Jeffrey Warren has extensively investigated using 555 circuits to trigger cameras via CHDK. This appears to be the least expensive option, [his notes are here](/wiki/dual-camera-kit-electronics). for completely DIY 555 circuits, [see Mathew Lippincott's note here](/report/adjustable-555-based-shutter-trigger-circuit). Chris Fastie has explored the 555 setup and two other options providing greater degrees of control. His 555 timer setup is beautifully constructed and costs less than $20: Chris has also useda tiny remote control purchased on ebay. [his notes and KAP forum discussions here.](/notes/cfastie/4-22-2012/tiny-remote-control) And an [AuRiCo](http://skyware.fam-engels.de/), or AutoRigController, [sold in the states by Brooks Leffler](http://www.brooxes.com/newsite/BBKK/BBKK-PARTS.html#AuRiCo). His notes are [here](/notes/cfastie/4-10-2012/almost-synchronous-shutters) and [here](/notes/cfastie/4-14-2012/good-shutter-synchronization). The AuRiCo is quite capable and very small and lightweight, but costs a lot (it can also control two servos). The one sold at Brooxes for US$64 is a special version that DOES NOT HAVE an output for USB shutter release via CHDK. So it requires a US$44 cable (same page) to work via USB. [Peter Engels](http://skyware.fam-engels.de/) sells a version that does have an output for USB triggering via CHDK.