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Hello Svorkoetter,

 

Could you advise whether it would do any harm if I was to drop a 470 ohm resistor with LED into the circuit between the battery + side and pin 8 of the TL072 - I

forget to turn things off and I find a glaring red light helps :) .  Also another silly question from me - which resistor is the Pot or variable one and do you have any recomendations for the pot or is it just bog standard. 

 

I am getting all the components together now.

 

Cheers,

 

Vic

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You could add a resistor and LED, but it would reduce the battery life by quite a bit. As it is now, the circuit will run for about 100 hours non-stop from a 9V battery. That will go down to about 15 hours if you add the LED as described, which isn't even long enough to do a power reserve test. By the way, I use a rechargeable 9V battery (8.4V actually) so as not to use up 9V batteries.

 

There is no pot in the current iteration of the circuit. I found that with the original circuit, I always had the pot turned up all the way, so I just left it out and replaced it with a fixed resistor, R5, which is part of the output filtering circuit.

 

One important thing to keep in mind is that the completed circuit should be installed in a metal enclosure, with the circuit ground connected to the enclosure. Otherwise, despite all the filtering, AC hum will be a problem. Also, if you're using jacks for the input and output, it's best to use shielded cable between the board and the jacks.

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Hello Svorkoetter,

 

Thanks for the quick response.  I actually had a small plastic enclosure in my shopping basket so you have saved me from wasting a couple of pounds.  I take your point about the LED and am surprised at the power drain it would cause and will ditch the idea.  Also I will build the amp as per drawing and use shielded cable as you suggest - I want to give myself as much chance of success as possible.

 

Many thanks,

 

Vic

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A quick update. I've just finished building an amplifier as per my posted schematic (which is slightly different than the one I built that is in the photos earlier), and it works fine. Also, I've found that if I use shielded cable between the circuit board and the input and output jacks, then a metal enclosure is not necessary after all (sorry Vic). The previous amplifier had more cabling due to the potentiometer in the circuit, and that one did benefit from a metal enclosure.

Edited by svorkoetter
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Here are some photos of the amplifier I just completed, built exactly according to the schematic and circuit board layout I posted earlier.

 

post-140-0-43715000-1413078432_thumb.jpg

 

I borrowed a trick from my electric R/C flying days, where electronic speed controls often used heatshrink tubing as an enclosure. Here the tubing just acts as a sub-enclosure to provide some strain relief:

 

post-140-0-46663800-1413078427_thumb.jpg

 

Here it is in the case:

 

post-140-0-71222300-1413078421_thumb.jpg

 

Vintage style labelling:

 

post-140-0-56026300-1413078413_thumb.jpg

 

Oh, by the way, I measured the exact current consumption to be 3.63mA. Adding a red LED and 470 Ohm resistor would increase this to 19.2mA, thus cutting battery life to less than 1/5 of what it would be without the LED.

Edited by svorkoetter
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Hello Folks,

 

Just spent 4 days looking round the pottery outlets in Stoke on Trent so I have been off the radar for a while.  By the way it was a waste of time.

 

I have ordered all my bits and await delivery.  I made one mistake, I thought I was ordering the enclosure from the UK but it transpires that it is coming from China.  Thanks  Svorkoetter for the guidance, the only difference I am making is I am bringing the shielded cable out from the board to a Jack for the computer rather than having two Jack sockets I will only have one for the mike.  I know it will have a long tail and not be aesthetically pleasing but it will save having an additional junction on the box and I will always be able to find the cable :D .  Hope that a two metre tail will not have any side effect.

 

PS my effort will not be as pretty as yours but hopefully it will still work.

 

Cheers,

 

Vic

Edited by Vich
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Soldering the output lead directly to the board is fine. Whether it's a 2 metre tail, or a 5cm tail with a 2 metre cord plugged into it, makes no difference. In fact, you could solder the lead from the mic directly to the board too, and do away with all connectors except for the plug that plugs into the computer. If I commercialize these, that's probably what I'll do, since connectors add both material and labour costs.

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Hello Svorkoetter,

 

I thought about connecting the mike direct but the cable on the korg does not "look" like it is shielded, it looks fairly thin.  I have not got it in my hands yet so I can not say one way or another - I may be getting over cautious.  I know that you have stressed the need to use the shielded cable in the enclosure when using Jack sockets so am I right in assuming that if the Korg cable is not shielded it could be detrimental if I soldered it straight onto the board.

 

I promise not to bombard you with any more questions.

 

Cheers,

 

Vic

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The Korg cable is definitely shielded (I cut the plug off of mine and installed a smaller plug). If it weren't, then shielding from the board to the jack would be pointless (and the microphone would be so sensitive to AC hum rendering it completely useless).

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Hello again,

 

Excellent,  I think I will wire direct then.  Just waiting on deliveries now.  I cut a bit of stripboard of some scrap I had lying around and just realised how small the board is.

 

Cheers,

 

Vic

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Here's a picture of the latest version of the hardware (I'd already posted a picture of the amplifier earlier, but here it is with the new microphone stand). I've got the cable coming out of the top of the stand now, which frees up one more side to allow testing in one more position.

 

hardware.jpg

 

I've also completed the first draft of the on-line manual for the software, which can be read here: http://www.watchoscope.com/manual

 

Comments or notification of errors are much appreciated.

 

The software itself needs some final cleaning up, so the download link in the manual does not yet work.

 

Going forward, I think I'm going to make the microphone stands by modifying a readily available Chinese case holder. I've ordered a few to experiment with.

Edited by svorkoetter
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Hello Stefan,

 

Just wondered where you sourced the piezo from, plenty of 27mm on the Bay but am I right in thinking you use the brass type rather than ceramic and is it 30nF / 30,000pF it seems a little harder to find those specs.  I have ordered the korg but thought I would get a piezo to experiment with - just want to give myself the best chance at success.

 

Cheers,

 

Vic

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I don't recall where I got it from, but I just ordered some off eBay too (in 27mm, 20mm, and 12mm) for further experimentation. The exact capacitance isn't all that critical. They're all going to be in the 15 to 30nF range.

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Svorkoetter,  What you have achieved so far is brilliant,  a low cost accurate timing machine must have potential sales in the thousands,  me for one.

 

Just a couple of thoughts,  batteries are a bit of a nuisance,  would it be possible to run it from a small DC transformer?  or seeing as it must be used with a PC would it be possible to run it from a USB port?

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Svorkoetter,  What you have achieved so far is brilliant,  a low cost accurate timing machine must have potential sales in the thousands,  me for one.

 

Just a couple of thoughts,  batteries are a bit of a nuisance,  would it be possible to run it from a small DC transformer?  or seeing as it must be used with a PC would it be possible to run it from a USB port?

 

Thanks for the kind words autowind. It'll be interesting to see what the sales potential is. I mean, how many watchmakers and WISes are there out there?

 

Yeah, I realize that batteries are a nuisance, but I didn't want to introduce yet another source of AC hum (from DIY people using a cheapo AC adapter). Also, if I do go commercial, adding an AC adapter introduces issues with European vs. North American (and others) voltages and plugs, and requiring CSA, UL, and CE approval.

 

In my own circuit, I use a rechargeable low-self-discharge 9V battery (it's actually nominally 8.4V, but that's fine).

 

To switch to USB power, I'd need to get a chip that works well on low voltages and whose output can swing from rail to rail. The TL072 is only good from 0 to Vcc-1.5, so on 5V USB power, it's cutting it a bit close wrt headroom. But I'll experiment with it.

 

And yes Nigel, you only need a single TL072 chip. Notice there's only room for one on the circuit board layout.

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