TP3 3 - 3.6 V not 1V Step 5

[baluba]

Hi, i have bought a minipulse pluse rev C from Silverdog, and now i am building it. I must mention that i do not have an oscilloscope.

At Step 5 when i want to measure the voltage at TP3 (between TP10 and TP3) i get somewhere around 3 and 3.6 volts... not 1 volt like it should.

The first 4 steps are ok, i verified everything and it is like it should be.

I have removed the coil and the voltage at TP3 is somewhere around 0.6 V. I have changed the damping resistor R1 with values from 200 ohms up to 1150 Ohms (original is 470 ohms) also i have put a 10 ohm resistor in series with the coil. I have verified all the soldering. The coil is 0.6 ohms and 380uH, no shield. Coil is connected to the middle pins, of PL2 and PL4.


The only problem i could see is that R14 and R15 are 21 ohms not 27 ohms, like in the schematic (the resistors are from kit package). For the moment i do not have a 27 ohms resistor...

I have mounted the missing parts from step 3... and looked over the list with the problems of Rev C...

What do you think is the problem?

[kt315]

that has no sense to measure impulse signal by ordinary tester, that measuring DC and AC voltage, but AC means clear sinusoidal voltage, not
impulse still with low off-duty factor. try to find an o-scope.

[baluba]

Monitor TP3 with an oscilloscope. If no scope is available, then measure with a voltmeter. There
should be approximately a +1V DC offset at TP3. Place a cola can close to the coil, and the voltage
should drop to around 150mV. A coin will only drop the voltage by a few 10s of milli-volts

This is my first metal detector... i have some experience in electronics but only in digital and some audio...

Should i go to the next step?

[Qiaozhi]

At Step 5 when i want to measure the voltage at TP3 (between TP10 and TP3) i get somewhere around 3 and 3.6 volts... not 1 volt like it should.

The 1 volt measurement quoted in the Build Document was based on a single build, and in practice this voltage can vary quite widely between units. I will correct this in the documentation when we go to REV-D. Since the preamp is AC-coupled to the sampling integrator, the RX signal gets referenced to 0V, so I would just carry on with the construction. The important point to check is that the voltage decreases in the presence of a metal target. I would suggest using a Cola can for the test.

[kt315]

then measure with a voltmeter.

digital voltmeter tester is going in strange manner if a signal has a specific impulse form. it shows in two tree and even more time of value than really is DC level.

[Chet]

Hi baluba,
21 ohms for R14 and R15 should be OK. To verify this check the voltages on pin 8 and pin 4 of U5. The voltages should be approximately the same except opposite polarities.


With the coil connected you are seeing 3-3.6 volts. This may be the coil receiving noise from florescent lights, computer or TV. Try to turn off everything that may be radiating noise to the coil.


And locate the coil on top of a plastic waste container away from the work area and away from any metal for all testing.


For now stay with 470 ohms or a little less for damping.


In another post, Qiaozhi recommended using a damping resistor adjustment tool: a 10k pot in series with a 200R resistor, both in parallel with a 1k2 resistor. This provides an adjustment range between 171R and 1k1.


The tool is normally used with an oscilloscope. At a later time you might try the following to adjust the damping without an oscilloscope.


Using the damping resistor adjustment tool in place of the damping resistor; monitor the TP3 voltage changes while moving a large target across the coil at a fixed height.


Try different damping settings until best voltage change is acquired. Then change to a smaller target and refine the damping setting.


After the best setting is acquired; remove the tool and measure the overall resistance. Choose a standard value (1 or 2 watt) resistor that is less than the value measured and install it as the damping resistor.

[baluba]

With the coil connected you are seeing 3-3.6 volts. This may be the coil receiving noise from florescent lights, computer or TV. Try to turn off everything that may be radiating noise to the coil.

I have a laptop and a lamp (i can adjust the brightness of the lamp) on my desk, maybe that is the problem. The laptop has the WiFi On.

21 ohms for R14 and R15 should be OK. To verify this check the voltages on pin 8 and pin 4 of U5. The voltages should be approximately the same except opposite polarities.

4,8V and -4,8 V at U5 pin 4 and 8



By the way now i am at Step 7 and i cant adjust any voltage from R6... TP4 sometime it shows 4,2 volt sometime 0,7 V, slowly go up and and down (it takes 4-5 seconds).

Either my multimeter is imbecile, or me...

[baluba]

nothing is happening when i put metal object near the coil, or move it back and forth, no voltage increase no voltage decrease...

[Chet]

Hi baluba,
For Step 7 you need to short PL3 again.


On step 8 you need the 50k Reject pot connected to PL3. The following additions should be in the next revision of the Build manual.


“A few minor suggested changes; add to Step 3, Q2 (2N3904), R9 (10k) and R5 (20k), add to Step 7, Short PL3 again, add to Step 8, connect 50k Reject pot to PL3.”


If this does'n work. Are you hearing the transmitter in Step 4? Be careful around the coil and transmitter any probing or mistakes will result in feeding high voltage or high currents in the direction of the U4, 555 chip. This normally lets the smoke out of the chip. They are cheap so it’s nice to have a spare around.

[baluba]

I have read and made the changes for each step before...

At step 4 i hear a sound in the headphones, if i put the coil near my head. Also at step 3 i measured the TP1 with the multimeter set on 2 KHz and i get 1012 HZ frequency

By the way, i use a 12 volt 7 amp lead acid battery, the voltage is 12,8 V.

[johnandles]

[QUOTE=baluba;199428]Hi, i have bought a minipulse pluse rev C from Silverdog,.........no shield. Coil is connected to the middle pins, of PL2 and PL4......

Coil connection should have a link for mono coil, if no shield connected. Check coil connection
Cheers
John

[Chet]

Hi baluba,
Since your meter can measure frequency let’s see what is working.


The voltage on pin 2 of U5 should be clamped by D5 and D6 to between +0.7 to -0.7 volts. Measure the voltage to confirm that the diodes are working. I don’t know the sensitivity of your frequency meter but try it here after the voltage checks.


You should definitely be able to measure approximately 1 kHz on the following test points:

TP1 = Transmit driver pulse; which you have measured


TP3 = transmitter pulse and flyback pulse which were clamped to +/- 0.7v by D5 and D6 but have been amplified by U5 to approximately +4v / -4v. This +4v and -4v signal should provide a measurable 1 kHz signal to your meter. The receiver signal is on the tail end of the clamped transmitter flyback pulse. Which will be separated from the influence of the large transmit voltages by the Q3 and Q4 timed gates.


TP4 = time delay for the gate used to cancel the earth’s magnetic field effect


TP5 = time delay for the target gate


TP7 = Earth Effect gate


TP8 = Target gate


A few questions; have you restored the coil circuity to the original configuration?
How is your coil wound?
Is the 380uh measured or calculated?


Johnandles just posted another point; to clarify the connections check the following;

One wire from the coil should connect to one of the outside pins of PL2.


The other wire from the coil should be connected to the center pin of PL2.


Then the center pin from PL4 should have a jumper to the center pin of PL2

[baluba]

The voltage on pin 2 of U5 should be clamped by D5 and D6 to between +0.7 to -0.7 volts. Measure the voltage to confirm that the diodes are working. I don’t know the sensitivity of your frequency meter but try it here after the voltage checks.

0.032 Volt, Freq 1020 hz (if i put the probe on TP10, and other probe in the air it shows 1020 hz )

TP3 = transmitter pulse and flyback pulse which were clamped to +/- 0.7v by D5 and D6 but have been amplified by U5 to approximately +4v / -4v. This +4v and -4v signal should provide a measurable 1 kHz signal to your meter.

3.38 V static, and when i swing a huge metal object... it goes to 3.42V...(it is not stable....), Freq from about 1.4 Khz to almost 1.8 Khz it goes crazy

TP1 1020 hz stable
TP3 1.4 khz to almost 1.85 khz it go crazy, sometime it stays to 1020 (if i approach my hand to multimeter )
TP4 0 hz 0.8 V static to 1.60 V if i put a metal object near by the coil
TP5 1020 hz stable
TP7 1020 hz stable
TP8 1020 hz stable

The coil is in original form 470Ohm damping and no series resistor.
25 cm diameter (10 inch aprox), 25 turns of 0.8mm copper wire, tied together with electric tape, PVC i think.
380uH measured with L/C meter

i have wired in the wrong way the coil... but i have written this in the first post.... i have found this connection diagram here http://www.geotech1.com/forums/attachment.php?attachmentid=30782&d=1406724328 there is no jumper wire, and no PL2 center and margin....

[baluba]

It works yeeeeee, it detects my weding ring 7 gram of gold, at 17-19 cm

[Chet]

Hi baluba,
My mistake on TP4, it should have been TP6 for a 1 kHz signal.

The good news from the small voltage changes you observed at TP3 and TP4 when you swing a large metal object over the coil indicates that the circuity is working.


The strange frequency results from TP3 is understandable because it is a mixed noisy signal that is hard for the frequency meter to lock in on.


The meter is also reacting to the high voltage flyback pulse that is being radiated from the coil. If you have a small AM radio you can probably tune into the coil frequency noise.


You should continue on to finish the audio circuity and see how it works at that time.


For final testing you need a clean electrical noise environment with the coil away from metal objects. I have to turn off everything in the house except old light bulbs and the refrigerator. You mentioned a light with a dimmer; the problem is that the dimmer is probably pulsing a transistor similar to the Q1 transistor used in your transmitter. The receiver is very sensitive to any stray noise.

Regards,
Chet