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12V DC Motor ON / OFF / Direction Control and Feedback

Discussion in 'Sensors and Actuators' started by OvaR8ed, Feb 24, 2012.

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  1. OvaR8ed


    Feb 24, 2012
    Concept / Outcome:
    I'm planning to drive a laser pointer slowly using a simple geared down 12V DC motor through an angle of about 70 degrees using output from PC. The gearbox is designed such that the laser travels very slow (i.e. 70 degrees = 5mins).

    I want to place LDR sensors at the limits ofthe sweep to detect the laser and give feedback to my software to stop the laser and initiate other stuff on the software side.

    Interface and I/O:
    I don't have a micro-controller so I'm planning to simply hack a USB keyboard for the chip. Windows already has drivers for it, it has 3 binary output channels (CAPS LOCK LED, NUM LOCK LED and SCROLL LOCK LED) and more input channels than there are buttons / keys.

    When the laser hits an LDR it opens a transistor circuit to switch a Solid State Relay connected to a specific key. This hotkey will stop the motor and change the state of one of the outputs to change direction of the motor. The motor can then be started again from the software with the other LDR waiting on the other side for the cut-off signal.

    Potential Problems:
    The LDR recieves a contious signal from the laser and the computer input only wants a short pulse so I decided to use a RC differentiator circuit to send a short pulse when the laser first hits the LDR. The motor stops over the LDR and even though the laser keeps the LDR circuit in ON mode, the signal (hotkey) to the PC will be off after the short pulse.

    The solid state relay that initiates the hotkey for the keyboard chip requires between 1-5V and 10mA to switch on so I designed the following circuit to send the pulse. (See pic/link below)

    Any comments and/or suggestions are very welcome, thanx!

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    That link you provided is broken. I presume the circuit you have attached has some relevance to it.

    The attached circuit seems unworkable, but I don't know for sure because there is some uncertainty in my mind as to where the +12V connects and whether all the ground connections are actually grounds.

    The chief problem would be that the collector is effectively shorted to your input. I would normally expect to see the collector connected to 12V and he LDR being one leg of a resistive divider between +12V and ground.

    Normally one might expect the load to be in the collector rather than the emitter as your arrangement adds significant negative feedback which would cause a linearly increasing voltage across the load over a wide range of illumination of the LDR. I suspect you want the load to switch from off to on over a relatively small range of illumination.
  3. OvaR8ed


    Feb 24, 2012
    Sorry for the confusion, the LDR circuit is not a single LDR, but it is an entire standard LDR Switch Circuit with 12V power supply using a transistor to switch a Relay on when the laser hits the LDR.

    The attached cicuit is the RC Differentiator circuit used to convert the contiuous current comming from the LDR circuit's relay to a single pulse of about 200ms long to use as a signal through the hacked keyboard chip (Like a hotkey) to the PC.

    Without the Differentiator the signal to the PC will be continuous causing sticky keys and all sorts of rubbish to happen...

    I wasn't sure about the Differentiator circuit hense the post. If anyone knows of an easier way to achieve the same result, please let me know.

    I'll try to work up a complete circuit to show the everything.
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