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Help getting my circuit to work - sound card osciloscope calibrator

Discussion in 'General Electronics Discussion' started by Robert Hill, Aug 7, 2015.

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  1. Robert Hill

    Robert Hill

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    Mar 5, 2015
    Hi all, I'm being driven mad by a cricuit i'm trying to build which just isn't working!

    Here's the schematics for what i'm building.
    This is the first part, the idea is that by turning the trimmer POT (VR1) you can set the desired level of DC at the DC+ and DC-
    upload_2015-8-7_23-31-31.png
    The problem I'm having is that the maximum voltage I can get at the DC points is 0.87 volts and no matter how much I turn the POT the minimum I can get is about 0.74 volts (TR1 gets mega hot at this point as well!). As far as I can tell the resistance of the POT is changing when I turn the screw.

    Anyone got any suggestions as to what might cause this issue? This part of the circuit appeared to work properly at one point but no longer does.

    The second part of the circuit joins on to the right of the schematic above, it is:

    This is an astable multivibrator whos peak to peak is supposed to be the same at the DC level set previously.
    However, the AC+ and AC- points never give the desired square wave. Is there a way to test this circuit to find out what has gone wrong? Anywhere obvious to start looking?

    Any help much appreciated as it is pretty disheartening given how much time i've put in that I just can't get it to work properly!
    Thanks,
     
  2. duke37

    duke37

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    Jan 9, 2011
    There is no multivibrator in your circuit.
    TR1 can only pass less than 10mA because of R4 and R5 so should never get hot.
    A simple multivibrator should not be powered with more than 5V since the emitter base voltage can exceed the break down voltage. Even so, I do not see how the current would be very high.
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    As stated repeatedly here your Sound Card Scope is strictly an AC instrument. Whatever that circuit is supposed to be (it has errors) it's going to have a DC output that can't be seen or measured by your scope.

    Chris
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hi Robert
    I have emailed Steve, the guy who designed this circuit. Hopefully he will be able to help. Have you double checked all the connections?
    Adam
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hi Trevor, I think he is referring to the other circuit being an MV.
    Cheers
    Adam

    MV.PNG
     
  6. Alec_t

    Alec_t

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    Jul 7, 2015
    Because of variation of the Vbe of Tr1 with temperature, the circuit is far from ideal as a calibrator. A TL431 adjustable reference voltage source could be used in a more accurate/stable supply for, say, an oscillator based on a CD40106 Schmitt inverter.
     
    (*steve*) likes this.
  7. Robert Hill

    Robert Hill

    111
    12
    Mar 5, 2015
    Hi guys,

    Thanks very much for the replies. It's hard work when you are trying to teach yourself stuff and need a hand.

    I've been doing a few tests on the circuit using my multimeter (so learning a few new skills there).

    - I discovered that I'd not cut one of the strips on the copper strip board so that the full 9v power was connected into R6, C1 and the collector of TR3. Therefore the DC+ and DC- points were subject to between 1-4 volts through TR1 and 9 ish volts through R6. This was the cause of the inability to modify the DC+ and DC- volts between more than 0.78 and 0.89 v. This also I believe was the cause of the very hot transistor.

    - I've tried to test the capacitors in the circuit. I did this by selecting the resistance setting on my multimeter and applying the probes to either side of the capacitor. Based on something I read online, if the capacitor is good, it will initially show resistance, but this will then drop quickly to nothing. If the resistance remains constant then the capacitor is busted. I found one capacitor had a consistent resistance, whilst the other dropped to nothing. Am I right in thinking the capacitor with constant resistance is busted?

    When I connect my voltmeter between the DC+ and DC- points I get 0 volts. However between a single one of these points and ground I get a voltage reading the same as that between ground and the emitter of TR1. When I connect an LED between either the DC+ or DC- points and ground the LED lights up, so there is obviously voltage/current available at these two points but none between them. Does this indicate the two DC points are or have somehow become connected? They are meant to measure the voltage across R5.

    @ Chris, yes, I know the sound card osciloscope can only 'see' AC inputs. The purpose of the DC+ and DC- in the above circuit is so that you can tell what peak to peak voltage level will be present at the AC+ and AC- points in the second part of the circuit.

    @Alec_t The calibrator should only need to be on for a few minutes at a time to set up the programme on the computer so perhaps the designer didn't think temperature effects would matter so much?
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Adam, is the first schematic supposed to be the power supply for this AFF?

    Chris
     
  9. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hi mate
    No its the calibrator circuit .I think
    Adam
     
  10. Robert Hill

    Robert Hill

    111
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    Mar 5, 2015
    Hi all,

    I got it working!!!! :p:D:p:D:)
    Can you tell i'm pleased?

    I ended up having to replace several transistors which had died and amazingly after looking over the board about 20 times I noticed one unsoldered wire.

    It now works perfectly!

    Thanks so much for the help and input, this really is the nicest forum I've ever been on.

    Hooray! Celebrate square wave come on:

    upload_2015-8-8_21-24-29.png
     

    Attached Files:

  11. Steve Garratt

    Steve Garratt

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    Jul 22, 2014
    Hi Robert,

    I'm sorry that you are having problems with my calibrator circuit but I'm please that you are sticking with it.

    It's not easy to test capacitors with a multimeter for anything other than leakage resistance and you can only do that reliably if you remove the capacitor from the circuit to do the test that you describe. You might have a faulty capacitor but I don't think that it's likely.

    R4 is supposed to hold TR2 switched on which means that a current should flow through collector and emitter and produce a voltage across R5. From your tests it appears that TR2 is not switched on at all. The most likely causes are that you have another tracking problem, TR2 is in the wrong way or that the transistor is faulty. Check the voltage with respect to ground on the base of TR2. It should be approximately 0.6-0.7 V.

    You are correct with your responses to Chris and Alec.

    Please don't get disheartened when you build circuits that don't work first time. I can tell you that hardly any of mine do and I always have to build a debug time into all of my projects. You will get better with practice but never imagine that you will achieve perfection. Nobody is that perfect.

    The problem is that there is so much that can go wrong including not making all the cuts in tracks, invisible solder bridges between tracks, components inserted the wrong way around, incorrect component type or value etc.

    I recently spent several frustrating hours on a circuit where I could see no fault at all. I could see that there were waveforms and voltages that were not right but the reason for it evaded me for hours. It turned out to be a solder bridge between two tracks that could not be seen even under a brightly illuminated magnifier. The debug session was hard but when I finally cracked it I was elated. Stick with it and you will get there.

    Steve
     
  12. Steve Garratt

    Steve Garratt

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    Jul 22, 2014
    Oh I am so pleased you got it working while I was composing my message above. You beat me to it.

    Steve
     
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Robert, that is something that is arguably undisputed. Glad you got it working! ;)

    Chris
     
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Steve, speaking for myself I hope you didn't get the impression that I was trying to stifle Robert's enthusiasm. Definitely to the contrary. Possibly if links to your designs had been posted we would have been more enlightened regarding how they were to be used. We didn't see the astable FF until Adam posted it.

    Chris
     
  15. Steve Garratt

    Steve Garratt

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    Jul 22, 2014
    Hi Chris. On the contrary I was simply trying to provide encouragement by confirming Roberts comments. I think the people on this forum are doing a great job and just wish I had more time to hang out here. I might have noticed Roberts post earlier and contributed quicker.

    Steve
     
  16. Robert Hill

    Robert Hill

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    12
    Mar 5, 2015
    Hi guys, I had thought i'd posted the second part of the circuit in my inital post but at 11pm at night I obviously messed it up.

    Steve, in the end it was TR2 (as you helpfully suggested) and one of the capacitors I had to replace (I'd been using a salvaged capacitor from something else which worked when first tested but must have died on more continuous use).

    Elated is definately the right word. Although debugging a circuit is hard work there is real satisfaction in getting it working. Now I can enjoy getting the box finished so it looks nice.
     
  17. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Well done! Glad you got it working.
    Adam
     
  18. Alec_t

    Alec_t

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    Jul 7, 2015
    Good result!
     
  19. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Keep butting that Dam Robert! ;)

    Chris
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Hi Steve,

    A bit of thread hijacking here...

    Is the circuit in the first post yours? What is it supposed to do? What should the output at "cal" be?

    EDIT: Please pretend I didn't ask that. We've discussed it before :)
     
    Last edited: Aug 9, 2015
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