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  1. KrisBlueNZ

    do you know these resistors?

    Hello Bob and welcome to the Electronics Point forums :-) I'm pretty sure the schematic is wrong in at least one place. Did you draw it up yourself? The relay coil is connected across Q1's collector and emitter, but it should be connected between Q1's collector and the positive supply voltage...
  2. KrisBlueNZ

    (Electro)Magnetic Levitation

    The lead-acid battery charger probably does not produce a clean DC output. They often have no smoothing capacitor, so their output voltage is just a full-wave rectified signal at twice the frequency of the AC mains supply. This is probably the reason for the buzzing sound. You need to power the...
  3. KrisBlueNZ

    (Electro)Magnetic Levitation

    1. What type of "12V battery charger" are you using? What kind of battery is it designed to charge? What is its rated output current? Why aren't you using a power supply instead of a battery charger? Have you measured the voltage going into the circuit when it's running? 2. Do you have access...
  4. KrisBlueNZ

    home made digital speedometer

    That display will work but it's not very bright - 3.9 mcd (millicandelas). I thought you would want a very bright display for visibility in daylight. Do you have any programming experience? If so, what language(s)?
  5. KrisBlueNZ

    Logitech Z5500 diagnosis help

    The regulator that's outputting 21V may be doing that because there's no load on it. (Or of course, it could be damaged, or it might be supposed to put out 21V because the two versions could be different.) Is it possible that there's a break in the feed from that regulator to another part of...
  6. KrisBlueNZ

    Logitech Z5500 diagnosis help

    Thanks for the pics. How does power get to the board? Is there a mains transformer in there somewhere? Or a separate power supply board? Or is it powered from an external power supply?
  7. KrisBlueNZ

    circuit help

    This is what I think it does and how I think it works. Some of the component values are hard to read. If you can clarify them and repost the schematic, that might help. I'm going to have to guess at some of the values. There are two separate functions: a voltage limit and a current limit. Both...
  8. KrisBlueNZ

    Component questions (transistor, capacitor)

    You can save a lot of space, and a bit of money, by using transistors with base resistors built-in. These are called "digital transistors", "pre-biased transistors", "bias resistor transistors" (BRTs) and "resistor-equipped transistors" (RETs) by different manufacturers. They are available with...
  9. KrisBlueNZ

    Unequal voltage source in parallel

    If they're isolated from each other, and rated for the same current, you can connect them in series and use a buck converter to convert down to the voltage you want. You might get some more helpful answers if you describe the situation in more detail. What are the natures and specifications of...
  10. KrisBlueNZ

    Open collector buffers VS Non Open Collector Buffers

    "Never attribute to malice that which is adequately explained by stupidity" https://en.wikipedia.org/wiki/Hanlon%27s_razor Also, see his signature (inserted, along with the avatar, by Steve, I expect).
  11. KrisBlueNZ

    Open collector buffers VS Non Open Collector Buffers

    If a standard logic device from the same family is being driven, I can't think of any reason to common up multiple open-collector outputs that are all carrying the same signal. If you're driving a load that requires significant current - an LED, or a relay coil - then that could be a reason.
  12. KrisBlueNZ

    Open collector buffers VS Non Open Collector Buffers

    It's possible that multiple open-collector outputs could be tied together to increase the drive current, but the usual reason is making an "OR-tied" signal as I described in post #2. Push-pull outputs in logic circuitry have a similar drive capability to open-collector or open-drain outputs...
  13. KrisBlueNZ

    Logitech Z5500 diagnosis help

    Yes, you could be on the right track with the missing 8V supply. You need to find where it comes from and why it isn't there. Can you upload a few photos so we know what we're dealing with?
  14. KrisBlueNZ

    Home automation using DTMF technology

    So you're talking about cellphones automatically answering incoming calls. If this is an option that's available with particular cellphones, then the manuals for those cellphones will describe how it works. I don't see any reason why a cellphone, or especially a smartphone, couldn't...
  15. KrisBlueNZ

    Help making small squeaker

    Good idea Bob! That could be done with a fast PIC with plenty of ROM for the waveform(s). You would need a pretty high PWM cycle frequency to accurately reproduce a squeak, but it would be doable. The dark detection could be done by periodic wake-up, enabling the LDR with an output pin and using...
  16. KrisBlueNZ

    IC voltage comparator

    OK, R15 is being driven directly from the output of an op-amp - a very low-impedance source. So I'm confident you can use an LM311 to replace the LF311 in that circuit.
  17. KrisBlueNZ

    Home automation using DTMF technology

    Please explain what you're talking about. "Auto answer mode" - are you talking about a dial-up modem that can be configured to answer an incoming call on a phone line? DTMF ("touch tone") is used for dialling. It is not related to answering calls. Do you want to generate DTMF audio or detect...
  18. KrisBlueNZ

    Help making small squeaker

    Hello and welcome to Electronics Point :-) Well, this is a new one! A device that makes random squeaks, put inside a PVC pipe, that can operate below freezing! I guess a good start would be an MP3 player, playing a single long track consisting of squeaks at random intervals. You could make a...
  19. KrisBlueNZ

    12V Battery Charger Schematic

    The circuit uses the LM338 to regulate the output voltage and current to the battery. The LM338's output voltage is determined by the voltage at its Adjust pin (the one shown at the bottom). Specifically, the output voltage is 1.24V (nominal) higher than the Adjust pin voltage. This 1.24V...
  20. KrisBlueNZ

    Open collector buffers VS Non Open Collector Buffers

    Thanks shumifan50 :-) I hope he can follow it!
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