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Return to BlogUSB Caps Locker Based on ATtiny45
Posted Feb 01, 2012 at 3:12 pm
Figure 1
The Caps Locker is a device that plugs into a USB port and takes control of a computer’s Caps Lock key which switches it ON/OFF at 30-second to 8-minute intervals.
The circuit was built around Atmel AVR ATtiny45 which requires very little modification to obtain the random Caps Lock activation working. It implements a USB HID keyboard and instead of doing anything useful, it sends scan code for the Caps Lock key after waiting between 30 sends and 8 minutes, causing the Caps Lock status to be toggled ON/OFF. At this instance, the Caps Lock light also toggles since the operating system controls the LED on the keyboard. It was intended for demonstration and experimentation purposes only and should never be used in situations that would result in loss of data.
The PCB used is a single-sided with a couple of jumper wires for simplicity. The PCB can also be modified to avoid using jumper wires by employing test clips directly to the ATtiny45 leads. To prevent connection problems due to oxidation and to help tin the USB pads, the parts are soldered onto PCB which was created in a tiled set.
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2 years ago: Thanks…it was really a spur of the moment thing, as you can see by the messy construction. but really easy to put together. obdev’s avr-usb works great. it only took a little while to dig into the easylogger code and disable the adc and button detection parts, randomize the timing, and have it only send scancode 57 (caps lock). if you want to do something similar, just make sure a report with keystroke is followed by an empty report. i thought of a few other interesting variations, trying to keep focus on “harmless” stuff. sending a “volume up” keypress is easy…could be quite effective. holding down shift for a while would be more subtle than caps lock. how about pressing “F1? every couple minutes to be extra helpful. mouse wiggles can have bad consequences, but should be highly entertaining on a gaming machine. wheel of death: mouse right at high speed, click mouse1 button at 10hz and roll mouse wheel for weapon select. anyway i’ll put some schematics and pcb and code up later for those of you who want to try this. the eagle part for the board-edge USB connector could be handy too, i’ll make it available.
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