If you don’t own a cat, hearing the sound of one meowing from somewhere in the house probably comes as quite a shock. The Cat Prank box built by [Reuben] promises to deliver such hilarity with aplomb. 

The idea is simple: hide the Cat Prank box in a cupboard or other space in a friend’s house, and it will meow from its secret location. When found, either the light sensor or motion sensor will trigger the yowling of an angry feline, with hopefully startling effects.

An Arduino Mini is the brains of the operation, paired with an XY-V17B sound module which plays the required animal wailings. There’s also a 433 MHz radio module that lets the prankster trigger meowing via remote control.

Code is available for those wishing to build their own. We’d love to see a mod with a time delay built in, so the device could be hidden and left to start meowing at some later date when the prankster is far away.

Similar work has graced these pages before, like the devilishly fiendish OpenKobold design. Just make sure your friends are receptive to such jokes before you go ahead and invest time and hardware in the prank!

Taking great pictures means making them more vibrant enhancing saturation and contrast. Ynformatic has published some tips to help you do that by creating a DIY device to control a polarizer using an Arduino Pro Mini, an iPhone, and a screen from an auto-darkening welder’s mask.

A phototransistor located facing the iPhone’s flashlight LED is connected to both an external interrupt pin and an analog pin. Short pulses on the LED cause interrupts in the Arduino code which are used to synchronize the polarizer. Long pulses on the LED cause the Arduino to enter calibration mode. The time interval between syncrhonization pulses is continuously measured and divided into three equal parts. On receiving a synchronization pulse the voltage is set to 0V for one part, to the 45 degree voltage for one part and finally to 5V for one part. Voltage for the polarizer is supplied from an Arduino PWM output pin. To get a reasonably stable output the PWM frequency was increased to 32 kHz and smoothed with a second order RC filter. The liquid crystal display will be damaged by a constant DC voltage so a CMOS switch is used to alternate the polarity. A 2 kHz square wave generated from a free running Arduino timer is used to drive the switching.

An iPhone app written in Swift is responsible for the user interface and image processing.

Explore the schematic in the picture below, while the full source code for the Arduino and iPhone can be downloaded from here.

Read the full article on MAKE