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  1. KrisBlueNZ

    Power supply failure alarm

    That circuit isn't going to work. The voltage divider that feeds the base will always turn the transistor ON, even when the circuit is receiving power. The 100K trimpot needs to be in series with R2, not with R1, and even then it needs to have a higher resistance. This circuit is not very well...
  2. KrisBlueNZ

    How can I build a simple counter with a HC393 chip?

    So you've already worked out that you want to divide the 5 kHz frequency by 256? That will give you an output frequency of 19.53125 Hz. The HC393 contains two 4-bit counters, each containing four cascaded divide-by-two stages. The clock frequency is divided by 2 at the QA output, divided by 4 at...
  3. KrisBlueNZ

    how to convert 5mV to 15V

    You will never get a useful answer to a question as vague as that. Exactly what are you trying to do? Give us LOTS of information. Everything you can think of.
  4. KrisBlueNZ

    Pass Transistors and Current Limiting

    Can you start by posting a schematic and full specification?
  5. KrisBlueNZ

    Project Help!!

    Yes, you'll need sixteen cells (24V divided by 1.5 volts per cell is 16 cells). You connect them in series (nose to tail in a line). You can probably get six-cell and four-cell holders. Alternatively you could use four 6V "lantern" batteries connected in series. These are modular batteries, base...
  6. KrisBlueNZ

    Project Help!!

    Cool, so it does! These look like they would work, assuming the magnetic characteristics are enough for your application. They are rated for 24VDC operation. The one with -030 in the part number has the lowest power consumption, 10W. At 24V the current will be: I = P / V = 10 / 24 = 417 mA. The...
  7. KrisBlueNZ

    Project Help!!

    I've never heard of an electromagnet that's permanently magnetised and becomes unmagnetised when energised. It would have to have a permanently magnetised core and be designed so that the electromagnet's field "cancels out" the permanent magnetism in the core. I don't know if it's even possible...
  8. KrisBlueNZ

    Reverse the voltage?

    Could you mount the IC upside down? You might be able to bend the pins around so the opposite side is facing outwards. Do you intend to use a USB joystick interface permanently? Can you give the part number for the USB joystick interface? If it works with old joysticks (from the 1980s and 1990s)...
  9. KrisBlueNZ

    Project Help!!

    I don't think a normally energised solenoid will be workable with a battery supply. The amount of current needed to hold the plate in the locked position will be prohibitively high. Perhaps a motor and gear system to move a locking bolt? Run the motor one way to lock it, the other way to unlock...
  10. KrisBlueNZ

    Op-amp help please!!!

    Re the argument between BobK and Laplace. Perhaps you can both agree with this description. Apart from some very special-purpose devices, op-amps have two power pins: positive and negative. They have two input pins (inverting and non-inverting) and one output, and they produce a voltage at the...
  11. KrisBlueNZ

    Reverse the voltage?

    As often happens on these forums, there is FAR too little information in the question. To the OP, please give as much relevant information as you can. At least a few paragraphs describing the situation. This should include, at a minimum: What you have, with part numbers, circuit diagrams, and/or...
  12. KrisBlueNZ

    Help with circuit for a 12v DC Timer

    That's a pretty amateurish project and the circuit diagram is not well drawn. The two capacitors are for smoothing or "decoupling". They remove noise from the voltage rails that they're connected to. There's one on the input to the regulator, and one on the output. This is normal practice...
  13. KrisBlueNZ

    Paralleling LEDs is OK ?

    Harald, is thermal runaway a factor with high power LEDs? If Vf drops as temperature rises, and the LEDs are (at least partly) thermally isolated from each other, the LED that starts by drawing the most current will draw progressively more and more as it gets hotter, and eventually end up...
  14. KrisBlueNZ

    Transformer Primary open circuit

    As Harald says, it could be a fuse blown, rather than the actual winding open circuit. Some transformers have various types of fuses built in. Remove the transformer and have a good close look at it. Perhaps you could take some close-up photos and upload them.
  15. KrisBlueNZ

    How to verify my ac-dc adapter is ok?

    If the fault took out a circuit breaker, it won't be on the secondary side. Most likely to be insulation breakdown in the primary. The primary measures 949 ohms, which sounds fine, but when mains voltage is applied, insulation can break down. Look carefully at the primary wires and all...
  16. KrisBlueNZ

    Monitor Proview DX 777NS model 786N-horizontal line

    That's partial "frame collapse". The part of the circuit that generates the voltage to move the electron beam up and down (the "frame" circuitry) is not working properly. There are many possible causes, but a likely one is failed electrolytic capacitors in the frame circuit, especially if the...
  17. KrisBlueNZ

    Question about Voltage and Current

    I posted an analogy that explains voltage, current, resistance and voltage dividers in a way that may help you have a more intuitive understanding of the issues you're asking about. It's in the thread https://www.electronicspoint.com/newbie-questions-t248766.html
  18. KrisBlueNZ

    How do I find the right transistor?

    I mentioned manufacturing because it's a way to tell whether a design is REALLY workable. For example, perpetual motion machine inventors are always demonstrating prototypes but never get their products manufactured in production batches. It's a quick clue to the fact that they don't actually do...
  19. KrisBlueNZ

    logic probe

    You're welcome :-) Good luck!
  20. KrisBlueNZ

    How do I find the right transistor?

    I had a quick look, they seem to be "silicon-germanium heterojunction" bipolar transistors made by NXP and Infineon. The ones I've seen are rated for a few volts VCE maximum! They are clearly designed for a narrow application niche, for communication equipment operating in the GHz range. I...
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