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SpaceCam

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Trust Spacecam 200 Modification.

The Trust spacecam 200 is based on the ov7620 cmos sensor which is also used in many other webcams. This chip is described as a camera on a chip as it contains all the circuitry needed to take pictures, set exposures, convert to digital, write to sram, etc. The only other chips needed is the SRAM and a USB bridge. The on chip timing circuitry allows exposures up to 1/30 th of a second however the facility for another device to control the length of exposure is given by the FREX pin.

Data sheet is here


What this mod does is control the signal to FREX. As long as FREX is logical high the chip will be exposed. This time is determined by a 555 timer and can be altered between 0.5-10 seconds with a potentiometer. In order to work the exposure process needs to be synced to the vertical sync signal hence the second lead to be soldered to the chip. The main tools required to make the mod is a fine tipped soldering iron and some solder wick if things do wrong!


1 Make the controller circuit below.




2 Remove PCB from camera and remove lens to expose chip

3 leads need to be soldered on to pins 4 and 16. The pin numbering is below.





(Tip: cut a small piece of insulation tape and stick it on the pcb next to the pin to be soldered to. Take a 4" bit of insulated cable and remove 1 strand of wire. Fold in 2 and twist to make a very small loop. Apply solder to the loop and solder to the pin without adding fresh solder. The solder used to connect the cmos to the board has quite a high mp so don't be afraid to used plenty of heat)

4 connect up to the controller board taking 5v and 0v from USB connections (red and black leads)


5 mount in project box.


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In order for the FREX signal to control the exposure the camera must be in progressive scan mode. The standard driver uses interlaced for all modes except 600x480x16. In order to get round this the driver needs to be modified as follows:-

1 download new 7620p.set here.

2 Disconnect camera

3 backup the copy in windows/ovtcam/ and replace with the new version.

4 remove the spacecam driver from control panel- imaging devices.

5 Connect camera.

In addition to giving a 24bit progressive scan mode the new driver also:-
Set all amps to max
Minimises the A/D converter range
Turns off image compression over USB
Turns off vertical and horizontal line emphasis.
Turns off contrast enhancement and noise removal.
Makes a nice cup of tea.

Limitations.

The cmos chip works by first filling the photo detectors with charge. These detectors are made to be leaky with the charge leaking away faster the more photons fall on them. Apart form inherently not being very sensitive the max exposure is limited by the speed at which the charge leaks out of the chip in the dark. This shows up as increasingly high backgrounds until the image becomes white. With out cooling the chip exposures of around 4 seconds are possible. Given these limitations the mod does produce a camera capable of catching some deep sky objects on a 25 camera.




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A further, although probably unworthwhile, refinement is to add cooling. My first look at cooling this camera comprised of filling the project box with dry ice. What I saw was a dramatic decrease in the background until condensation shorted out the camera! I now have a slightly more elegant solution.




The cmos is a high density 40 pin device and is unlikely to come off the board easily so the normal cool finger peltier solution is not really on. Instead I built a masking tape dam around the camera PCB and filled the back of the board with epoxy potting compound. This stuff has good thermal conductivity and good electrical resistance. When it hardens it provides a flat surface to mount a peltier. A cross section is below. When adding a peltier to a camera remember that apart from pumping a small amount to heat they also produce a lot of heat so a big heatsink and fan is in order. Although the background decreases sensitivity is not dramatically increased.



For a description of a very nice spacecam 200 mod and some better pictures see the QCUIAG site.


 

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Copyright 2006 Steve Chambers. All Rights  Steve@pmdo.com

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