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  1. KrisBlueNZ

    Problems substituting a relay

    I also don't agree that the circuit is unsafe, provided that the control signal is optocoupled. If I was going to replace this circuit I would use a relay with a mains-voltage AC coil, and switch it using an optocoupled triac device like the MOC3063. The 3063 is rated to carry 100 mA RMS so it...
  2. KrisBlueNZ

    Problems substituting a relay

    You should stick with a 24V-coil relay if possible. The current needed by the circuit will be halved so you'll have a much better safety margin. You've increased the 1 uF cap. Have you checked the power dissipation in the zener? CDRIVE, I agree there are flaws in the circuit. It would have...
  3. KrisBlueNZ

    Problems substituting a relay

    I don't think it's such a bad design. CDRIVE, I would have liked to hear what specific problems you see with it; you just said that it's "awful". It's a tidy way to obtain a small amount of power from the AC line, the topology is optimal [correction - see edits at end], and component values are...
  4. KrisBlueNZ

    How to find short on PCB

    A digital multimeter on ohms range may not work for testing diodes, which have forward voltages in the range 0.6V to 1V. Does the meter have a diode test range? Otherwise, make a continuity tester with a 9V battery, an LED, and a series resistor of around 1K. Each diode in the bridge should...
  5. KrisBlueNZ

    Pink noise generator circuit mod

    That would be good. The circuit draws hardly any current so the amp won't mind the extra load. It will have a positive rail that you can drop down to 12V or 15V with a 78L12 or 78L15 regulator, which accepts up to 35V input voltage. If the amp's positve more than 35V you can drop it down with a...
  6. KrisBlueNZ

    Pink noise generator circuit mod

    In addition to the issue raised by CocaCola, here are two other issues you need to think about. First, yes you will need to increase the output level of the pink noise generator, to suit the input requirements of the amplifier inside your powered speaker. You can do this with a fixed-gain...
  7. KrisBlueNZ

    4049 hex inverting ic

    gorgon, it's certainly true that you can't expect good frequency accuracy or repeatability from an oscillator based on a Schmitt trigger IC, but you can't expect particularly good accuracy or repeatability from any oscillator that relies on the value of a capacitor anyway. If you need...
  8. KrisBlueNZ

    LED Component Question: Total Newb Here

    Good idea :-) I've just checked on Digikey for high-brightness blue LEDs and the lowest forward voltage specified is 3.2V. I guess at 3V they would probably still illuminate slightly, and anyway a new 3V lithium cell will have a terminal voltage above 3V, so that method would probably work...
  9. KrisBlueNZ

    LED Component Question: Total Newb Here

    It looks like all the LEDs are connected in series, which means a failed LED or a broken track anywhere will prevent the whole lot from working (like a string of Christmas lights - the older type). You can unplug the board and test each LED individually, using three 1.5V cells in series along...
  10. KrisBlueNZ

    4049 hex inverting ic

    Very good advice as usual (*steve*). The Schmitt trigger oscillator is a favourite of mine because it uses only 1/6th of an IC (or 1/4 of a quad Schmitt NAND gate, if you already have a need for one or more NAND gates), and there are often other uses for the other gates, such as buffering the...
  11. KrisBlueNZ

    op amp question

    As duke37 says, an op-amp cannot produce an output voltage that is outside the voltages supplied to it. If you look up the data sheet on an op-amp, the output voltage swing will be specified. For example, the Fairchild data sheet for the old 741 op-amp, at...
  12. KrisBlueNZ

    4049 hex inverting ic

    If you're after multiple oscillators, you can get six out of a hex Schmitt trigger inverter like the 40106, 4584, or 74C14. No, not necessarily. The 4049 can drive an LED easily enough, and a lot of other circuit elements. It can't supply a great amount of current though. Whether a buffer is...
  13. KrisBlueNZ

    convertin an output current range into a non-linear voltage range using load resistor

    Let's see if I understand you. You have a data acquisition board that connects to some sensors that convert some physical quantity to a variable current. This current is fed through a settable resistance (this is called a "shunt resistance") (a digital potentiometer in this case) that converts...
  14. KrisBlueNZ

    Tillman Preamp J201 Replacement

    Also, Digikey have J201 in SMD for USD 0.55 http://www.digikey.com/product-search/en/discrete-semiconductor-products/jfets-junction-field-effect/1377093?k=J201 They also list the through-hole version but it's listed as non-stock.
  15. KrisBlueNZ

    4049 hex inverting ic

    That's not a complete circuit diagram. It should have worked, though, because the 4049's outputs have an inherent resistance that will prevent damage to the LEDs. BTW none of those resistors in the star are needed. There are much better ways to drive LEDs though, including directly from the 555...
  16. KrisBlueNZ

    2 LED's circuit (not so simple as it sounds)

    BobK: No, the important difference is in the behaviour when the probe is not connected. The OP specified that in this case, neither of the LEDs should light. With your circuit, both of them will light when the probe is not connected to anything.
  17. KrisBlueNZ

    2 LED's circuit (not so simple as it sounds)

    A simple answer is to generate a 1/2 VCC rail using a voltage divider from +14V to 0V and connect the LEDs in reverse parallel, one side to the voltage divider and the other side to the probe. . . . . . . . . . 1K . . . . . . . 1K +14V --------/\/\/\/\-----------/\/\/\/\/----------0V . . . . ...
  18. KrisBlueNZ

    HOW TO CONNECT BATTERY MONITOR

    There are a number of problems with that circuit, including the fact that it simply will not work. Pin 10 of the LM3914 does not go to 0V when the top-of-scale LED is illuminated - it only goes to about 2V below the positive rail (the exact voltage depends on the LED colour, and the LED current...
  19. KrisBlueNZ

    Help needed for simple problem

    FWIW I designed a little sawtooth oscillator for this job, mostly as a way to test the simulation features of circuitlab.com (which are very very slow). It seems to work per the simulation but I haven't built it up myself to check. The ramp is not truly linear because C2 is charged through a...
  20. KrisBlueNZ

    Motor control diy

    A quick search on maplin.co.uk found this geared motor kit, which could be just what you want: http://www.maplin.co.uk/geared-motor-kit-24813 In general you'll want a DC motor, so you can reverse it by reversing the polarity of the voltage supplied to it. You can do this with a "DPDT...
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