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  1. KrisBlueNZ

    amplify ultrasound signal

    You could look at H-bridge drivers like L293, L298, L6201/2/3, L6205/6/7 from STMicroelectronics. These provide a complementary drive at several amps and operate up to around 50V. The output seen by a tweeter conneted across the H-bridge is actually a bipolar square wave, which will probably...
  2. KrisBlueNZ

    hih4030 voltage drop

    In my experience, a typical successive approximation ADC has a fairly high input resistance but requires a low driving impedance at AC, because of the current spikes injected back into the driving source during the switching involved in the successive approximation process. These spikes are very...
  3. KrisBlueNZ

    infrared LED

    Can you place a reflector on the other side of the door? How about two narrow beams of IR light using different modulation frequencies, reflecting off a surface on the other side and received by one or two receivers. If the reflector isn't perfectly smooth there will be some scatter so the...
  4. KrisBlueNZ

    Basic transistor Guitar Amp

    Your circuit is a small-signal amplifier. It can't drive a loudspeaker. Its output impedance is too high (the collector resistor is 1k). You could scale down all the resistors and replace the transistor with a big one running at a much higher current, then you would have a Class A amplifier...
  5. KrisBlueNZ

    infrared LED

    duke37, nice ideas! Re avoiding bogus triggering, obviously put the emitter and detector as close to the items being detected as possible. Also, if these reflective surfaces are always facing exactly the same way, you could angle the emitter and detector at say 30 degrees off the perpendicular...
  6. KrisBlueNZ

    hih4030 voltage drop

    Since it's specified as a resistance, not a current, I assume it's the minimum resistance.
  7. KrisBlueNZ

    Transistor Question

    Yes, bipolar junction transistors do require SOME base current, but how much current the base will draw depends on the circuit configuration. The comparators in the 555 are made with bipolar transistors (at least, they were on the original devices), and they will draw a very small amount of...
  8. KrisBlueNZ

    infrared LED

    duke37: snap!
  9. KrisBlueNZ

    infrared LED

    I guess a parabolic reflector would be best, or failing that, a tube. Are you using pulsed infra-red? This is the best way to distinguish your reflected light from ambient light. It's the method used in infra-red remote controls. You pulse the LED at a fixed frequency and use a receiver with a...
  10. KrisBlueNZ

    hih4030 voltage drop

    Yeah, it wasn't where you'd expect it to be.
  11. KrisBlueNZ

    hih4030 voltage drop

    Figure 9 on the data sheet says that the minimum output load resistance is 80k.
  12. KrisBlueNZ

    5V Logic from 12V

    Steve's advice is good as usual, but I have a few points to add. 1. The noise filtering capacitor, and the capacitance of the zener diode (which can be in the nanofarad range) will both cause a delay on both the rising and falling edge of the input as seen by the micro. Steve's suggested values...
  13. KrisBlueNZ

    RS-485 (MAX487 / MAX485) - On-chip fail-safe feature

    Re guaranteed high receiver output on undriven RS-485 bus. The MAX48x data sheet says only that the receiver has a fail-safe feature that guarantees a logic high receiver output when the inputs are disconnected; it doesn't say how this works. My comments on offsetting the differential input...
  14. KrisBlueNZ

    Simple voltage limiter question

    I think you're misunderstanding what regulators do. A regulator will do what you want. If the input voltage to the regulator is low, the output voltage will be low as well. A regulator can't create voltage from nothing. You should also consider the regulator's dropout voltage. If a regulator has...
  15. KrisBlueNZ

    Current Reduction

    Sounds very much like a problem with the circuit. Two small 9V batteries can't supply enough current to do much damage, but I guess your three 6V batteries can. If the circuit is supposed to operate from 18V, then using three 6V batteries should not cause the semiconductors to lose their...
  16. KrisBlueNZ

    DC motor speed control using 8051 and l298

    I agree with CocaCola's comments. Regarding port pullups, IIRC the standard 8051 port architecture (this is an 8051 device not an AVR) have pullups by default if the output is driven high. Driving a port pin low forces it low with a hard pull-down, and means it will always read low; driving it...
  17. KrisBlueNZ

    3V 2 LED flash circuit

    D'oh! Yes I meant astable. The reason I suggested leaving the collector resistors in, and adding LEDs with resistors in series with them across the collector resistors, is that without a resistor directly from collector to VCC the collectors will not pull fully high when the transistor turns...
  18. KrisBlueNZ

    DC motor speed control using 8051 and l298

    There are some obvious problems with your circuit diagram. 1. The two capacitors from the crystal to ground are shown as 1uF but they should be in the picofarad range, typically around 22-33 pF each depending on the specifications of the crystal. 2. The way you've drawn a battery connecting to...
  19. KrisBlueNZ

    Help on circuit

    If I understand your post, you need a circuit that drives a single 12V bulb from two inputs, each of which is either energised (steady +12V) or de-energised (no voltage) in the following way. When the LIGHT input is energised the bulb is ON, except if the FLASHER input is also energised, in...
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