My brother wanted to mod an Xbox 360 controller so that one of the triggers could be used in a rapid fire fashion.
There’s things online that suggested he could literally wire one end of one of the LEDs to a switch and the other side of the switch to the middle pin of the variable resistor that creates the variable trigger in the controller. Presumably the crystal on the board is supposed to be hooked up to drive the LED in the controller. But it didn’t seem to work. I didn’t have an oscilliscope handy so I couldn’t see what the signals were doing, so I wasn’t sure if it should work or not anyway. So it was back to the drawing board…
Luckily on his search around the Internet, he’d come across this blog
The principle is simple enough, take a 555 timer, stick a few components on it to set the interval of the timer and hook the timer output up to the controller.
A little measuring around things suggested to me that the variable trigger is simply just a variable resistor. The resistance between ither CW or CCW and the common pin were 2K at one extent and 3K at the other extent. I guess that’s a nice simple way to make a variable trigger work. Knowing the resistances and such, I then measured the voltages between CW and GND, CCW and GND, common and GND. One result was 0V, one was about 1.6V and commond was about 1.3V. That kinda makes sense, as the voltage of the common pin moved between 0V and 1.6V presumably the system would think it was either fully on or fully off. Nice and simple.
Now the Xbox360 controllers are USB, so i’d got 5V and GND to power the 555 chip. I knocked the circuit up quickly, using a 100K variable resistor in place of the 22K and hooked the chip up to the USB 5V and GND pins to power it. Again i’d not got an oscilliscope, but using a voltmeter I could see the output of the 555 was oscillating to some extent.
I was mildly concerned about hooking up the 5V output of the 555 directly to the common point of the triggers variable resistor as the controller was only providing 1.6V maximum as far as I could tell. But seeing as the 3.3V line that my brother had hooked up to it based on the ‘one wire and a switch’ method hadn’t broken anything, we gave it a try anyway.
A few minutes later (and after tweaking the variable resistor a bit) my brother was happy that he would empty the entire clip of what appeared to be a single shot hand gun in Call of Duty in around 1 second, rather than the 3 or so seconds it would take to mash the trigger frantically otherwise.