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How do we find the correct resistance for a component — Parallax Forums

How do we find the correct resistance for a component

MunifTheGreatMunifTheGreat Posts: 20
edited 2012-01-02 10:02 in General Discussion
This is a project of my own I want to do.
It's a laser security system.
I have programmed the circuit as the same as a pushbutton.
Here is a schematic representing the circuit.
ALARM.jpg

My question is how can I figure out what resistors to put between the photoresistor
How can I determine the resistance if I'm testing this circuit in the dark with a laser ( unknown specifications)
Will it work?

Thank You.
__________________________________
Sorry, I speak better french than english and I'm new in electronics.
ALARM.jpg
503 x 397 - 28K

Comments

  • LeonLeon Posts: 7,620
    edited 2011-12-31 06:13
    Why have you got two resistors on either side of the photoresistor?

    Photoresistor data?
  • UnsoundcodeUnsoundcode Posts: 1,532
    edited 2011-12-31 07:13
    Hi , take a look at this link http://www.acroname.com/howto/photoresistor/photoresistor.html

    This is just one example out of hundreds you can find with a Google search.

    In this example the wire marked V is where you would connect to your Stamp pin P14. The only modification I would make would be to add a 220 Ohm resistor from P14 to the connection V, this would be a precautionary measure to prevent excess current damaging your Stamp chip

    Jeff T.
  • Mike GMike G Posts: 2,702
    edited 2011-12-31 07:23
    I'm not sure exactly what you are trying to do but... Use an RC circuit with a known capacitance to calculate the resistance. The idea is to measure the time it takes to charge or discharge the RC circuit.

    Resources

    Measure Resistance and Capacitance -·Updated (2/2/10) - found on the following link.
    http://forums.parallax.com/showthread.php?89958-Propeller-Education-Kit-Labs-Tools-and-Applications

    http://forums.parallax.com/showthread.php?111176-PE-Kit-Tools-Measure-Resistance-and-Capacitance&p=791692#post791692

    Plus we need to know what model (part number) photoresistor you are using.
  • paulvivepaulvive Posts: 35
    edited 2012-01-02 07:58
    Use Ohms Law To calculate
  • Spiral_72Spiral_72 Posts: 791
    edited 2012-01-02 08:26
    I'll probably be a good idea to use a current limiting resistor on that speaker. Unless it's a very tiny audio emitter you'll likely have to drive it with a transistor or op-amp or......

    The photoresistor circuit you have drawn might not work depending on what you have in mind:
    P14 as an input would do nothing as the photoresistor doesn't generate a voltage
    I suppose you could set P14 as an output, then current limit it to 5ma or so with the photoresistor at the lowest resistance (bright light?) Use ohm's law for this.... then you would monitor the voltage drop with another pin, or you might be able to read the state of P14.
  • tonyp12tonyp12 Posts: 1,951
    edited 2012-01-02 10:02
    P14 should be a input, the first resistor should not be inline like that, but instead pulled up to 3.3v (or 5v if that is Vss) and a 10k value should be OK.
    The second one you don't need at all.
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