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  1. Arouse1973

    A replacement for a photodiode

    Ok I have had a long think about this and have come up with two things that might work. The first one is to use an opto isolator. The second one is a bit off the wall and I haven't seen this done before but it seems to work. It uses a 4 terminal FET as a diode. Current is controlled by a...
  2. Arouse1973

    A replacement for a photodiode

    This is proving a little more difficult than I first thought. Leave it with me. I am trying to keep it simple. Adam
  3. Arouse1973

    automatic doorbell

    Yeah I did think off the CD4060 for simplifying this but was unsure whether it would work, you just proved it won't :) Adam
  4. Arouse1973

    A replacement for a photodiode

    Yep sure give me a few hours. Adam
  5. Arouse1973

    automatic doorbell

    Although being of an analogue mind, I do like neat ways of doing digital things without the use of a micro. Adam
  6. Arouse1973

    automatic doorbell

    Yes I liked the 40 KHz generation, nice use of a counter to provide accurate timings. Adam
  7. Arouse1973

    automatic doorbell

    That's a neat circuit Kris. Adam
  8. Arouse1973

    A replacement for a photodiode

    Yes current sink not source. Adam
  9. Arouse1973

    38 khz IR diode driver

    The maximum current for the 2N2222 is 800mA which would require a clip on heat sink or large copper area on the PCB connected to the emitter. The maximum current for the LED is 1050 mA which is too much for the transistor. Now this does depend on whether your pulsing the LED or if it's on...
  10. Arouse1973

    Transistors switch on output pin or input pin

    Yes Kris this did cross my mind about temperature tracking but how poorly implemented it is I discounted it. The compensation done this way I think should be from a constant current source or in the feedback loop of the log amp. Another option I have also seen is a PNP transdiode on the output...
  11. Arouse1973

    Transistors switch on output pin or input pin

    Some op-amps can not take voltages too close to either supply rail, so I am guessing this is ensuring that the input of the op-amp always sits 0.6 Volts above the input signal. But I didn't design it and as we can only see part of the circuit and we don't know what it does. Thanks Adam
  12. Arouse1973

    A replacement for a photodiode

    Ok look at using an adjustable regulator as a current sink, all you need is one resistor and the regulator. Adam
  13. Arouse1973

    Transistors switch on output pin or input pin

    The voltage across a diode is a log function of the current through it. The second diode keeps the input of the second opamp approx 0.6 Volts above the output of the first opamp. You will have to ask the desinger of the circuit as to why. But I guess it because of the opamps inputs
  14. Arouse1973

    Transistors switch on output pin or input pin

    I have never seen a variable diode what do you mean.
  15. Arouse1973

    Transistors switch on output pin or input pin

    It is across the output and input,look again. Adam
  16. Arouse1973

    What kind of relay is this?

    A diode clamp is very aggresive, it pushes the same current that is in the coil into the supply. This might not be allowed so caps are used to store the energy which is the slowly used by the coil until the energy is gone. Adam
  17. Arouse1973

    Transistors switch on output pin or input pin

    Hello The first part is a log amp used for smoothing out very large variations in input signal to a more realistic level. Used in measuring light that varies from dull to very bright is one application. The second transistor is diode connected, its a diode basically and makes sure the input to...
  18. Arouse1973

    What kind of relay is this?

    The first one, cap across diode limits any flyback from the diodes inductance. The second one cap and resistor is a soft suppression, this stores the flyback energy in the capacitor rather than dumping large current into the supply. This could be used for very large coils which would make the...
  19. Arouse1973

    pulse generators

    Hi sorry I cant help you any longer. Thanks Adam
  20. Arouse1973

    A replacement for a photodiode

    Could you use a high power, narrow beam width LED driven from a pulse generator circuit? What photodiode are you using? Adam
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