Interference problem of the circuit

[DJSUOL]

I make three PCBs of the SURF PI metal detectors to try to build a metal detection system to detect three different areas: top, middle, and bottom. I use one power supply to provide 12V to these 3 circuits at the same time. However, I found the interference occurs when I use three circuits together. I use 3 pairs of Tx and Rx for each circuit. If I just connect only one circuit, it can work well. But if I use two circuits together, the speakers will sound together. The PCBs are put close to each other, is the interference caused by it or the interference is from the coils? When I measure the power supply, I found it is not DC but a changing signal that is similar to the output of NE5534. Could someone tell me how to solve it? Thanks for the help.

[waltr]

You must Sequence the three detector so that ONLY ONE is active at a time.

What you are trying to do is the same as having three people shout at you at the same time.

[DJSUOL]

Why the circuit is wrong if I want them to work together? I use 3 different demo circuits for the related three Rxs and they are separated with each other.

[DJSUOL]

Actually, I think the SURF PI has a settling time and if I active each one, in turn, I think the wrong warning will increase, how can I solve it?

[dbanner]

How much seperation between Rx coils? Also, one spkr or three separate spkrs? It seems that the Rx coils might be cross receiving. Only way around this might be to sequence the three circuits as Walter suggested.

[DJSUOL]

I use one speaker for one of the circuits and I use another two LEDs to make a display. Actually, I take apart the test coils away from the other two but this problem still occurs.

[kt315]

one generator for all 3?

[DJSUOL]

3 555 circuits

[kt315]

this is your problem

[DJSUOL]

So should I use only one 555 circuit to drive 3 cirucuits

[DJSUOL]

When I close the switch, I can hear one short time sound and it becomes calm. Is this correct? In addition, why I cannot use three generators?

[dbanner]

I strongly recommend you put a block/ function diagram of your setup along with some technical information on how you intend to implement it( physical drawings), only then can others offer some advice/ diagnostic information for you.

[dbanner]

Did you use computer power supply for the 12 V?

[Qiaozhi]

It seems obvious to me that your 3 Surf-PI circuits are not synchronized, with the result that they will interfere with each other.

Probably the simplest solution would be to drive all 3 MOSFETS from a single oscillator. Just removed the 555 timer on two of the boards, and connect pin 3 of U1 on the 3rd board to pin 3 on the other two.
Alternatively you could time domain multiplex the 3 circuits so that only one is active at any time during the TX period. This will be more complicated to achieve. Personally I'd go with option one as a first attempt.

[dbanner]

Synchronization of the three circuits from a single timer in my opinion does not prevent cross-interference. What is needed is shielding between high, mid and low coil arrays and shield separation inside the chassis. Or sequencing of the three circuits.
Alternatively, a single TX with three separate Rx, with Rx gain diminished somewhat to zone the responses.