Promote Systems has what you need. Buy Promote Remote Control with bulb ramping kit. It is the best and most powerful solution on the market.

What makes it "best and most powerful"?

It's got more features than a swiss knife!

1. Single Shot mode
2. Stack focusing (dream feature for macro photographers)
3. High Dynamic Range (up to 9 EV stops, much higher than any internal bracketing of any camera on the market)
4. Bulb Ramping (if you're into time-lapse photography, this feature alone is worth the money. It's a Godsend)
5. Manual Shutter Hold (for bulb shooting)
6. Bulb HDR Ramping (this feature is cutting edge)
7. Time-lapse
8. Time-lapse HDR (makes a really tedious task super easy and intuitive)
9. Hyperfocal calculator

Device is also weather sealed and it's got a large backlit LCD. Promote Remote controls the camera via shutter release cable + USB cable and in case of Bulb Ramping, it is connected to the camera via shutter-cable + PC Sync cable. P.R. has a complete control over your camera, its auto-focusing mechanism, mirror-locking, etc.

I own pretty much ALL remotes out there and have tested them extensively over the past 2 years. Promote Remote is a super-robust, professional workhorse for a serious photographer and in terms of features and performance, it is in a league of its own. Promote System's tech support is top notch and firmware updates keep adding the new and exciting functionality to the already impressive list of features. In short, there's no other device on the market that can even remotely compare to Promote Remote.