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I ordered several solenoid's from American Science and Surplus to see what I could come up with. The next problem I ran into was the 555 timer circuit I used in the first digital flight camera. That circuit would open and close at equal time intervals. While that would work if you wired directly to the shutter release in most cameras, it wasn't a good option using a solenoid because it kept the solenoid activated too long, causing the solenoid to heat up. It would likely work fine in a rocket where you take a picture every second or two, but in my balloon project I'm looking at two hours worth of pictures, so I needed a longer delay between pictures, as well as just a short pulse out to the solenoid.
After searching the web for some viable options, I modified a circuit and came up with the circuit shown below.
Parts List:
Reed relay.......... 275-233 (Radio Shack)
6.8 K Ohm 1/4 watt resistor
330 K Ohm 1/4 watt resistor
16 volt 100 uF electrolytic capacitor
1N914 silicone switching diode
555 timer IC
8 pin IC socket
2) two junction terminal blocks and the
Printed circuit board or perf board. I've been making my own PCB's for some time now using a laser printer, here's the image file for the PCB.
This is a top view showing component layout and an x-ray view of the circuit layout.
Here's the camera and timer as set up and operational on my work bench.
As you can see in the picture above, the solenoid is a pull style, where a push style would have been much easier to mount. I know, it sort of has the junior high science project look to it, but it works. The plywood base is attached to the camera using the camera tripod mount, the aluminum bracket on the top is just hot glued to the camera. I'd like to run a couple of screws into the camera case, but I'm not sure if I want to risk running a screw into the electronics. The lever arm is a slice of fiberglass from a copper clad board, a small machine screw glued to the lever arm contacts the button on the camera.
My first attempt at this used a rotary solenoid, but that solenoid didn't have enough power to make it reliable. This system has been turned on several times and just clicks away... I decided to slow down the picture taking to about one picture every 26 seconds, I could go much faster, but in a balloon I didn't see the need to have a picture that often. Not much changes minute to minute on a balloon flight, at one picture every 26 seconds, a 2.5 hour flight would give me 360 photos! Still more than I'd likely ever want to look at. All that's left is to wire a battery pack to the solenoid, I'll use my 8.5 amp hour 3.6 volt lithium batteries for that. The timer uses only about 9 mA between pictures, then about 25 mA for the 1 second pulse on shutter release. So I think I'll just use a regular 9 volt alkaline battery to power the timer.
I'd need to do some playing around with the capacitors and resistors to change the timing duration, with the diode the way it is, I don't think the standard formula for calculating the time will work here. But I think going from a 100 uF cap to a 200uF cap should give you about 50% more time. Likewise, doubling the large resistor should increase the time about 50%. It works the same the other way, halving the large resistor should give you about 50% less time.