In our exploration of BFO technology, it was discovered that the presence of a metal target was capable of altering the inductance of the search coil. In order to detect this small change in inductance, a reference oscillator was used to produce a beat tone that could be heard in a set of headphones. Perhaps you have been wonder-ing whether there are any alternative methods that could be used to detect this change in inductance, and thereby eliminate some of the instability problems of the basic BFO design. If so, then you would be correct.
There is a type of metal detector known as “off-resonance”, which uses a differ-ent technique to sense the change in inductance. An oscillator is used to drive the search coil (similar to the BFO) but via a high resistance, and the output voltage is rectified before being applied to the input of a comparator, where it is compared to a reference voltage.
In some “off-resonance” designs, the rectifier output is applied directly to the input of a voltage-controlled oscillator (VCO) (Figure 4-1) so that the operator can effectively “hear” the change in inductance caused by the presence of a metal target.
This method has the advantage of providing some elementary ferrous/non-ferrous dis-crimination. Unfortunately there is little (if any) improvement in performance/depth over the standard BFO. Perhaps the only real advantage is the simplicity of imple-menting this type of design, given that it only uses one tuned oscillator.
However, there is one very good use for the “off-resonance” detector — as a simple handheld probe, where performance/depth is not an issue. Also, the ability to compare the search oscillator output voltage to a reference source, and give a yes/no indication of a metal target, is a distinct advantage. The reason why this method works is very simple. The search oscillator is designed to produce a sine wave output
Fig. 4-1: “Off-resonance” Metal Detector Block Diagram
Rectifier VCO Speaker Search
Coil
Search Oscillator
“It was all too complicated and, where it is too complicated, it meant that someone was trying to fool you.”
— Terry Pratchett (The Fifth Elephant)
that has an amplitude which varies with frequency. Therefore, we only need to moni-tor the amplitude of the oscillamoni-tor, rather than directly comparing its frequency to a reference source.
Search Oscillator
The BFO in Chapter 3 was designed to oscillate close to 100kHz. If you have constructed this circuit, it will have become apparent that the discrimination capabili-ties are less than perfect. The main culprit is our old friend the “skin effect”, which tends to make all metal targets look alike regardless of their thickness. In order to combat this problem, the “off-resonance” detector uses a much lower frequency, which allows the signal to penetrate the target below the surface layer. Search fre-quencies from 30kHz down to only a few kilohertz can be used in practice.
For this design we are going to choose an operating frequency close to 25kHz, and (just to be different) we will use a Hartley oscillator, instead of the Colpitts we implemented last time.
Since we are designing a probe circuit, there is no need for a large search coil. In fact, a large search coil would be a disadvantage, as the probe search head must be capable of being inserted into holes where a normal sized detector coil would not fit.
If we also choose to use two standard RF chokes (similar to the one used in the refer-ence oscillator of the BFO) then it just so happens that these will fit quite nicely into the end of the oval pvc tubing that is readily available from most DIY stores. Hence we have a simple and sturdy construction for the probe head.
The RF chokes used in this design were both 1mH. Note that the 4.7mH choke used in the BFO reference oscillator has too large a diameter to fit within the tubing.
Next, (referring to Figure 4-2) we need to calculate a value for C1 that will provide an operating frequency of approximately 25kHz.
The Hartley oscillator equation is:
Eq 4-1
Fig. 4-2: Hartley Oscillator
L2 R2
C1
R1 +V
Q1
L1
C2
Search
Coil #1 Search
Coil #2
f 1
2 L1 L2 + C
---=
Which can be rearranged as follows:
Eq 4-2
For a frequency of 25kHz, and L1 = L2 = 1mH, then C = 20.3nF.
If we decide to use the nearest standard capacitor value of 22nF, we can recalcu-late using the first equation to arrive at an operating frequency of 24kHz. The final circuit for the search oscillator is shown in Figure 4-3.
You may have noticed that the biasing arrangement for our Hartley-based oscilla-tor is rather unusual. The purpose of R2 and C2 is to cause the bias voltage of Q1 to be dependent on the oscillator frequency. Consequently the bias voltage on Q1 is sen-sitive to any changes in the inductance of L1 or L2, with the result that the collector voltage of Q1 will vary in amplitude. This means that a decrease in inductance gives an increase in frequency and also an increase in amplitude at the collector of Q1.
Likewise, an increase in inductance will result in a decrease in frequency and a decrease in amplitude.
For the circuit values shown, the cutoff frequency should be near to 28kHz, but in practice it is slightly higher (33.9kHz) due to the combined effects of the other com-ponents, layout, and component tolerances; whereas the search oscillator is designed to resonate at 24kHz. Hence the name “off-resonance”.
At this point we could simply rectify the oscillator output, and feed this DC sig-nal into a comparator (for comparison against a reference voltage) before driving an audible or visual warning device. Although this would be one possible solution, there are a number of potential issues to consider. Firstly, the oscillator is liable to drift due to changes in component tolerances, or simply due to temperature. Secondly, we would also have the problem of supplying a stable reference voltage. The reality is that these “drift” problems means the user is constantly having to make manual adjustments to compensate.
Fig. 4-3: Search Oscillator for Probe
L2
Automatic Tracking Circuit
What if there was some way to automatically track these slow moving changes, such that the user never has to make an adjustment?
Let us first consider the circuit shown in Figure 4-4:
The purpose of the envelope follower is to measure the amplitude of the search oscillator. Depending on the value of C3, you can change the time constant of the cir-cuit such that the output of IC1b “follows” the amplitude at either a faster or slower rate. This circuit is a modified version of a precision peak detector, but without the complication of providing a reset circuit to discharge the capacitor. Also, note that the opamp is being driven from a single-ended power supply, and therefore D1 and R3 are needed to block the negative part of the waveform that is fed to the non-inverting terminal of IC1a. It can then readily be seen that IC1a, D2 and C3 would form a sim-ple peak detector, if the cathode of D2 were connected to the inverting input of IC1a.
The opamp IC1b (unity gain amplifier) was added as a buffer to prevent the sensitiv-ity pot (R7) from discharging C3. Since there is no reset circuit in this design, the charge on C3 gradually leaks away due to the input bias current of IC1b. The design uses two of these envelope followers, one with a fast time constant (C3 = 10nF) and another with a much slower time constant (C4 = 10uF). By feeding these signals into a comparator, the slow tracker will act as a reference signal for the fast tracker, effec-tively eliminating problems due to drift, and creating a simple switch-on-and-go detector. Whenever a metal target comes within the influence of the search coil, the fast circuit will respond first; and the voltage at the inverting input of the comparator will rise above the voltage at the non-inverting input (i.e. the output from the slow tracker). This will cause the comparator output to go low, which can then be used to activate a warning device.
The output of the LM311 comparator can drive loads referred to ground, the pos-itive supply or the negative supply. In the case of the Probe design, the comparator is being operated in the single-supply mode, and pin 1 (which is effectively the refer-ence) needs to be connected to ground (same as pin 4).
If we now combine all these parts of the design together, the result will be as shown in Figure 4-5.
R1 should be adjusted for maximum amplitude at the collector of Q1. Finding the correct operating point will also ensure that the oscillator starts reliably when the probe is turned on.
Fig. 4-5: “Off-Resonance” Probe Circuit
R2 56k C1 22n
R1 5k
+V Q1 2N3906 L1 1m
C2 100p
D2 1N4148 C3 10n D3 1N4148 C4 10u
R3 100k
D1 1N4148 R7 1k
C5 100p
+ R4 2k2 R6 100 IC4 78L05
IC3 LM311 +V VB1 9V
C6 100u
+V
+V +V
2 3
1
8 4
6 5
7 2 3
1
8 4
6 5
7
3 2
7
8 4 L2 1mSearch Coil
IC2b CA3240 IC2a CA3240
IC1b CA3240 IC1a CA3240 SW1
1
The threshold level can be adjusted using R7. After setting this multi-turn poten-tiometer. there will be no need for further adjustment, since any variations due to cir-cuit tolerance, temperature or supply voltage will be automatically handled by the combination of slow and fast tracking circuits. Note that although the slow tracking circuit is much slower than the fast one, it will eventually catch up. Which means the LED will turn off if you hold the search head next to a metal target for a sufficiently long time. You can also add a piezo buzzer across D4/R5 to provide a quite loud audio response when a metal target is detected. It must be the type that beeps when you con-nect it to a battery, and not the piezo speaker type.
IC4 is an LM78L05 3-terminal regulator that provides a stable 5V supply, giving a very long life from a single PP3 9V battery.
The on-off switch is a SPDT type that connects the negative terminal of the bat-tery to the system for the “on” position, and resets the slow tracking circuit in the
“off” position. This allows C4 to be quickly discharged via R6 when the unit is turned off, thus resetting the threshold. Figure 4-6 shows the completed prototype (built on Stripboard), together with a close-up of the probe head assembly containing the two 1mH RF chokes. The complete circuit will fit comfortably within a handheld ABS case that features a battery compartment suitable for one PP3 battery. The case used for the prototype shown also included a set of clip-in battery terminals that were sup-plied as standard.
Fig. 4-6: Completed Unit
There is little point to be gained in compiling a chart of metal targets against dis-tance, given that the probe head is a miniscule 6 x 12mm. Typical detection distance for all targets is between 1” and 1.5”, which is perfect for this application.
Experimenting with the coils wound on a ferrite rod can provide good results, and you may find it interesting to try some larger coils as well. There is no need to place both coils inside the search head. L1 can be a choke placed inside the enclosure, and L2 can be a mono loop in the search head. However, if both coils are placed close together (as in the original prototype) then the coils need to be in anti-phase, other-wise the oscillator will fail to start.
The search oscillator could also be redesigned using a Colpitts, so that the search head requires only a single coil without a center tap.
This design has a number of advantages for the beginner who wishes to construct their first detector, such as:
1.Simple search head construction
2.Simple to build and setup
3.Absolute minimum of adjustment
4.Switch-on and go
5.Very stable design
6.Low power consumption
Construction Details
The component placement and PCB layout are for illustrative purposes only. You may need to adjust the layout to accommodate components available in your area. In particular, please note that transistor pinouts can vary depending on the country of manufacture, even for what appears to be an identical part number. See Chapter 13 for more details.
The PCB layout view (Figure 4-7) is from the underside of the board. The actual size of the PCB is 2.2” x 2.5” (55.9mm x 63.5mm). Note that there are 2 jumpers required in this layout. The parts placement (Figure 4-8) is shown from the top-side.
The 3-dimensional view (Figure 4-9) provides an idea of what the final product will look like in real life. Note the connectors are designated as follows:
PL1 = search coils
PL2 = SW1a
PL3 = SW1b Parts List
Resistors: (5% 1/4W)
R1 5k multi-turn pot
R2 56k
R3 100k
R4 2k2
R5 150
R6 100
R7 1k multi-turn pot
Capacitors:
C1 22n
C2, C5 100p
C3 10n
C4 10u elect., 10v
C6 100u elect., 10v
Inductors:
L1, L2 1m RF choke
Diodes:
D1, D2, D3 1N4148
D4 LED
Transistors:
Q1 2N3906 (or any general purpose PNP)
ICs:
IC1, IC2 CA3240
IC3 LM311
IC4 LM78L05
Misc:
SW1 SPDT switch
Fig. 4-7: PCB Layout (single-sided)
Fig. 4-8: PCB Parts Placement
Fig. 4-9: PCB 3D View
CHAPTER 5