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Electrical Engineering help me!

  1. May 25, 2004 #1
    For a robotics project i have to make an electrical circuit in which 5 light sensors need to light 5 corresponding LEDs.The LED must only turn on when the light sensors detect the infrared light from a laser pointer and afterwards must stay on permanently(assuming the battery is still running).We have a 9V battery that supplies power.We can use anything to make the circuit as long as its rather compact.We can use standard electrical parts like resistors transistors diodes etc.

    Can someone please tell me of a way to do this or perhaps design a circuit to do this and tell me about it.
  2. jcsd
  3. May 26, 2004 #2


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    Ok, here's an idea and you can use google to find the answers.

    Step 1: Get light converted to electricity. Your light sensor is a photodiode or photovoltaic or something like it - it can be used to detect light and create a corresponding voltage output.

    Step 2: Once that voltage level (light level) crosses a threshold, you want it to activate the circuit. A voltage divider with potentiometers gives you the ability to adjust the levels respectively to adjust where this voltage level is based on light levels (room lights and laser pointer light levels).

    A 555 timer chip has inputs that can respond to voltage levels. If you take the 555 wired as a monostable it will produce a single pulse once a certain voltage crosses a threshold on one of its inputs.Then the output of the monostable 555 can be used to turn on a transistor.

    Step 3: Use the output to create a latching circuit. When this transistor turns on, you wire another transitor to turn on as well and they keep each other on and the LED as well.

    Many many other solutions exist. But this one has some handy parts in it that can be used elsewhere to boot. And a 556 chip has two 555s on one chip so its easier and cheaper too, FYI.

  4. May 26, 2004 #3


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    I hope you will take Cliffs suggestion and look into what he's given you. If you understand these steps and the devices to accomplish them, you might find designing circuits can be lots of fun.

  5. May 26, 2004 #4


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    Here read these:
    http://www.doctronics.co.uk/voltage.htm [Broken]
    http://www.doctronics.co.uk/ldr_sensors.htm [Broken]

    If you make it halfway down this page (the rest you don't need), he gives a circuit that will satisfy your needs in a different way. You get to use digital logic with the NAND gates, so its just merely another way to do it.
    http://www.doctronics.co.uk/biscuit.htm [Broken]

    Last edited by a moderator: May 1, 2017
  6. May 27, 2004 #5


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    I would search the major semiconductor manufacturers (Lucent, Motorola, Dallas, National, Linear, TI, and there are others that I can't think of right now) for transimpedence amplifiers (TIAs) or O/E chips.
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