Question about surf pi design

[DJSUOL]

Could someone tell me the function of the 2N3906 transistor and why the delay signal should be taken from the collector of it? When the transistor is on, I found the pulse becomes 3.5V to -5V, how does this happen? In the delay circuit using 14093B, is the output trigger at the falling edge and the pulse width is determined by the series resistor and capacitor?

[Carl-NC]

The 3906 level-shifts from +12/0 to +5/-5. The pulse widths are set by the 51k|1nF RCs, delays set by the other RCs. NAND gates are used as inverters.

[DJSUOL]

I built a circuit of surf PI and it can work well. However, if i keep conducting the power supply for about 5 minutes, the IRF9640 transistor can not work. Could you please tell me why this happens?

[Carl-NC]

Overheating?

[DJSUOL]

So how can I solve it? What is the reason for overheating?

[kt315]

mosfet is in open state. no impulse on the GATE.

[DJSUOL]

Hi Mr. Carl, I made 3 PCB of surf PI circuit to try to build a metal detector door, and I use 6 coils 3 TXs and 3 Rxs. I use one power supply to provide 12V to these circuits at the same time. However, If I just connect on a pair of coils to one of the PCB, it can work well. But when I connect another Tx coil to the second circuits, The first circuit keeps warning. Do you know what the reason for that? Is the interference from the other PCB or from the coil wire? Thanks for your help.

[kt315]

surf PI uses the monocoil. give pics what you meant written 3 TXs and 3 Rxs.

[DJSUOL]

I just change the circuit with 2 coils 1 Tx and 1Rx. During the test, it can work well. Since I have 3 circuits, so I have 6 coils