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  1. KrisBlueNZ

    Clamp a floating input

    Good! Yes, that relay sounds good. I like the brand name - close enough to Sanyo to be confusing, not close enough to get sued! The statement about the FET is really just for thoroughness, and to warn you that the signal on LOW/DRL must always be as you described. I'm not suggesting a bigger...
  2. KrisBlueNZ

    Clamp a floating input

    Hi, sorry for the delay. Here's a simple suggestion. The voltage on C1 represents the state of the LOW/DRL input. Whenever LOW/DRL is low (0V), C1 is discharged quickly through R1 and D1. Whenever LOW/DRL is high, C1 charges relatively slowly through R2. So while LOW/DRL is low, or has...
  3. KrisBlueNZ

    Valve (tube) amplifier schematic help

    Normally you should ground circuits for safety, but that can cause earth loops (see http://en.wikipedia.org/wiki/Ground_loop_%28electricity%29 ). In this case your power supply is double insulated so there shouldn't be a safety issue, and it will be grounded through the amp whenever it's plugged...
  4. KrisBlueNZ

    Valve (tube) amplifier schematic help

    That's cool that you want to get into electronics. It's a really interesting subject! But of course I'm biased :-) It's great that you have a multimeter and an oscilloscope. You can do a surprising amount with an old analogue scope, even something with only 5 MHz bandwidth and a single trace...
  5. KrisBlueNZ

    Clamp a floating input

    OK, don't worry about making a mistake with the measurement. As long as you understand it now, everything's fine. But can you tell me, is there a third state, with the control signal steadily low? When the light is completely OFF? The way I'm imagining the whole setup is that the light has...
  6. KrisBlueNZ

    Clamp a floating input

    Thanks for those answers. That cicuit looks OK apart from the 220 uF capacitor from the op-amp output to ground. I don't know what you expect it to do, but it won't, and it's a bad idea. I'd also change the zener to a lower voltage, say 5.6V, and increase R3 to get, say, 10 mA through it. This...
  7. KrisBlueNZ

    oscilloscope trace`

    Try rotating the oscilloscope (looking down on it from above, rotate it clockwise or anticlockwise slowly) and seeing whether the trace can be made horizontal. The trace is affected by the Earth's magnetic field. You might have to have the scope facing in a different direction. You can also try...
  8. KrisBlueNZ

    Backlight problem on lcd tv

    More on the dual MOSFET device. It looks like Fairchild, Diodes Inc and Vishay all make similar parts in the same package, but nothing I found on Digikey or Mouser is rated for 16A/12A. They are rated for more like 8A. The STU404D may be over-rated for that application and you might be able to...
  9. KrisBlueNZ

    heating up of the MC34063AP IC

    Steve has explained the situation thoroughly, as usual. You can increase the operating frequency by reducing CT. 470 pF gives an operating frequency of about 33 kHz but the MC34063 is specified to operate at up to 100 kHz. The CT value for 100 kHz operation is about 180 pF but this value comes...
  10. KrisBlueNZ

    Backlight problem on lcd tv

    That's very helpful. I've attached the schematic to this post, but I had to use JPG format (not good for line drawings) and a fairly low quality factor so it was small enough for the electronics point system to accept. It's readable though. The controller chip is the STR-H2003, and the devices...
  11. KrisBlueNZ

    Clamp a floating input

    OK, thanks for that description. It sounds like you need to identify the negative reference for these voltages. The 14.x VDC voltage sounds like it is relative to the chassis, right? If so, I would expect the 8.2V p-p square wave is probably relative to chassis as well? When you measure the...
  12. KrisBlueNZ

    I (a newb) would like to design a small & simple GPS tracking system for my 2 cats

    I can't add much here; everyone else has made good suggestions and given good information. My approach (and I think it's already been suggested here) would be to get a small, light GPS receiver module, like the ones used in smartphones, and a tiny radio transmitter using an unlicensed frequency...
  13. KrisBlueNZ

    What is the function that transistor in the circuit?

    Great! That's sweet. Your cheque is in the mail BTW :-)
  14. KrisBlueNZ

    Laptop thermal fuse

    I agree with Jotto's advice. It's probably shutting down because it's overheating; it's probably overheating because the cooling system is inadequate. I have a Toshiba laptop that has the same problem. You can get software that displays the core temperature(s) on the screen, so you can see how...
  15. KrisBlueNZ

    What is the function that transistor in the circuit?

    As soon as the signal becomes too loud (i.e. amplitude exceeds 1.4V peak to peak), we want the gain to be reduced. One peak in the waveform will cause the transistor to conduct for, say, 1 ms, which will reduce the capacitor voltage slightly, and reduce the gain slightly. When the next cycle of...
  16. KrisBlueNZ

    What is the function that transistor in the circuit?

    Q3 will only conduct on the peaks of the signal, so each time it conducts, it needs to discharge the 10 uF capacitor by a significant amount. That's why the resistor from Q3 collector is 1.5k. When the overload has gone, the gain needs to return to normal, but not too quickly. That's why the...
  17. KrisBlueNZ

    heating up of the MC34063AP IC

    That diagram is straight out of the data sheet. What value are you using for CT? Are you sticking with 470 pF? That will run the MC34063's oscillator at about 50 kHz. Why are you using a 15 uH inductor? The data sheet suggests a 220 uH inductor with CT = 470 pF. 15 uH is much too small. What...
  18. KrisBlueNZ

    heating up of the MC34063AP IC

    15 microhenrys is very low. The MC34063 isn't designed to run above 100 kHz. There's no link in your post.
  19. KrisBlueNZ

    What is the function that transistor in the circuit?

    The transistor won't be damaged as long as the base doesn't go below about -7V, which it cannot because the power supply voltage is only 3V so the maximum signal at Q2 collector is only 3V peak to peak. If you remove the diode, the AGC will not work as cleanly as it should, because when the...
  20. KrisBlueNZ

    What is the function that transistor in the circuit?

    Q3 doesn't conduct on the negative part of the signal at all. It conducts when its base goes positive by 0.7V with respect to its emitter (which is grounded). The diode conducts on the negative part of the signal, and forces the negative peaks of the signal voltage to be no further negative...
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