Laser trip wire – the bare essentials

[Gordon] sent us a tip about this simple laser trip wire system after reading yesterday’s post on a more complicated laser security unit. That build did a lot to provide functionality, such as a system to disarm the trip wire, and a robust light detection circuit. This time around there’s more happening with smoke and mirrors than with electronics.

[The Timmy] built this simpler version based on a laser trip system from Afrotechmods (video of that one is embedded after the break). He had a bag full of small square mirrors which he attached to a wall with some poster putty. A laser module shines a beam of light onto a cadmium sulfide sensor after it bounces around the optical network for a while. That CdS sensor controls an N-channel MOSFET, switching it off when light is detected and on when the intensity of the laser is absent. This example just turns an LED on and off, but since it uses logic-level voltages you can choose to add a microcontroller to the mix if you have other plans in mind.

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Input device using LED and four photoresistors

[Julien] built an input device that uses reflected light detected by some photoresistors. Placing your hand above the device will reflect light from the LED back down onto the cadmium-sulfide sensors. The resistance of those sensors is read by four ADC pins on a Teensy microcontroller and translated to mouse movements. In the video after the break you can see that this works rather well in controlling the cursor. The source code is available on pastebin but we’re also going to host the code for posterity.

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Controlling an AC drill using one PWM connection

This peculiar setup allows [Ben Krasnow] to control an alternating current device using one pin on a microcontroller. He’s experimenting with a power drill and has relocated the trigger circuitry that makes it spin. On that board he found a variable resistor combined with a capacitor which control a triac, actuating the speed of a drill. [Ben’s] solution works great and isolates the drill from the control circuitry. He replace the variable resistor with a cadmium sulfide photoresistor (basically a variable resistor whose resistance depends on the intensity of light). Pulse-width modulation is used to adjust the brightness of an LED shining on that photoresistor and thereby affect the speed of the drill. This is such as simple alteration to the drill we’d call it MacGyver-esque.

See a demonstration after the break.

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Laser harp

[Jared] had a bunch of lasers left over from a previous project that he put to use by producing this laser harp. The look of it reminds us of a very small Koto or perhaps an Autoharp (although the chords can’t be changed on this model).

We’re so glad that [Jared] spent the time to produce such a fine looking body for the instrument. The strings that would traditionally produce the sound on a harp have been replaced with laser diodes shining at Cadmium Sulfide photo resistors. When a beam of light is broken, an Arduino detects the change via the CdS cell and plays a sound through an Altec Lansing speaker inside of the case.

Unfortunately there’s no video available but we’re pretty sure it makes a “pew-pew” sound. There is a link to download the source code but it points to the overview page instead of downloadable code. From the fritzing diagram the CdS cells are part of a voltage divider which provides digital logic to the Arduino. That should be pretty easy to replicate even without seeing [Jared’s] code and we’re sure you can source other Arduino instrument projects for tips on wave shield or midi functionality.

[Thanks The Cheap Vegetable Gardener]