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  1. KrisBlueNZ

    Power supply failure alarm

    You've made the same mistake as before - you're trying to use the red power bus for two separate signals, but you can't, because the connection runs all the way from the left end to the right end. You need to move the three right-most connections off the red power bus and put them on a separate...
  2. KrisBlueNZ

    Power supply failure alarm

    You can use two 4.7 ohm resistors in series.
  3. KrisBlueNZ

    Power supply failure alarm

    Oops I uploaded too soon. That's not the best way to arrange the circuit. The 555 could be damaged on power-up. Here's a better arrangement. All the comments from the previous post still apply.
  4. KrisBlueNZ

    Power supply failure alarm

    Power supply failure alarm with oscillator This circuit combines the 555 oscillator with the power failure detection. While power is present, C1 is kept charged via D1 and R1, and voltage through D2 prevents the 555 from oscillating. When power fails, there is no longer any voltage on D2 so the...
  5. KrisBlueNZ

    Power supply failure alarm

    Since you've told me you're using a 555 to drive the piezo, I've got an easier way to handle this. I'll upload a schematic soon.
  6. KrisBlueNZ

    Power supply failure alarm

    I mean you can't put both ends of it on the same horizontal bus bar, because that shorts it out. You need to move the right hand end onto one of the normal vertical sections of the breadboard.
  7. KrisBlueNZ

    Power supply failure alarm

    Your diode is shorted out because the power bus connects all the way along from the left edge to the right edge. You need to move the diode somewhere else, you can't put it on the horizontal power bus bars.
  8. KrisBlueNZ

    Power supply failure alarm

    Yes, your diode is shorted out. You need to move that stuff somewhere else.
  9. KrisBlueNZ

    Power supply failure alarm

    Ahem... The twin strips that run sideways... they're connected together along the whole length, not just in groups of five holes like the other strips are! It's probably a good idea to add a 10 ohm resistor in series with the diode anyway.
  10. KrisBlueNZ

    Power supply failure alarm

    That location is on the hard drive of your computer! I can't access it there. You can attach it to your post - use the "Go Advanced" button, I think it's called, and click the paper clip in the top middle of the editing window.
  11. KrisBlueNZ

    Power supply failure alarm

    When you first connect the circuit to the power supply, there is nothing to limit the amount of current that flows through the diode. When C1 is discharged, it will draw quite a lot of current when it starts to charge. Maybe the diode is getting damaged each time you test it. Try adding a 10 ohm...
  12. KrisBlueNZ

    Power supply failure alarm

    That's not right. Pull the diode out and try again. If the lamp goes out immediately now, the diode is shorted. If it still goes out slowly, there's a short somewhere else.
  13. KrisBlueNZ

    Power supply failure alarm

    So if the monitor circuit is connected across the lamp, when you disconnect the battery, the lamp fades out more slowly than if the circuit isn't connected? That's definitely not right. That sounds like the diode is shorted. If the diode itself is OK, maybe there's an error in your breadboard...
  14. KrisBlueNZ

    Power supply failure alarm

    Can you take some zoomed photos of your breadboard?
  15. KrisBlueNZ

    Power supply failure alarm

    Yes, the triangle in the diode points in the direction of current flow, for "conventional" current, that is, current that flows from positive to negative. Yes, the diode blocks the discharge of the capacitor. Without it, the capacitor will discharge back into the load that you're monitoring, and...
  16. KrisBlueNZ

    Power supply failure alarm

    That circuit will beep when the voltage across its inputs falls. It won't beep if you just disconnect it from the power source. To test it, you have to make the voltage across the input connections of the circuit go to zero. You can do that by shorting the input connections together. If you want...
  17. KrisBlueNZ

    Power supply failure alarm

    So you tested it using a battery as your power supply, and a light bulb? That will work, but you need to connect the circuit across the light bulb and disconnect just the battery. The light bulb has to stay connected across the input to the circuit. I don't understand what you mean about the...
  18. KrisBlueNZ

    Power supply failure alarm

    OK. I just checked out the NTE159 data sheet. It has a fairly low gain. You would be better off using a BC557/8/9 with a B or C suffix. Are you testing it by turning the power supply off, to simulate power supply failure? Does the power supply have a load on it? If it doesn't, the output...
  19. KrisBlueNZ

    Power supply failure alarm

    That circuit will work. It does exactly what john monks suggested earlier in the thread, it removes the resistor from the base to ground, which was what was holding the transistor ON before. What is your "beeper" circuit? It's a 555 oscillator, but what is it driving? It may need a lot more...
  20. KrisBlueNZ

    Power supply failure alarm

    Here's a simpler circuit based on your one that doesn't need an adjustment. For a longer warning time, increase the 4700 uF capacitor and/or use a piezo buzzer with a lower current consumption. The 22K base resistor can be changed as well. It needs to provide enough base current to keep Q1...
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