Search Coil E-MOSFET Colpitts Oscillator

 Here a search coil design and operataion using enhancement MOSFET based Colpitts oscillator is illustrated. The search coil is used as the inductor of a Colpitts oscillator. The Colpitts oscillator was build using 2N7000 N-channel enhancement MOSFET. To design the search coil, the oscillator and the enhancement MOSFET amplifier online calculators were used. Although other biasing method like drain feedback E-MOSFET biasing method could have been used here voltage divider biased E-MOSFET amplifier design was chosen because of stability of this biasing technique relative to other biasing methods. Search coil like this can be used for metal detection.

What is the frequency used by BFO? 

 BFO stands for Beat Frequency Oscillator and it is used in metal detectors. The frequency of the BFO is usually set to around 400 kHz. This is the frequency that detects objects made of metal and creates a signal. 

How does BFO work?  

BFO stands for Beat Frequency Oscillator and it works by creating a signal at a specific frequency and sending that signal through the search coil. The search coil then generates an alternating current which is amplified by the amplifier. This signal is then sent out, and when it encounters metal objects, it will generate a change in frequency. This change in frequency is detected and used to locate the object.

What is the search coil oscillator made up of?  

The search coil oscillator is made up of a search coil and an amplifier. The search coil is wound around an iron core and connected to an oscillator circuit. This circuit produces an alternating current, which is then amplified by the amplifier to create a signal that can detect metal objects.

Below picture shows the search coil Colpitts oscillator build on a breadboard. 

e-mosfet based colpitts oscillator search coil

The oscilloscope showed sine wave of 100KHz.

oscillator waveform on oscilloscope

The design starts with finding out the capacitor value and the inductor value for the Colpitts oscillator. To do this the online Colpitts oscillator calculator was used. Using the calculator we find that if the two capacitor values are 40nF then for 100KHz oscillation frequency the required inductor value is 126.65uH.

Colpitts oscillator calculator

Next we have to make the search coil of 126.65uH inductance. For this AWG 26 enameled copper wire of 0.4mm diameter wire was used. To calculate the number of turns required for 126.65uH inductance wound around with 10.5cm diameter ring the online Air Core Inductance Online Calculator was used.

air inductor calculator

The calculated number of turns was 25.61 but later on experimenting 27 tight turns gave 100KHz. So 27 turns was used to make the search coil. The following shows the search coil.

The next part is to design the N-channel E-MOSFET amplifier. For this the online Enhancement MOSFET Biasing And Amplifier Design Calculator was used. The following shows the calculated values for the resistors and capacitors for frequency of 100KHz, 5V dc supply, and various E-MOSFET parameter inputs.

e-mosfet amplifier design calculator

The circuit diagram of the designed search coil based Colpitts oscillator is shown below.

search coil e-mosfet colpitts oscillator design

 There are some difference between the calculated values and the actual values used. The calculator showed R1 of 2.50KΩ but a practical value of 2.7KΩ was used. The drain resistor was replaced by a 1mH RFC inductor. The coupling capacitors calculated by the calculator was not used instead both the input and output coupling capacitor of 0.1uF were used. The source resistor R3 calculated value was around 7Ω but during the testing it was found that a 56Ω gave 100kHz signal and so 56Ω was used. Also it was noticed that the oscillator without the bypass capacitor oscillated with higher output peak to peak voltage(5.86Vp-p). However a 10nF bypass capacitor was used as it give rise to smoother sine wave waveform.

The following video shows the enhancement MOSFET based Colpitts oscillator search coil in operation.

The following video shows simulation of the above designed search coil Colpitts oscillator in Multsim circuit simulator.

  So here it was shown how to design E-MOSFET Colpitts oscillator based search coil. Such search coils are used in metal detection for security and other application. They are used in beat frequency oscillator based metal detector. Colpitts oscillator is used for search coil instead of Hartley oscillator because it is easier to design the Colpitts oscillator as for using Hartley we need two inductors.

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