2nd B.E.A.M. bot attempt - Herbie Photovore

Discussion in 'The Projects Forum' started by skullforger, Jan 26, 2009.

  1. skullforger

    Thread Starter Member

    Nov 3, 2008
    14
    0
    Hi,

    After finishing the first one (it works very well, thanks to this forum!) I've started building the circuit of the second one. It is similar to the one in this post http://forum.allaboutcircuits.com/showthread.php?t=9139&highlight=photovore

    I've attached a picture of the circuit I'm trying to build (I'm sorry for the quality).


    I've built the circuit on a breadboard (except for the trimpot, I've used another 47k resistor instead). When I connect the battery, nothing happens. When I test it with a multimeter, I have value 0 (ground) on pin 5 and 6 of the LM386 (the pins that normally should drive the motors). But when I disconnect the motors, the values on pin 5 and 6 are okay (3 to 6 V, depending on the light conditions)! I think I'm missing a clue here, but I find it very strange! Normally if a positive lead has 5 V and you connect it to a motor of which the negative lead goes straight to ground, it should turn, shouldn'it?

    Any help would be kindly appreciated :)
     
    Last edited: Jan 27, 2009
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Will the motors run when you connect them individually across the battery?
     
  3. skullforger

    Thread Starter Member

    Nov 3, 2008
    14
    0
    Yes. Could it have something to do with the LM386 that can't keep the voltage high enough the moment a motor wants to drain power?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Pin 6 should be connected to the battery + (Vs).
    Pin 5 is the output of the amplifier.
    Pin 4 is ground, connect to battery -.

    You should be able to vary pin 5 from somewhere between ground to nearly Vs by changing the amount of light that hits the two CdS cells (one getting lots of light, one nearly dark). If you can't get the output to change, take a look at the input side and see what the outputs of the CdS cells are doing (pins 2 and 3 of the 386 amp).

    Being CdS instead of photodiodes, they will respond pretty slowly. Photodiodes or phototransistors would respond much more quickly.

    Since you are using two fixed 47k resistors instead of a pot, you will not be able to adjust the tracking of the 'bot once you do get the motors running. You could use a single 100k trimpot with the wiper tied to V+, one end of the pot to one CdS cell, and the other end to the 2nd CdS cell. Start off with the pot centered.
     
  5. skullforger

    Thread Starter Member

    Nov 3, 2008
    14
    0
    Thank you so much! Your explanation of the Pins made me realize that I didn't supply the Vs. It works now!

    I have bought 2 photodiodes, as you said they would react quicker. What is the best way to set them up in your opinion: photovoltaic mode or reverse biased mode? (My book depicts 2 different setups, in reverse biased mode, they seem to act like 2 photoresistors, needing power. In photovoltaic mode they don't need power because they generate a voltage themselves by sunlight? Did I understand this right?)
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Use them in reversed biased mode. You will have much better control over the voltage range that way. In photovoltaic mode, it's anybody's guess what your output will be.
     
  7. skullforger

    Thread Starter Member

    Nov 3, 2008
    14
    0
    Ok. Thanks for your help!
     
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