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

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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!


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