I have Nebulosity 4 for controlling my QHY camera, but the Nikon D3100 is not supported. For that camera, I need to run a shutter control app. It allows me to setup a sequence of photos with configurable times between to allow for saving the image. Some work by interfacing to a commercial DSLR trigger adapter, like ShoeString Astronomy's DSUSB/DSUSB2.
If you want to do it yourself, some will also work with almost any USB to serial converter that brings out the RTS signal. That is what this project is about - making your own using a USB to Serial converter.
In order to get color shots, I have to use the Nikon D3100 DSLR. The factory automation software does not support the D3100. The solution is DSLR Shutter from Stark Labs. It is a free program for Mac and Windows that can use a generic USB to Serial converter to control the shutter by toggling the RTS line.
The converter is a little special, because it has all of the modem control lines brought out to headers on the edges of the board. You need this kind of converter. ebay is a great source for them. They cost around $5.00 from sources in the USA. This one came from MDFLY Electronics.
The converter has the serial lines on a header on the end opposite the USB connector. It isn't used and it occupies space I don't have, so I unsoldered the header from the board. The lid to the enclosure won't go on with the voltage selection jumpers in place, so they were removed as well. If I had used a larger enclosure these changes would not have been necessary.
On the Nikon, the shutter won't work if the focus is not pressed. To get around this the photo line is connected to ground. That causes the voltage on the focus line to go to -2.7V. Then when the focus line is grounded using the opto-isolator, the shutter is opened. When the focus line is ungrounded, the shutter closes and the camera saves the picture. Seems backwards, but that's the way it works. You can't hook the shutter and focus lines together directly because one has +2.7V on it and the other has -2.7V. With a lens attached and auto-focus enabled it works the other way - focus is closed then the shutter. With auto-focus off this circuit will control the camera with a lens attached.
On other DSLRs the shutter may behave differently. Some I know only require you to close the shutter switch. You might experiment with your camera, but keep in mind you are shorting a pin to ground - make sure it is the right pin. I would recommend finding out as much as you can about the way the shutter remote works on your camera before attempting this. Starting with a cheap shutter release cable is almost the only sure way for some cameras.
If you can't tell the pinout with the shutter release cable and a meter, look at some more popular DSLR Shutter Release Pinouts.
A 4N35 opto-isolator separates the camera from the USB to Serial converter. Opto-isolators use an LED as input and a phototransistor as the output. When the LED is on, the phototransistor is on and conducting. What we are doing is connecting the phototransistor to the focus line, the LED to the RTS line, and by doing so, controlling the shutter with the RTS line. If your camera is not a Nikon, you will need to measure the voltage on the shutter or focus line (whichever runs the shutter) and connect pin 4 of the opto-isolator to the most negative voltage and pin 5 to the most positive voltage.
Forgive the nail polish and the flowable silicone sealant. The sealant holds the board in and the nail polish hides the LEDs.
The extra resistor is due to my using too large a value when I designed it. During testing I saw that the opto-isolator did not have enough gain to pull the focus line to ground, so I paralleled another resistor. The 470Ω resistor in the schematic is correct for my use. Your value may need to be lower, but try it with 470Ω first.
All in all there are three wires and one resistor added to a cheap USB to TTL Serial converter, and a 4N35 opto isolator (also available on ebay). It shouldn't be too difficult once you figure out your pinouts.
The cable is cut from an aftermarket Remote Shutter Release Cord for the DSLR I'm using. Yours will be different. I had to open the shutter release case, see which pins did what (it was marked on the board) and then remove the cable and wire it with a 3.5mm stereo plug on it. Only two pins are used, but it is easier to get a stereo plug and jack. By using the plug and jack it made it possible to change cameras without having to re-wire the circuit. One just needs to build a new camera cable.
Since there is no interface to the computer, other than the shutter, I have to store the images on a flash card in the camera.
Dark frames have to be made the old fashioned way, throwing the lid on the front of the telescope. There is no way to make an exposure with the shutter closed on the DSLR.