# Hi Guys just want to see some opinions or tips to improve this circuit ( bfo detector)

#### Tylereng123

Joined Sep 6, 2017
9
I am getting a reasonable output wave from the speaker when I have a slight change in Inductance between the search and reference coil, but I am trying to figure out a good way to eliminate noise when there is a slight difference in frequency between the two coils so that the speaker does not output a sound when there is no metal present across the search coil, as I understand even a slight difference in frequency will lead to a sound from the speaker due to increasing difference in voltage of the two waves. This is the circuit I've been simulating

#### Tylereng123

Joined Sep 6, 2017
9

#### BR-549

Joined Sep 22, 2013
4,928
I'm trying to understand what your saying.

#### Tylereng123

Joined Sep 6, 2017
9
Hi BR- 549 I am just trying to figure out a way to test whether or not the speaker will make a sound when there is no metal present in the area of the search coil

#### BR-549

Joined Sep 22, 2013
4,928
Did you build this circuit....or are you simulating it?

#### Tylereng123

Joined Sep 6, 2017
9
I built this circuit on a software called Tina and am testing it before I print it onto a PCB board, do you have any advice?

#### BR-549

Joined Sep 22, 2013
4,928
Does that mean you built this circuit........or does that mean you simulated it?

#### Tylereng123

Joined Sep 6, 2017
9
I simulated it I guess?

#### BR-549

Joined Sep 22, 2013
4,928
Alrighty.......I'll let one of the sim ops help you with that. From what I can tell......you should get a constant tone out.......when not in proximity of metal. Metal should cause a change in tone.

What do you mean with the word "noise"?

#### Robin Mitchell

Joined Oct 25, 2009
819
When he says noise he means that if the frequencies of the two circuits is slightly different then the system will make a crackling noise as the two wave forms will not be identical and thus will result in a drift. The op-amp would detect these small changes (as one waveform will be high while the other will be low) and therefore result in a change on the op-amp output.

The solution would be to synchronise the reference oscillator with the sense oscillator. Not sure how to do that but that would be my two cents on it!

#### Tylereng123

Joined Sep 6, 2017
9
Thank you, by noise I mean the output of the speaker, I need to find a way to ensure that I will not have any output from the speaker if there is no metal inside the area of the search coil.

#### Robin Mitchell

Joined Oct 25, 2009
819
Or you could have a circuit that could reject short pulses!

#### Tylereng123

Joined Sep 6, 2017
9
By rejecting short pulses how would this prevent noise form occurring when no metal is present?

#### BR-549

Joined Sep 22, 2013
4,928
You need a modified squelch circuit. You will need real circuit parameters......and measured effects of metals on oscillators. With these parameters....you can develop a strategy on the "squelch" circuit.

#### Dodgydave

Joined Jun 22, 2012
11,245
I am getting a reasonable output wave from the speaker when I have a slight change in Inductance between the search and reference coil, but I am trying to figure out a good way to eliminate noise when there is a slight difference in frequency between the two coils so that the speaker does not output a sound when there is no metal present across the search coil, as I understand even a slight difference in frequency will lead to a sound from the speaker due to increasing difference in voltage of the two waves. This is the circuit I've been simulating
View attachment 134486
No it wont work, both 555 outputs will be at positive supply, as the threshold pin is not used, Also the discharge pin is tied to positive and will just blow the 555 up.!!!

#### DickCappels

Joined Aug 21, 2008
10,123
Per Dodgydave at least disconnect pin 7 from the power supply -let it float because it's not doing anything and floating won't hurt anything, but it will probably lengthen the life of your NE555's and that of your batteries.

There is more than one way to deal with the "noise" when the two oscillators are at nearly zero beat.

You can use a sharp high-pass filter to reduce the lower frequency beats. This would work much better when hetrodyning sine waves than square waves because of the harmonic components in the square waves, but it might be worth a try.

Another approach is to injection lock one oscillator to the other when they are at nearly the same frequency. To that take the square wave from pin 3 of U1 through a small nonpolarised capacitor ( like a .01 uf ceramic cap so you don't have to worry about polarity) to a resistors (start with about 20k) to pin 5, the VCO input of U2. Now, when the two oscillators get close to the same frequency U2 will lock in phase and frequency with U1. The larger you make that 20k resistor the more sensitivity you will get.