic signal will be heard in the headphones. Transitioning to the mode with a longer pulse delay ensures the exclusion of the influence not only of the magnetic properties of the soil but also eliminates the device’s reaction to various extraneous objects (rusty nails, foil from cigarette packs, etc.) and subsequent futile searching.

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