The TP1 Waveform

[1843]

Regarding the Q3 drive, -3v is a little low, should be more like -1v, but it should still be OK.

- Carl

I can see it from -1V to -VB only when the R9 be changed with a 1K.
Is it better that remain 220-Ohms?

[Carl-NC]

I can see it from -1V to -VB only when the R9 be changed with a 1K.
Is it better that remain 220-Ohms?

I just realized that in Opt.1 there is R15=10k, which will cause the levels you are seeing. If you short R15, The level should be close to -1v. But none of this really matters, -3v is fine too. Yes, leave R9 at 220 for fast pull-down.

On TP4, this is the output of an opamp, which probably does not swing rail-to-rail, so +/- 4v or so is normal, and will vary amongst opamps.

I still cannot explain the funky TP1, so just leave R37 open for now.

Are you still getting the bizzarre audio response?

- Carl

[1843]

Yes

[1843]

Yes Carl,

[1843]

Yes Carl,
The audio is bizarre since the TP9 is varying. My HH every 2 seconds makes a beep.

[1843]

Carl,

I found that whether uninstalling the OP-Amp or set the sample pulse width at minimum causes that sound be good.
Also turning the second delay pot changes the sound.

What do you say?

[Carl-NC]

You might have interference. Try rotating the coil 90 degrees, or moving it to different locations, and see if it gets better. Or, take it outside.

[1843]

waveforms

I took it outdoor, but in each condition its sound was bad.
The problem is around the IC6 and IC7.

[Reg]

Hi,

You might be having instability with your 12 V power supply. You might try turning R 35 to maximum resistance and see if that causes the 1 second beep to stop or change.

I have run into a similar problem if the audio is too loud. It will cause the 12V to drop, which then causes the pulse to change and that causes flucuations at TP 4 and ultimately, causes a problem somewhat like you are having.

Watch your 12V with your scope on A/C and see if the voltage is dipping when the audio beeps.

Your pic of TP 4 is correct. I am not sure how Carl got the pic he did. The voltage should be higher on the left, then drop negative and then ramp back up to 0V just like what your detector is doing. The higher voltage you are worried about is the signal you get at TP 4 when the pulse is occurring. If you also look at the pulse, you will see the pulse current on when the voltage is higher.

Let me know what you find if you adjust R 35 to maximum resistance and what you see if you check the 12V supply. I suspect you will see the 12V dipping when you hear the beep.

Reg

[Reg]

My HH TP 4

Hi,

Here is a pic of my TP 4. As you can see, the voltage is higher on the left, then drops negative and finally ramps back up to 0V.

This is a proper pic of what you should see.

Reg

[1843]

I have no problem with the power supply because I use a 8 Amperes battery.
I cannot get the main sampeling switch such as Carl's.



Does my TP3 look correct?

[1843]

Power Supply Changes

In which VOLTS/DIV I must see the power supply changes?
When it is set at 100 or 200mV, the waveform has changes.

[Reg]

How much of a reflection do you see when the unit beeps and does the reflection seem to last the entire beep time? Even a 1/4 volt can cause big problems if it doesn't recover quickly. I never did measure the actual voltage drop to see just how little of a voltage could cause the problem, but I suspect even a drop of 100 to 200 mv could cause problems if the gain is high enough. In most cases the deflection is only in the tens of millivolts and only lasts for a few usecs when a regulator is used. This short of a time doesn't allow for the oscillation.

In the case where the voltage drops and remains there, the voltage drop causes a change in the pulse which is reflected in the return signal. This change is amplified sufficiently causing the audio beep, which in turn starts the process all over again.

YOu might want to turn R 35 to maximum resistance (or at least, increase the resistance to well over half) and then adjust R 41 for less volume and see if either or both pots cause the beeping to stop or change significantly. If either causes beeping sound to change dramatically or the beeping to stop, then the problem is most likely the stability I mentioned.

Also, increasing the value of C1 and C10 will usually change the beep rate. I personally use Tantalum caps for the caps in the power supply sections also. Tantalums are about "25 times" better than electrolytics when it comes to filtering higher frequencies out of the system. This info came from an article written in one of National Semiconductor's application notes. I never tried to verify the accuracy of the info but have noticed the power supplies are much more stable. I also add a large Tantalum cap across the two larger electrolytics.

Like I said, I had a similar problem on my HH quite a while back. In fact I think I posted something about it before, but don't remember just when or in which thread. What I also recommended at that time was to simply build or buy an external amp to power the audio if a speaker is to be used. Some headphones will require additional power also and cause the same problem, so a simple external amp for them is also recommended. Personally, I have found the cheap Radio Shack mini amp that sells for about $12 works well and is hard to beat.

Regardless, a good regulator on the main power supply is the best solution. Even a heavy duty battery doesn't cure the problem.

Now, if you add a series resistor in series with the FET, maybe a 30 ohm to 40 ohm, you reduce the pulse load and this helps. However, even with this resistor, if the audio signal draws too much current, it will still oscillate. The cure is to reduce the audio current and limit it from ever drawing too much if a regulator isn't used.

Reg

[1843]

Hi,
The sound does not depend on the coil, sound volume etc.
When I remove the MOSFET or coil, again HH beeps...
Only when I remove the OpAmp it sound will be good.
The reflection depends on MOSFET pulses..and they are slight(about 10mV)
I don't know why...

[Carl-NC]

First, Reg is right, my TP4 waveform does not look right. I suspect I took the photo before properly setting R18, and didn't notice it was wrong when I included it in the article.

Second, all your waveforms look fine. It appears you have the sample pulse width set to maximum, so you might want to narrow it a bit.

Third, if the beeping is still occurring even without the coil connected, then you have a circuit problem, and it should not be too difficult to track down. Use the oscope to probe all the nodes in the signal path, esp. around IC8 where everything should be DC. Look for jumping voltages that should not be jumping. Start removing components, beginning with the speaker and working backward, until the jumping voltages cease.

Carefully inspect for solder shorts... I recently had a wisp of solder (visible only with a magnifier) that was causing an intermittent short in the pulse generator... drove me nuts for 2 weeks.

- Carl