Sensor and Detector: Laser Pointer Triggered On/Off Switch

 

 Figure 1. Left: Controlling a presentation with a laser pointer. Right: Turning on/off an alarm clock or lamp from bed.

'Laser Pointer Sensor'. - Remote control is commonly performed with either modulated infrared emitters or radio-frequency wireless transmitters. During a boring presentation, it occurred to me that the presenter could control a slide show by aiming their laser pointer at a list of commands with bullet-point targets. Alternatively, someone lying in bed could set the clock alarm/snooze (across the room) or turn off lights by simply aiming the laser dot at the correct spot on the desired object. This would eliminate the need to have special remotes for each manufacturer. 

This article demonstrates a simple circuit with two off-the-shelf logic chips to allow a laser to turn on and off an LED. Obviously, the electronics would need to be expanded to control an appliance or support additional sensors. However, this example project is perfectly functional for controlling a low-voltage DC device, such as starting a robot.  

Schematic of the Laser Pointer Sensor

The full circuit is divided into two parts for learning purposes:

1. Detecting a laser aimed at either the “on” or “off” sensors.
2. Setting the correct output state, and keeping that state even after the laser pointer is turned off.

Here is the first wiring diagram that handles laser detection: 

 Figure 2. A schematic showing two sensors, two trimpots, and a TI TLV3702 comparator chip.

IC1 is a comparator chip that compares the voltage across R2 (a light sensor) to the voltage across R22 (a potentiometer). When the laser hits R2, the sensor’s resistance drops, which causes the voltage across it to drop. When the laser is not targeting R2, the sensor’s resistance increases (to normal) and the voltage follows.

If the voltage of R2 is lower than R22, then the comparator chip pin 1 outputs low (0 V). If the voltage of R2 is higher than R22, then the comparator chip pin 1 outputs high (5 V). Incidentally, if you wanted the opposite result, you could just swap the inputs to pins 2 and 3. 

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