The shutter circuit appears quite simple on a Polaroid 450. I'm not an electronics engineer, but I know enough to understand that a photocell provides varying resistance depending on the amount of light that reaches it. If we can find out the resistance to shutter speed ratio, we can control the shutter speed by simply bypassing the electric eye and feeding the shutter circuit the resistance necessary for a particular shutter speed.
I realize that others have developed digital shutter control for these cameras, but it requires either purchasing a circuit board and other parts, or buying the components yourself, assembling the circuit and programming the chip, etc. Plus, when I checked the program used to fire the shutter, I didn't like how the shutter worked: it energizes the shutter solenoid when you cock the shutter and stays energized until you take the photo. And you need to completely rewire the shutter circuit, gutting pretty much everything in place and replacing it with the digital circuit.
So here's a method that an average person can do with basic soldering skills. The most difficult part is removing the electric eye and soldering wires in its place. If you want to retain the option of an automatic shutter, replacing the electric eye for that option.
First, I came up with a shutter to resistance ratio with both the open and closed aperture settings. This is the "indoors" (open) vs "sunny day only" (closed) setting indicated above the lens:
|Resistance||Shutter speed||Resistors||accumulated resistors||total accumulated|
This table gives you 2 ways to build the array: one where each value is built by combining resistors, and the other where each accumulated value is the sum of all the ones before it. You'll notice the math doesn't add up. This is due to resistor tolerance. When building an array using accumulated values, you should build it on the fly, otherwise the accumulated value go off from the official resistor values. The advantage of building the array with accumulated values is that all the resistors fit under the 12 point switch, whereas the individual values stretch out to one side making the array quite a bit bigger.
Notice the values above 1/8 second are pretty much linear in their ratio. Below that, it departs from a linear relationship. I've tested this on the original model 100 and the model 450 and the values are the same.
|Diassemble the shutter body. This will involve 3 screws and removing the aluminum sticker indicating the aperture setting. Unscrew the shutter circuit from the back of the body. You may need to cock the shutter to get at one screw. Desolder the electric eye from the plastic ribbon cable. Note: even Polaroid did not allow their technicians to mess with the plastic ribbon cable, opting rather to replace the whole thing if it wasn't working. Their repair manual wouldn't even allow their technicians to desolder the power cables coming from the battery compartment. The reason for this is because the ribbon cable melts when struck with a soldering iron. The key here will be to use as little heat as possible for removing the leads to the electric eye.|
|Get some earphone cable from a non-working headset or earphone set and trim it to length. I found this is the best cable to work with because it's thin and strong. You will need to carefully strip the cable, but because each wire is individually insulated, you're going to have to burn off the insulation by surround it with solder for around 10 seconds or so. You'll see the solder stick once the insulation burns off. Then, solder the leads to the electric eye terminals.|
|An option here is to wire this in a way that gives you the option of automatic exposure. Look at the wiring diagram and wire the electric eye in this manner.|
|Bend one of the terminals of the electric eye 90 degrees so it doesn't get soldered to the circuit board again, and attach a lead to it.|
|Make a cut in the plastic that the electric eye sits in to make room for the 90 degree wire, then solder the other electric eye terminal and one of the leads to the circuit board, and another lead to the other terminal on the circuit board.|
|Run the wire along the battery cable to the battery compartment.|
|Pull the wires to the top of the battery compartment. You'll have 3 leads: two for the electric eye, and the other to switch between using the electric eye or the resistor array.|
|Using the above table, solder the accumulated value for each resistor(s) in series, connecting one switch terminal to the next in a cascading manner. Start with a 470 ohm resistor, soldering it to terminal 12, leaving the other end of the resistor free to solder to the lead to the electric eye. The accumulated value will be the sum of all the resistors in series, depending on the position of the switch. Because of this, you just need to solder the leads from the electric eye to the middle terminal and the resistor soldered to the 1/1000 sec terminal of the rotary switch.|
|If you would rather build the array with individual values for each switch point, solder the appropriate resistors to each terminal of a 12 point single pole switch. You'll likely need to combine values to reach the necessary value.|
|Cover each line with heat shrink tubing to prevent any shorts. This was the way I did my first 2 manual conversions, but having done an array in series, I would recommend that method.|
|Fold the resistors 90 degrees and solder all the leads together. This will connect to one of the leads which goes to the electric eye terminal. Connect the other lead from the other electric eye terminal to the middle terminal on the sliding switch. Then, connect the lead from the electric eye on one end of the switch and the resistor array to the other. This will allow you to switch back and forth between manual and automatic shutter.|
|I would test the array before putting everything back together. Test the 1/2 second and compare it to a known good shutter. If it is slow or fast, you can adjust it by moving the trim POT on the side of the shutter circuit. The POT at the top of the camera seems to fine adjust the timing. Counterclockwise makes the shutter slower, and clockwise makes it faster.|
|I mounted the rotary switch on the side of the camera which allows it to stay in place when opening the battery door. Drill a hole and insert the rotary switch shaft.|
|Make a shutter speed indicator and mount it, then screw the swtich in. This particular manual shutter speed control gives the camera shutter speeds in 1/2 steps under 1/60, for shooting paper negatives. A 12 point switch should give you 1/1000 sec to 1 sec and bulb for a normal camera.|
|Here's the finished paper negative camera. Notice I made an aperture scale reminding me of the apertures on the different settings and ASA speeds. When the aperture is in the closed position, I need to decrease the shutter speed by 2 stops. Under 1/4 second I have to look up the shutter speed on a table. Here's a table with aperture values vs ASA/open/closed settings:|
|When using the closed settings, the shutter speed is 2 stops slower, so for instance, 1/125 with the open setting will be 1/30 with the closed setting.|
|The closed setting will shoot 2 stops slower like with the model 350/450. Additionally, with the open setting, ASA 150 and 300 will shoot 1 stop faster than the shutter speed indicated. Best to avoid those settings and just use f/8.8 with ASA 75, then use the closed settings with an additional 2 stops.|
|Here's a finished camera with speeds from bulb to 1/1000 sec.|
Feel free to contact me with any questions or news about your build!