CCPI Build

[pusateric1]

Hello all, I’m progressing along in my build. I’m building the crossbow classic pi and I’m at the point where I’m setting my frequency and pulse width, I’m using a bench top power supply set at 12v. LT1054cp chip and irf840 were used. can anyone help or explain why my signal looks weird. It’s not a nice square wave like I’m used to seeing. Thanks in advance

[Nightryder]

It's kind of hard for anyone to tell you what exactly is wrong with little info. But I'm going to take wild guesses here:

If you are confident about the capacitors (minimum voltage losses if not 0) , try tracing the issue by checking the voltages. Then replace the timer with another one and see if that goes well. Sometimes I get unexpected waves on my oscilloscope because its prob is set to 10X.

I highly recommend you to check the YouTube channel of the member Surf Detector

I was hesitant to share a build document with you because there are several revs of this project and I'm not sure which one I have, so I didn't want to confuse you. Besides, I have it buried somewhere in one of my usb drives. If you still need it, I'm going to dig it out for you.

[Qiaozhi]

Build document from member mschmahl:

[pusateric1]

Thanks I will check the voltages and change the timer again. This is rev c. , the build document I was using only has 9 pages. I was reading inside the metal detector, and in the pi section I read that not all transistor pin outs are the same. I’m wondering if the 2n3906 and 2n3904 could be wrong. I imagine The 78l05 and 79l05 are correct though because I was getting -5v at r16 and +5 at at pin 8 on u8. I only installed them based on the silk screen pattern as I read that info after I made this post and never verified pin out before installing.

[pusateric1]

Verified pin outs on all transistors and they all match the silk screen and schematics so it’s not that. I started populating a new board from scratch and I’m checking as I go. I’m new to using the o-scope so it could just be user error. I won’t know till I completely finish the board and see the end result so I’ll continue to the end and see how the finished board functions.

[pusateric1]

Finished the board and was successful, I am now looking for a suitable coil as this will be mostly a beach machine. Is there any advantage to an open pi coil design like the 12x15 sef detech vs a flat wound coil like the coiltek elite series? I see the flat wound coils on detectors geared towards nugget hunting no so much coin shooting and beach detecting like I will be doing. I’m looking for commercially produced coils geared more towards the gpx line as I already have a shaft that will that coil ear mount spacing.

[Qiaozhi]

This might help (see attachments).

[pusateric1]

Finishing up the project and when I went to turn the detector on resistor 24 let out magic smoke. It’s a 3r3 fed from the irf840 for the TX circuit. Could it be because I’m using the irf840 instead of the irf740? I designed and 3d printed the enclosure and it’s coming along so I definitely don’t want to give up.

[Carl-NC]

The 840 should work just as well.

[Qiaozhi]

As Carl said, the IRF840 will also work.
Was the coil connected when the 3R3 started to release the magic smoke?
If so, does it still happen when you disconnect the coil?

[pusateric1]

Yes the coil was connected. I’m using a gpx coil With the 470r resister on pl3 between 0v and rx.

[pusateric1]

[Qiaozhi]

What happens if you disconnect the coil?
Does the magic smoke stop?
If it does, then the TX oscillator may have stopped oscillating causing the MOSFET to be turned on permanently.

[pusateric1]

When I replace the resistor and don’t have the coil plugged in there’s no smoke and just a solid tone. I’ll look at the tx circuit and see if that’s it. Do you think replacing u5(ne555) or u7(ne5534) would help?

[Qiaozhi]

I would put the scope probe on the NE555 output pin to see if it's oscillating before deciding what to replace.