Hammerhead Question

[Ray-NM]

Carl I was looking at the gate pulse that is derived from the 555 timer.
I found that the pulse going to the gate of Q3 takes about 10usec's to reach 0 volts or -Vbb. I see that the pulse from the timer is fast enough but at the collector of Q1 the switched pulse takes about 10 usecs to reach 0volts. The gate of Q3 switches and about 4 volts. Shouldnt the falling pulse be faster than that ? I am using 2n3904 and 2n3906 in the circuit.
I am guessing that there is a problem with Q1. Am I on the right path?
Anyway thanks for your thoughts ahead of time.
Ray-NM

[Carl-NC]

Yup, that's way too slow. Check the value of R9, make sure it's 220 ohms and not 2.2k or 22k. Sometimes those red and orange stripes look brown.

- Carl

[Ray-NM]

I'll dig out my magnifying glass and have a good look.
Thanks
Raynm

[Ray-NM]

I replaced both q1 and q2 and the resistors and the 555 and still have the problem. I will find it. Maybe a cold solder joint or a hairline bridge somewhere.
RayNM

[bbsailor]

Troubleshooting: Try This

Ray-NM,

Before you start the procedure below, do the following. Use an ohm meter and verify the value of all TX circuit installed resistors. Ensure that the pins of Q1 and Q2 are installed correct.

1. Disconnest R5 end from Q1 to isolate the 555 timer.
2. Apply power to circuit.
3. Measure the voltage on 555 between B+ (negative probe connction) and pin 1 for 5V.
4. Observe the 555 output at TP1 with a scope to verify that the pulses are present at near a 5 V amplitude, and operate through the full range of R2 and R3 adjustments.
5. Turn off power
6. Remove MOSFET
7. Reconnect R5 to Q1.
8. Turn power on and check again for voltage between B+ and pin 1 of the 555. Voltage should be the same as before.
9. Observe the signal across R9 driving the MOSFET gate. It should look like the previous observation at TP1 but with a different amplitude of pulses.
10. Install MOSFET and observe the signal on the MOSFET gate again. It should be the same as observed in step 9 above.

This should help you track down the faulty component or a wiring mistake in the TX circuit. Let us know if this helps.

bbsailor

[diminute]

OK

Desconeta el GATE del MOSFET, luego chequea la forma de onda, está mal entonces sospecha de la polarización de Q1 y Q2... si está bien, entonces tu MOSFET no es el ideal, tiene mucha capacidad de entrada.


Are you using a IRF740 ??? or other with a elevated input capacity ???


Bye

Diminute

[Ray-NM]

Yes I am using the irf740.
Raynm

[Ray-NM]

Found the problem

It seems that the irf740 was the culprit. Must have been some excess gate leakage that caused the problem. I removed it and looked at the gate signal and it was normal again. Replaced the mosfet with a lower DS voltage rated mosfet. I used a irf 510 since it has a lower value Coss of around 100 pf.
Was able to remove the damping resitor I had in paralell with R11. I will go back today and remove R11 and select a higher value resistance. The voltage rating on the irf510 is 100 volts so I will have to be carefull I do not exceed that by too much. I did find that the sensitivy has increased by using the the mosfet with a lower Coss. The signal is flat topped so the resistance of the coil is limiting the pulse. Just like everyone said it would. I did not measure the delay yet but will later today. I think what happened is at sometime or other I must have stressed the Mosfet passed it limits in my intial attempt to get the highest leve pulse out of it. At the time I really did not understand the importance of the shorter pulse delay.
Its cold here and in my shop it is about 20F. So I have to warm it up with the wood stove.
RayNM

[bbsailor]

Try the IRF820

Ray-NM

Try using the IRF820.
500V
2.5A
COSS @25V = 61pf, @12V = 80pf
CISS, low
R-on = 2.5 ohms

Surplus Sales of Nebraska has them for $0.45 each.

With the low CISS, the gate drive should be faster than the IRF740.

Couple this IRF820 MOSFET with a 5.4 ohm AWG 30 10.5" diameter coil with 19 turns (320uH) to 21 turn (360uH) and add between a 2 to 10 ohm series resistor between MOSFET and the coil, and you will have a coil with a 32 uS to 20 uS time constant so it will operate well over the pulse width of the Hammerhead and approach max current. Just measure the battery current draw, observe MOSFET heat and the circuit stability with different size series resistors and set the resistor value accordingly.

Radio Shack has 1 Watt 10 ohm metal oxide resistors that you might want to try. If it gets too warm, use two in parallel while experimenting. This still makes a coil (as noted above) TC of between 24.6 and 27.6 uS.

Glad you found the problem. Now you can get down to some serious experimenting.

bbsailor

[Ray-NM]

BBsailor Thanks for the help and encouragement

Thanks again. Also thanks to Reg.
What I am waiting on now other than the fine tunning is waiting for Carl and see if he produces the PCB's Not sure if he has enough takers yet.
Have not seen him on line lately so he may be too busy at work or on a vacation.
On another note I will get the IRF820 and try it out. I think when I am finaly finished with this detector and all the modifications I plan I will have a good gold detector.
RayNM

[Ray-NM]

did some checking and found I can now detect a 23 grain gold nugget at about 3 inches, thats with 2 flourescent lights on and the tv tuned to gold fever on the outdoor channel and 1 oscope.
pulse width is 100usec with a pulse rep rate every 700 usec.
I have a delay of between 9 and 10 usec. My damping resistor is 670 ohms.
This is using a 2 coils one for transmitt and one for receive. I dont need to damp the receive coil. 21 turns of #30 enameled wire 8 inch diameter. I am still using the irf510 since I have not ordered the irf820 yet. So that tells me that pulse of 120v is at least adequate for testing at this time.
Before I removed the irf740 I could barely detect the gold even when it was touching the coil. So a definate impovement.
The one thing I do not like is the option I chose for the audio using the vco.
I think I will setup a white noise generator and gate that as my audio.
That way it will sound more like the whites gm4 that I use. My ears are tuned to that sound and the sound the gold makes.
I know a little long winded but I am excited now.
RayNM

[bbsailor]

120V Peak Pulse

Ray-NM

With a 120V peak pulse, that tells me that you may be saturating the MOSFET as the IRF510 is rated at 100V. 100 uS pulse width is pretty long for detecting small gold. That pulse width is OK for coins, maybe even about half of optimum for coins. Readjust the value of the damping resistor (100 to 200 ohms higher) as it should be a higher value with less MOSFET capacitance.

Try this. Place a resistor equal to the value of the coil rsistance in series with the coil. Measure/observe the peak flyback voltage again. It should have a sharp or pointed peak, not a flat-top peak which appears brighter than the rest of that trace. If it is a flat-top and brighter, you are still saturating the MOSFET.

Raise the Hammerhead frequency, reduce the pulse width to about 50 uS, reduce the delay and measure the distance of your gold sample again.

If you can operate the Hammerhead at the lowest delay, while waving a small gold target under the coil and still get a response signal, you may have more delay room left in the coil, but not in the circuit. Reduce the main delay limit resistor R42a to either 1200 ohms or maybe even 1000 ohms to get a shorter delay range. You can reducing the value of R42a and operate at a lower delay until the coil stops responding when you reduce the main delay control pot. Set the delay just a little higher than when the coil stops responding. This will be the most sensitive operating point with that coil, for small gold, sampling a little higher on the early response curve for that target.

Now the fun can begin.

Keep good notes so you know what effect various changes make.

bbsailor

[Reg]

Hi Ray,

A couple of the reasons I used one of the Newer designed FET's was because of noise and lower capacitances of the FET. Generally, many of the new designs have lower capacitances

Now, something you mentioned should be cleared up. You mentioned you had to be careful to not exceed the 100V of the FET used or something to that effect. The FET's have a built in Zener that will simply limit the voltage at that point, so there is no need about worrying about exceeding the voltage. In other words, it is not a problem and won't cause damage or harm. However, using a lower voltage FET will make the design a little slower because the FET will take longer to recover. Using a higher voltage FET of even 250V up to 500V will speed things up a bit. Use a higher voltage FET and the decay time is reduced. Use a very low voltage FET and the decay time is lengthened considerably.

Now, as another little bit of wisdom, if you keep the pulse length short, maybe 50 usec or so, you will still generate a pulse that is more than adequate for gold detection. This pulse length should work great on gold up to an oz or two before showing much if any loss. On the ML's, the short pulse is used to detect the small gold and it is only about 50 usec long. Now, I have used pulses even shorter than that, down to 30 usec or so and that works fine also.

Ok, why else use a short pulse? Well, it makes shortening the delay easier and improves battery life. The short delay is more important when trying to detect small gold than more current.

If you want to know the current into the coil, simply place a resistor in series with the FET and measure the voltage across it. If you use a 1 ohm resistor, then the voltage across the resistor will represent the current in the coil. So, 1/2 volt will mean there is 1/2 amp current, etc.

I mentioned the current measurement technique just in case one wants to know how much peak current is going into the coil at any given pulse length.

Reg

[Ray-NM]

Ray-NM

With a 120V peak pulse, that tells me that you may be saturating the MOSFET as the IRF510 is rated at 100V. 100 uS pulse width is pretty long for detecting small gold. That pulse width is OK for coins, maybe even about half of optimum for coins. Readjust the value of the damping resistor (100 to 200 ohms higher) as it should be a higher value with less MOSFET capacitance.


bbsailor

I would think that the 120v peak is due to the lower inductance of the coil I am using. 21 turns of #30 enameled wire.
I will try your suggestions.
Thanks Ray

[Ray-NM]

Made some changes

I changed the preamp feedback resistor R13 to 100k then changed R26 and R27 to 1meg. This increased the detection range from about 3 inches to about 4 inches for the gold nugget with less noise. A nickel is now at about 6 or 7 inches. This is all still with an irf510 installed for Q3.
That definately helped. Also I changed R30 and R32 to 39k instead of the 150k and 100k that are in the option 1 schematic. This allowed me to set the threshold to absolutely quiet. Yeah I Know that decreases the sensitivity but
I was not able to reduce the click click to zero until I did that.
Waiting for Carl to order the new pcb's before I place an order for more parts to populate it with and to try some of the other Mosfets that bbsailor suggested.
RayNM