Hardware Modifications For Triggering Capture

H. G. Dietz

Department of Electrical and Computer Engineering
Center for Visualization & Virtual Environments
University of Kentucky, Lexington, KY 40506-0046

Initial release: May 29, 2007

Fortunately, many digital cameras support capture of images under computer control, and software for control of them is discussed in Software For Tethered Capture Control. Unfortunately, not all digital cameras support triggering of capture via their computer interface -- for many, the computer interface does nothing more than make the camera appear as a USB mass storage device. Here we discuss capture control for digital cameras that explicitly do not support it.

It used to be that SLR film cameras generally had a threaded shutter button so that you could use a mechanical cable release with them, and there were plenty of electrical control hacks using a solenoid to simulate the action of a cable release. Kapture Group, Inc. sells a very fancy solenoid unit with various triggering facilities; it really isn't very difficult to build your own (like this or this), and can be even easier using a modern servo (as often is done for kite-based aerial photography).

However, that's not needed for most digital cameras. The key observation is that, even if the camera's firmware does not provide protocol for triggering the shutter remotely, virtually every digital camera has a shutter button that uses an electrical contact to trigger capture of an image. Thus, it is possible to provide tethered control by simulating the electrical contact that the shutter button provides. More precisely, most shutter buttons provide two electrical contacts:

  1. The half-press contact triggers focus and locks exposure
  2. Continuing for a full-press triggers the shutter

So, do you have to open your camera to get to these contacts? It depends. Most DSLRs have 3-conductor connectors that allow you to place your own switches in parallel with the camera's built-in shutter button. Unfortunately, camera companies seem to have avoided use of standard connectors for most of these, so you might have to spend $25 to $60 to get the manufacturer's dumb remote cable just so that you can cut the connector off it.

Typically, one of the three wires is ground and the other two are logic high values (often around 3.3V), so you might be able to use a transistor instead of a switch -- but be sure to check the voltages from the camera before you hook-up your circuit. If you bought one of the dumb remotes, it also wouldn't hurt to confirm which wire is which by first using a meter to check which wires get connnected when you press the remote release button. Because the manufacturer's cables are so expensive, there are wiring diagrams and other details for some camera remote release cables on the WWW:

It happens that the astronomy field has had to deal with computer triggering of camera shutters via a remote cable because that is the only way to use the "Bulb" exposure setting. DSLRs commonly limit timed exposures to 30 seconds, which simply isn't long enough for faint deep space objects. One might think you could get away with just pressing and holding the shutter button, but vibration is a terrible problem for telescope photography. Further, a technique called image stacking is used to improve the image quality by combining many such exposures -- so one normally needs to capture a lot of images. As discussed in the DSLR_Shutter documentation, four solutions have become common:

  1. Parallel port triggers using the least significant bits of the data output (D0 or D1 -- pins 2 or 3 on the connector)
  2. Serial port triggers using the Ready-To-Send (RTS) line, which also can be used with USB-based RS232C dongles
  3. The DSUSB adapter, which implements USB control directly
  4. The Hap Griffin cables, which include cable for Nikon DSLRs using their NIR remote support (because some Nikon DSLRs do not have wired remote connectors)

It is worth noting again that KAP fanatics have done a pretty good job of providing solutions (that happen to be light enough to be flown by a kite). Gentles makes an impressive variety of remote controls for use in kites and other radio controlled applications. The radio control outputs normally used to drive servos are quite easy for a computer to generate -- a pulse coded modulation (PCM) 5V control signal with millisecond-range pulses.

In any case, it is worth noting that the above techniques only solve the problem of triggering exposures; you still need something else to download the images. Fortunately, just about every modern digital camera can do that via a USB connection. The catch is that many cameras will disable (ignore) the shutter while connected as a USB disk drive to a PC. It may be sufficient to logically disconnect the USB, however, for some cameras, it is necessary to electrically break the USB connection. This can be done fairly straightforwardly, for example, using a relay to interrupt the USB power line.

The Aggregate. The only thing set in stone is our name.