Where does B- exactly go to on this schematic?

Where does B- exactly go to on this schematic? Beat Frequency Oscillator EASY




There are two 220uF capacitors near the top of the schematic in each corner. The negative leads are labeled B-

Where do they go?

Re: Where does B- exactly go to on this schematic? Beat Frequency Oscillator EASY

In Circuit B+ and B- refer to power Supply. If Battery is used then +ve terminal and -ve terminal should contact with B+ and B- points respectively.

In other word,
B+ is Power (Vcc, Vdd etc)
B- is Ground (GND, VSS)

Hope it will be helpful.

Re: Where does B- exactly go to on this schematic? Beat Frequency Oscillator EASY

mishrashashi said:

In Circuit B+ and B- refer to power Supply. If Battery is used then +ve terminal and -ve terminal should contact with B+ and B- points respectively.

In other word,
B+ is Power (Vcc, Vdd etc)
B- is Ground (GND, VSS)

Hope it will be helpful.

Click to expand...

Thanks. That is what I figured. I just didn't know if those negative leads (B-) needed to go on the negative rail in any specific location i.e. between two components.

I got the circuit to function the first time. The loudspeaker's frequency was around 10khz (just judging from experience). Anyways, the frequency started to drop, & as it was dropping, it sounded like the loudspeaker was distorting. Now I am getting no audible noise out of the loudspeaker. I have taken the circuit apart & put it back together 7 times already & still no function.

Try again tomorrow I guess

What are you expecting to get out of it? The schematic looks like it's a metal locator and is only designed to 'squawk' quietly. It isn't designed for low distortion or high volume (quite the opposite in fact!).

Brian.

betwixt said:

What are you expecting to get out of it? The schematic looks like it's a metal locator and is only designed to 'squawk' quietly. It isn't designed for low distortion or high volume (quite the opposite in fact!).

Brian.

Click to expand...

I expect an audible noise from the loudspeaker when I place a metallic object near the search loop coil. That is not what's happening. I am getting 0 audible noise, no matter how I adjust the reference coil.

*There's actually a nut that goes around the reference coil. This of course, can be moved up & down to change the frequency of the reference coil to MATCH the search loop coil's frequency. The closer you get the frequencies, the quieter the "squawk" gets. Go figure, it's a beat note. I can't get A note lol. I am getting static noise when I move the wires about.

You see where the schematic drops down on the lower left? I accidentally took one of the negative leads from one of the electrolytic caps & ran it there. Well, I got an audible noise! Of course, this was done by accident. I thought finally, I did something right! Then I corrected the circuit, hooking BOTH the negative leads from BOTH caps down on the negative rail & it worked like a champ. I got a much higher frequency & when I placed a metallic object near the search coil, the frequency climbed near 20khz ...just like it was suppose to.

Then the frequency started to drop & the loudspeaker started to crackle (distort) & I got no audible sound. The end, lol.

Posting a video tomorrow!

Both capacitors should have their negative ends (B-) joined to the bottom B- line which is also the battery negative side.

I suspect the problem is actually your battery voltage dropping. Without any bias stabilization on the loudspeaker driver it could be drawing quite a lot of current, certainly far more than a small battery could sustain for long and as the oscillators have no voltage regulation, their frequencies will be very dependant on the battery voltage. The circuit should only be used with crystal earphones or a high impedance loudspeaker (at least 64 Ohms) or high impedance headphones. If you are using a standard 8 Ohm loudspeaker it is more than likely passing far too much current.

I'm curious how you know the frequencies, are you using an oscilloscope / frequency counter or guessing it audibly. Very few people can hear 20KHz and even 10KHz is getting close to some peoples limit. For best results you should hear something around 1KHz which is approximately mid piano keyboard tone. The output is actually the difference between the two search coil frequencies which could be much higher.

Brian.

Assuming I had a bad capacitor, I started swapping them out individually. When I took the capacitor out (labeled green) I got an audible noise. I lack terminology, but I'll call it unwanted back around noise.

Then I took the capacitor out (labeled blue) & the frequency increased. -The frequency was so high, it was barely audible.

Put both the caps back in & I get no audible noise at all.

Not sure if that helps, but at least I know the device is functioning (Yes, I tested it out w/ metallic objects when I had the caps out)

I've spent the past few months on diyma & DIYaudio. I'm under 20 years of age.

I have a 3.5mm jack & a crystal set on hand & will test it out in the morning.

The driver I am utilizing is 16 ohm -I thought for sure his would be sufficient in terms of efficiency. It's a 1" full range parts express buyout.

EDIT: There really wasn't much of a difference in terms of dB/loudness/output when I swapped out the 16 ohm w/ an 8 ohm HiVi full range

Both those capacitor are there to filter out high frequencies although the filter design isn't very good. They are supposed to make the frequency response roll off somewhere in the mid audible range so the higher frequencies of the oscillators is less prevalent then the beat note between them. Removing them lets more of the oscillator signals and noise in the previous stages reach the output, that's why it seems 'busier'.

The whole design looks rather like it was put together from parts someone had lying around, the values are not chosen well and no attention was paid to stability. A crystal earpiece or high impedance headphones should work at the output but normal loudspeakers will be very quiet and seriously overload the amplifier. With no current stabilization, the output transistor can potentially pass as much DC current through the loudspeaker as it will allow, the resistance of a typical small 8 Ohm speaker is about 5 Ohms so from a 9V supply it could try to pass almost 2 Amps, about 100 times more than a small 9V battery is designed to produce. In practise the current wouldn't be as much as that but exactly what it would be is highly dependent on the characteristcs of the transistors. A good design would tolerate component differences far better.

Brian.

Crystal earpiece grants same results other than the sensitivity (greater loudness).

^Keep in mind this is with one or both of the noted caps removed. With the circuit complete, as it was intended, I'm still not getting any audible noise.

I'll post a video as soon as I can. In the mean time, I'll be on Youtube figuring out how to test caps, resistors, & transistors.

Thanks for the help. I have a lot to learn. My design would consist of a battery, single coil, & an ammeter lol

My internet connection is too slow to view Youtube video so I'll have to take your word for what is happening.

Firstly, you should reconnect both those capacitors, they are vital to correct operation. If I had it in front of me, the first thing I would check is the battery voltage, especially as it is likely you have put it under strain with the small loudspeaker. My next diagnostic step would be to check both the search coil oscillators were running and on roughly the same frequency. What you should hear is a beat note which is the difference of the two oscillator frequencies, for example, if one was at 25KHz and the other 26KHZ, you should hear a 1KHz beat tone. It's the imbalance between these caused by metalic objects that make the tone change but if one is wildly off frequency or not running at all, the beat will not be heard. If you haven't got test equipment there are some crude checks you can try so confirm they are working:
Disconnect one of the coils so its oscillator stops running. Place a portable AM radio close to the coils and tune around, you should pick up a strong but silent radio signal which changes tuning slightly if you touch one of the coils. Reconnect the coil and repeat the test using the other oscillator. If they both produce a signal, the chances are that one is just too far off frequency from the other to produce an audible difference.

If you want to improve the amplifier, use a small audio output transformer to connect the speaker rather than using it direct. It should make a volume difference of many times and reduce the current consumption as well. To be honest, the whole design could be greatly improved.

Brian.

Betwixt, you were correct.

The schematic calls for 10 turns of #30 AWG magnet wire for the search coil. If I can remember correctly, it's suppose to oscillate at 104khz.

I didn't tell you I actually used #24 AWG magnet wire instead of #30AWG. I can't imagine why this would be an issue, as long as I wrap BOTH reference AND search coil with the same gauge wire. Anyways, I ended up wrapping another 10 turns on the search coil giving me a total of 10 turns. Now I can hear audible noise from the loudspeaker.


Sorry about the back round noise! (My brother purchased an external mic for his Canon Rebel & the noise is horrible! Anyone?)


Some things I've noticed,

You want the coils to oscillate so close together that you can BARELY hear an audible noise from the loudspeaker. BUT, you need to make sure the noise is audible so you can detect any frequency change. SENSITIVITY! If the coils are oscillating too far apart, you'll find that the beat note will be higher in frequency & your "detection range" will decrease dramatically ...& that's not good when the device is only good for about 5" lol.

(Okay, the "detection range" of the device isn't exactly decreasing, but the human ear is only good from 20hz to 20khz. I guess if you can hear past 20khz, then you don't have to worry about this)

*Most of the time, when you place a metallic object near the search coil, the frequency (what you hear from the loudspeaker) will increase gradually. When I was messing around with the circuit, I found that it got quieter & then stopped, & then started to increase again as you moved a metallic object near the search coil.

I couldn't resist getting out & yes, that's a rake, lol.

**broken link removed**

Improve sensitivity?

Here's how I see this,

The audible band (20hz-20khz) is mediocre compared to the search coils' band ...which is maybe infinite? The device will only function if the coils are oscillating within the audible band. Okay, let me rephrase that. -The device will function, perhaps at any frequency, but it's worthless until it's audible to us.

I see improving this device two ways,

1. Increase the magnetic field. So more turns of wire.

2. Increase the bandwidth. Toss out the speaker & throw in a meter. The analogy here is, "perhaps we can see more than we can hear." Now you might be able to detect changes that you couldn't hear before?

I've got an app that lets me download from Youtube so I'll try to watch later, otherwise I get one second of video and 10 seconds of pause - it doesn't make it easy to watch!

Your perception of a tone change is more sensitive than a meter so listening is better than watching a needle but for best results you should constrain the frequency between around 600Hz to 6KHz, if you go lower the change is proportionately smaller in comparison with the tone itself and if you go higher the filter starts to reduce the volume. Very few pepople can hear 20KHz, I can *just* hear 19KHz and my hearing is pretty good. The majority of people struggle beyond about 15KHz and the limit tends to drop as you get older.

There is a design flaw in the oscillators as they are, when two oscillators are coupled together they try to lock to the same frequency so if you tune them close to each other the chances of one or the other beng pulled apart to create any beat note at all is reduced. The problem is made worse by Q1 and Q2 bases being connected to Q3 which allows electrical interaction as well as the magnetic interection between the oscillators. It would be nice if the two coils tuned in opposite directions when something ferrous was moved nearby but they will both move in the same direction so you get a dead spot in the detection range somewhere physically between them.

The number of turns and the wire diameter is relatively unimportant as long as both coils are wound the same. You actually need some difference between them to generate the beat note but that difference is quite small so the minor discrepancies in their construction should be enough.

Brian.

In your video I see a cheap little speaker that might produce sounds from 200Hz (if it had an enclosure) to about 4kHz.
Since it is small and has no enclosure then it produces sounds from about 1kHz to 4kHz.

An expensive very good speaker will be pretty big and be in an enclosure designed for it (or headphones) will produce sounds from about 35Hz to 20kHz.

Audioguru said:

In your video I see a cheap little speaker that might produce sounds from 200Hz (if it had an enclosure) to about 4kHz.
Since it is small and has no enclosure then it produces sounds from about 1kHz to 4kHz.

An expensive very good speaker will be pretty big and be in an enclosure designed for it (or headphones) will produce sounds from about 35Hz to 20kHz.

Click to expand...

Hi Audiogurur, I follow you. I've spent some time on DIYaudio & DIYMA. Actually, those HiVi's were from Zaph's build.

Maybe I should model those PE buyouts in a t-line:lol:

Well I stayed up until 5 a.m. to put it on a perfboard. 5 HOURS! What a tedious process. I screwed up on my first one ...90% the way through:???:

In order for it to fit inside the enclosure, I had to sand the ends down. -My mounting tabs are gone now, hence all the hot glue:wink: The components aren't moving anywhere, although I'm not sure if I'll be able to troubleshoot it:shock:

I like breadboarding better. Maybe I'll sit on Eagle tonight (probably for 8 hours) & try to PCB

I normally make my own PCB if it's single sided or use a commercial PCB company if it has to be double sided with plated holes. My experience of building like that as soon as the glue sets you find a 'left over' resistor on the floor that did something critical in the circuit!

Good luck with it, let us know if it works!

Brian.