Pinpointer - Minimax-PP II

[andreapas79]

I made the same with stripboard.. the signal looks like better but, after two 4148 the signal is near zero, before 4148 i've 1,4V sine wave...it's normal this value?
I used 2n23906 and 2n3904, 490pf and 10nf

[Nandor]

Hi
I just measured mine again. so the Oscillator has 135volts peak to peak at 15,6kHz. After the diodes it drops to 64volts DC negative! When detection occures it drops to DC -54volts.

Hope this helps

[andreapas79]

yes thanks a lot... i dont understand why it give me only 1,45V...how is your coil? I've a ferrite rod 10mm x 55mm with 450turn 0,31 enameled copper wire, 7,95 mH 6.8ohm...i dont understand why the voltage is so low.
I've attached a photo of a quick measurement

[Nandor]

Well it should be a clean sine wave, I think it's a transistor that works bad on your board. Try to make the pcb it will work, it's an easy pcb to make, I drawed it with a marker pen and ruler.
I tried many coils, it works with allmost anykind of coils. If you use thick wire it will consume more power, 0.31mm is good and also 8mH it's only frequency thats changes. The best I could get was around 5cm for a big coin, but I don't think that it needs more detection range because we need pinpointing not a locator.

[andreapas79]

Thanks for your help. I have not yet build pcb because I'm experimenting, but if this gives all these errors i build a pcb. It's really strange from 150v to 1,4V.... could the 2n3904/6 replace BC807 / BC817 without problem ? can't that be the mistake, right?

[Nandor]

Well transistor pinout can be a problem, try the pcb with matched pair of transistors. Measure their hFe with a multimeter and try to choose 2 indentical one. Like PNP with 190hFe and NPN also with 190 hFe, or any other other just it has to be close to each other.

[andreapas79]

effectively the value is different, pnp 134, npn 198... could be it the problem?..... if yes now i understand why ne555 to generate function to pilot coil..

[dbanner]

Under third wave lockdown, so I decided to make minimax pinpointer.
I didn't have an active buzzer, so I made CD4060 add on board posted on previous page to generate the sound. Someone described this version sounds like a parrot, and well, it does! Just awful.
I needed a beep beep circuit and I found just the one. It works extremely good.
Based on 556 dual timer. Now minimax is sounding like the real deal. All I needed to do was to change C1 to 390nF. If like me, you don't have an active buzzer in your junk box, but you have couple of 555 timers, then build this circuit, it is the best beeper for minimax.

[Nandor]

[ta2cjj]

MMBT4401 : 2N4401