USB Remote Notes

Enable built-in CHDK remote support First, ensure you are running a reasonably up-to-date CHDK build. Some early builds did not support remote shutter release control via USB.

Now enable remote support via the CHDK menu: Main Menu > Miscellaneous Stuff > Remote Parameters > Enable Remote

Note. If you enable the "Sync Delay" then the shutter will release after you stop the voltage to the USB port. This was done because it was found they could sync the shutters on 2 cameras for stereo imagery much better as the voltage dropped. (It can also be useful if your switch uses poor-quality contacts, in which case the rise in voltage when it closes may not occur quickly enough to trigger CHDK's remote function.) Under most single-camera circumstances, however, you won't need this unless you need to sync the shutter release more accurately to your voltage trigger signal for specialized reasons.

USB Remote Cable

Using a USB cable, a low voltage power supply, and a push button switch, you can construct a device that will allow you to remotely trigger the shutter of your CHDK-enabled camera. Remote triggers are most often used in low-light situations when pressing the shutter button on the camera could introduce picture spoiling camera shake, but may also be useful in other circumstances - for instance if you wish to trigger the shutter with your foot, or with an electronic signal from another device.

In order to trigger your CHDK camera remotely, you will need to: connect a suitable triggering device to the USB socket of your camera; ensure "Enable remote" to have been selected in CHDK; run a suitable script; place your camera in shoot mode (i.e. not playback mode, movie mode, alt mode, etc) with CHDK still running; operate the trigger!

Homemade USB remote cable
A popular method for remotely triggering CHDK-enabled cameras involves constructing a simple triggering device from a 3V CR2032 coin battery, a USB extension cable, and a push switch. The camera's USB cable is then attached to the camera at one end and to the triggering device by the other end. Below is a circuit diagram of such a switch.

Your triggering device should incorporate a power supply - a battery, for instance - that has its negative (or ground) terminal connected to pin 4 of a mini-USB cable and its positive terminal connected to pin 1of a mini-USB cable via a momentary push-to-make switch.

The power supply should provide a voltage that is no greater than 5V (or you may damage your camera), but which must be greater than or equal to the minimum voltage specified on the Camera-Features page. See: http://chdk.wikia.com/wiki/CameraFeatures

 
If you are cutting up a commercially-made USB extension cable, then wires 1 and 4 are red and black, respectively, according to the USB standard. It seems that even cheaply made USB cables follow this standard.

Note that the 3V CR2032 battery might not be enough to trigger your camera; if not, you may need to use a different power source, e.g. three 1.5V AAA cells connected in series.

Credit: CHDK User Guide and the Dedicated Individuals who developed it.