Questions on Pinpointer Frequency.

[ToddB66]

Greetings !

My questions on PP Frequency are as follows:

#1..What circuit components are used to create the frequency of a PP ?

#2.. How do you test to see if the frequency is correct; is an oscilloscope required or can frequency be checked by a less-expensive method ???

Thanks in advance for your help on these questions.

ToddB66

[Beenthereuk]

Are you asking about a specific pinpointer circuit or about frequency generators in general?

In general, there are many different ways to create pulsetrain (digital) or sinewave type signals. Such circuits are called multivibrators and oscillators. And there are many different kinds.

As for setting the frequency on any one of these, it may be possible using a direct method like playing it through an audio amp (if it is meant to produce a frequency in the audio range) but in practice, a scope is a must for any of this kind of work.

[ToddB66]

Questons on Pinpointer frequency.

Are you asking about a specific pinpointer circuit or about frequency generators in general?

In general, there are many different ways to create pulsetrain (digital) or sinewave type signals. Such circuits are called multivibrators and oscillators. And there are many different kinds.

As for setting the frequency on any one of these, it may be possible using a direct method like playing it through an audio amp (if it is meant to produce a frequency in the audio range) but in practice, a scope is a must for any of this kind of work.

Beenthereuk.......Thanks for your reply !

My earlier questions were of a general nature, but if you don't mind I would now like to get into specifics. I am a neophyte in electronics and my first project is to build a Velleman Metal Detector Kit #k7102 and morph it into a Pinpointer, adding a pvc probe-tube to a plastic enclosure box and then mount the ferrite coil inside the front end of the tube.

Attached is a diagram of the circuit and I have purchased an optional buzzer(Radio Shack #273-074, 3-16 ,4.1 VDC, 4.1 kHz, 7mA) to be integrated in the circuit, adding an audio signal to the LED visual.

I am very interested in building electronic gadgets and have purchased a Hakko FX-888D Soldering Station, Silver-Bearing Solder, De-soldering Wick/Braid and a small tin of Lead Free Tip-Tinner. In addition, I have an Archer thru-hole Experimenter Printed Circuit Board for soldering practice before beginning the metal detector kit. I also bought two books for study purposes, i.e. Inside the Metal Detector, by authors George Overton and Carl Moreland (Advertised on this website.) and Getting Started in Electronics, by author Forrest M. Mims III.

Now to my specifics. Looking at the attached circuit diagram, (a) can you briefly explain to me how the frequency is created, (b) what circuit component(s) is/are involved and (c) is the attached circuit a multivibrator or oscillator type ???

Aside from my multi-questions above, please feel free to offer any other comments you think would be helpful.

Again, thanks a-bunch !
ToddB66

[Beenthereuk]

That is an LC tank type oscillator, specifically, an Armstrong variant (inductively coupled feedback) and they produce a sinewave. See these for more on how they work:
https://www.youtube.com/watch?v=1NAESKov17w
http://www.allaboutcircuits.com/vol_2/chpt_6/2.html


L1 and C1 are the LC tank, T1 provides the gain to ensure oscillation, L2 couples in positive feedback from T2's emitter. T2 is configured as a common emitter amplifier with a max gain of around 200x and T3 lights an LED and sounds a buzzer on detection. The carrier produced is loaded when metal is brought near the coil because it sets up eddy current in the metal itself. This causes a level shift in the circuit which in turn alters its visible output. Although a simple circuit, I would still have to simulate it all to explain exactly how it works.


The frequency the oscillator runs at depends mainly on the inductance of the coil and the capacitor used with it but is also affected by other capacitances in the circuit which are not immediately obvious (e.g. intrinsic capacitances of the transistor used, parasitic capacitances due to wiring and pcb, etc). However these things tend to run anywhere from 100 kHz to a few MHz and so you would definitely need an oscilloscope to see the signal produced.


My best advice is to get a cheap scope off ebay (try to get one with a minimum 10 MHz bandwidth) so that you can see exactly what your circuits are doing and also, if you want to learn more, I tend to recommend Floyd's books to new students:


http://www.pearsonhighered.com/floyd/


Above all else, have fun!

[Davor]

It is a tuned collector (Armstrong) oscillator. Frequency is largely irrelevant as the mode of operation is detection of oscillation envelope change related to losses incurred by vicinity of metallic objects and the eddy currents in them. This kind of devices is often called "absorption" because the oscillator working amplitude is set by resistive means, and they react to just about any metal.
The other kind of amplitude detecting device is off-resonance, where oscillation happens somewhat away from the natural resonance of a tank due to the phase shift in the oscillator feedback. Such devices have discriminative purposes, and are not so common.

[ToddB66]

Questions on Pinpointer Frequency.

Beenthereuk and Davor........Thanks much ! You've given me a lot to think about and learn from.

Beenthereuk.....I saved Floyd's books to my Favorites list; would you mind suggesting a good one for me to start with ?

ToddB66

[Beenthereuk]

This is probably the best to start with: http://www.amazon.co.uk/Electronics-.../ref=pd_cp_b_2

His older book on devices is the next best IMHO: http://www.amazon.co.uk/Electronic-D...yd+electronics

And there just happens to be a PDF copy here: http://hristotrifonov.files.wordpres...n-by-floyd.pdf

Older versions of these are usually much cheaper...

[ToddB66]

Questions on Pinpointer Frequency.

Beenthereuk......I found a used hardcover of Thomas L. Floyd's Electronics Fundamentals, Circuits, Devices and Applications, Fifth Edition, copyright 2001, in Good Condition for $4.59 US, Free Shipping and no taxes on the Internet @ Thrift Books and ordered that on Sept.28, 2014. It was received in 4-days on Oct. 02, 2014.

I think this will more than serve my home study needs.

Thanks for getting me connected with Floyd !

ToddB66