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  1. Alec_t

    Low voltage disconect circuit

    Too high a value. Try 10k. Also, you need a resistor (try 1k) between pin6 and the transistor base. Edit: Arouse beat me by a whisker to the base resistor suggestion ;).
  2. Alec_t

    Help please, im lost

    From the output wattage and voltage you can determine the Q1 maximum collector current. Assume the worst case beta for Q1 (check its datasheet) to determine its maximum base current. Do the same for Q2. You can then size R2 to supply at least that current. Pick a zener diode with a breakdown...
  3. Alec_t

    RF noise from a chip dc-dc step up module, fix?

    Do the motor and relay coils/contacts all have snubbers/RF-suppression-components attached?
  4. Alec_t

    DC Voltage multipliers?

    DC-to-DC converters which boost voltage exist: but they work by using the DC input to create high-frequency higher-amplitude oscillations which are then rectified.
  5. Alec_t

    led driver selection for project

    Having seen the unit it's not surprising. At 12V DC each LED will draw about (12-3)/2.7 = 3.3mA, so the 10 LEDs together need 33mA and the whole unit will dissipate 12*33 = 396mW. Cheap white LEDs are typically rated for up to 20mA, so a mere 3.3mA is nowhere near that.
  6. Alec_t

    Relay Behaving Strangely

    Are the two transmitters set to the same frequency?
  7. Alec_t

    led driver selection for project

    Heck, it's only 2 more screws to remove :). My money says there's no regulator. Each LED has its own 2.7k current limiting resistor, so peak current per LED with a 16VAC supply would be about 7mA (assuming the LEDs and resistors are not all in one series string). Maybe the LEDs have some inbuilt...
  8. Alec_t

    Variable resistor separate from control circuit?

    Oops. A reverse-biased diode should be across the optoisolator's LED. I'll edit the schematic in post #4
  9. Alec_t

    Variable resistor separate from control circuit?

    Here's a possibility, using an optoisolator to provide a zero-crossing reference to the micro.
  10. Alec_t

    RF noise from a chip dc-dc step up module, fix?

    SMPS units are notoriously noisy. I may be wrong, but I doubt you'd see much difference in the RF emissions. The output caps on the two units are clearly different, and the inductors are different (so perhaps the switching frequencies), but who knows which combination would be worse for RF...
  11. Alec_t

    Sound to light unit

    That circuit would be very marginal even with red LEDs. Blue LEDs have a higher forward voltage (Vf) than red ones, so you would need to arrange them as 2 parallel strings of 2, rather than 1 string of 4 as shown. Each string would need its own current-limiting resistor. The resistor value would...
  12. Alec_t

    tube (valve) help

    If the resistance measured between the heater pins (4 and 5) of the EF89 is not less than or equal to about 30 Ohms then the valve is dead.
  13. Alec_t

    tube (valve) help

    Er, where are the results of your tests?
  14. Alec_t

    Multiplexer IC for analog inputs

    As asked previously, what signal frequency?
  15. Alec_t

    Diode - Temperature

    Correct. The 'amplified diode' voltage appears between the collector and emitter terminals in the circuit shown.
  16. Alec_t

    SurgeProtector Strange behaviour

    Who knows? Since we don't have a schematic of either the surge suppressor or your device we're working with both hands tied behind our backs :D.
  17. Alec_t

    Trouble with solder

    Looks to me as though (a) you didn't let the legs of the components get hot enough for the solder to flow along them or (b) the legs weren't clean before soldering.
  18. Alec_t

    Defective PCB/scroll saw

    If there's any chance the saw has been dropped or knocked recently, pay particular attention to the solder joints of any of the heavier components. Any high Wattage components are also suspects for bad solder joints.
  19. Alec_t

    Diode - Temperature

    The junction voltage of a diode changes by about -2mV per deg C rise, but if you want to experiment you can use the base-emitter junction of a transistor in an 'amplified-diode' configuration as shown in the attachment, to give a greater change. This behaves like a diode with a 0.6V junction...
  20. Alec_t

    SurgeProtector Strange behaviour

    IF the surge protector includes an inductor and IF your circuit draws a rapid burst of current I guess it's possible that a back-emf voltage spike is generated which gets through to something critical in your circuit?
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