555 based photovore questions

Discussion in 'The Projects Forum' started by alex12342011, Mar 19, 2011.

  1. alex12342011

    Thread Starter New Member

    Mar 19, 2011
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    I am bread boarding the circuit in the image below. However I am confused about the line going through the 555 timer. Does this mean that pins 2 and 6 are connected? Or is it just pin 2 which goes in between the two photodiodes?

    Currently, the problem I am having is that the motors just stay a constant speed. Even without the connection to pin 3, they are both spin as if directly connected to the battery, unaffected by the chip. How is pin 3 even supposed to be connected? The two motors look like they are connected in series already, as if directly supplied by the battery. I am supplying the timer with power from 2 aaa batteries and the two motors are 1-3 volts each. I also have a cmos 7555 chip which I tried, it does the same thing. I am pretty sure its my wiring which is off.

    Any help is much appreciated, thank you.

    [​IMG]
     
  2. alex12342011

    Thread Starter New Member

    Mar 19, 2011
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    I am using photo transistors by the way.
    Labeled "photo transistor rx 5mm 880n"

    Checking the resistance of each, with a 25watt flourescent desklamp 3 feet away from me I get:

    0.6MΩ and 0.8MΩ

    Right under the lamp I get

    Roughly 0.9KΩ for each

    While we are at it, each motor read 1.5Ω
     
  3. alex12342011

    Thread Starter New Member

    Mar 19, 2011
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    Anyone know about the pins?
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    I am not speaking as to your schematic, I don't know if it will work. The 555 often has pins 2 and 6 cross connected, as this makes the 555 into a Schmitt Trigger. It is a very common configuration.

    555 Schmitt Trigger
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    3v is too low for a standard transistorized 555 timer to work.
    CMOS 555 versions have very limited current source ability.

    If you were using a 6v supply with 6v motors, you'd have much better luck. I can't think of a good way to fix this one offhand.
     
  6. alex12342011

    Thread Starter New Member

    Mar 19, 2011
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    Thanks for the replies. What about something along the lines of this circuit? If the solar panel is replaced with two aaa batteries. I can only power the circuit using aaa batteries, with a maximum of 2.

    [​IMG]
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Just how much voltage do you expect those photodiodes to generate? A 555 has two set points, 1/3 Vcc and 2/3 Vcc. You will have to exceed 2/3 Vcc for a 555 to flip.

    I don't see that solar cell having that much power or voltage. How much voltage are you getting? Wookie's comment about 555 power requirements are right on.

    What power you've generated will be shorted through the base emitter junction of the transistors. A BE junction looks like a diode, and has no real current limiting characteristics. Some resistance required.

    Enough negativity. What is the voltage of the solar cell when driving the motors under load (dragging the chassis along) and how much current is being used? You need some basic numbers to make this work.
     
  8. alex12342011

    Thread Starter New Member

    Mar 19, 2011
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    I don't want to use the solar cell, I wanted to modify that circuit by using aaa batteries. Also the photodiodes will be varying resistance allowing more or less current to pass.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Then it won't work. The pin 2 and 6 combined inputs are voltage inputs. There is no current to be had there unless you provide it.

    Just curious, using the advanced search function (not the standard one, it is broken), have you searched for photovore? We get a lot of those kinds of threads here, I think you'd be happy with the number of hits. I suspect the site you are getting the schematics from are pretty off base, as in wrong.
     
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