Dez
Bodger Extraordinaire
I recently bought a meter for my SP, and I was overjoyed to see that it appeared to respond to light, and be fairly accurate in room light. This was great! These things are almost never working at all, and I never heard of buying one from ebay that was actually accurate!
I subsequently found out that it was accurate ONLY at medium light levels, being way off in bright daylight where the cell just didn't have enough juice to move the meter needle far enough. I took it apart and found that someone somewhere had replaced the original cell with a new selenium cell, which really didn't fit properly and had itself nearly expired. I resolved to see what could be done with a modern Silicon Blue cell. I found some nice cells in the Edmunds catalog, 25 x 10 mm, which is a bit small, but decent, at a cost of next to nothing.
The cells are very thin and fragile. The first thing I did was to cut a brass plate, 33 x 20mm, which will fit nicely in the meter, and with a little bit of adaptation, will make proper contact with the ground side spring in the meter. The original cell is very thick, so I will need to come up with some kind of spacer, maybe a penny(?), to make good contact. I soldered the cell at one end, and held the rest to the plate with silicone glue around the rest of the perimeter.
It probably isn't a great idea to talk about this before I find that it acually works, but I want to ask a question of anyone who has done something like this before. The cell is much more sensitive than the original selenium one, so it will need to be padded down with a series or parallel resistor, or more likely a combination of both. The response to light is likely quite different from that of a Se cell, and probably neither of them is anywhere close to linear.
So my question is whether these old Se meters are based on the idea that the cell is a voltage source or a current source, and whether the same thinking can apply to the silicon cell. I know that I can get this thing working reasonably well across the light range with a lot of trial and error, mostly error, but I am hoping that someone who has already done something like this can point me in the right direction.
Cheers,
Dez
I subsequently found out that it was accurate ONLY at medium light levels, being way off in bright daylight where the cell just didn't have enough juice to move the meter needle far enough. I took it apart and found that someone somewhere had replaced the original cell with a new selenium cell, which really didn't fit properly and had itself nearly expired. I resolved to see what could be done with a modern Silicon Blue cell. I found some nice cells in the Edmunds catalog, 25 x 10 mm, which is a bit small, but decent, at a cost of next to nothing.
The cells are very thin and fragile. The first thing I did was to cut a brass plate, 33 x 20mm, which will fit nicely in the meter, and with a little bit of adaptation, will make proper contact with the ground side spring in the meter. The original cell is very thick, so I will need to come up with some kind of spacer, maybe a penny(?), to make good contact. I soldered the cell at one end, and held the rest to the plate with silicone glue around the rest of the perimeter.
It probably isn't a great idea to talk about this before I find that it acually works, but I want to ask a question of anyone who has done something like this before. The cell is much more sensitive than the original selenium one, so it will need to be padded down with a series or parallel resistor, or more likely a combination of both. The response to light is likely quite different from that of a Se cell, and probably neither of them is anywhere close to linear.
So my question is whether these old Se meters are based on the idea that the cell is a voltage source or a current source, and whether the same thinking can apply to the silicon cell. I know that I can get this thing working reasonably well across the light range with a lot of trial and error, mostly error, but I am hoping that someone who has already done something like this can point me in the right direction.
Cheers,
Dez
Highway 61
Revisited
I'd say that the Se cells are a current source and the Si cells are a voltage source, so that this Si cell has to be fed with a battery and included in a voltage source circuitry, not only exposed to light, to deliver a proper response...
Of course I can be wrong and specialists will tell.
Why didn't you decide to look for another Se cell ? Any cell from a still working handheld lightmeter would do it given that you resize it to the size of the original one.
Of course I can be wrong and specialists will tell.
Why didn't you decide to look for another Se cell ? Any cell from a still working handheld lightmeter would do it given that you resize it to the size of the original one.
Dez
Bodger Extraordinaire
Back to Selenium
Back to Selenium
"Why didn't you decide to look for another Se cell ? Any cell from a still working handheld lightmeter would do it given that you resize it to the size of the original one."
I tried my Silicon Blue cell, but the response of the Silicon cell is wildly different than that of the Selenium, and no matter what I did, I could not manage to get it anywhere near accurate over the range. So, I took a photocell out of a thoroughly defunct prewar Contax in which the meter was miraculously still good, and went at it with hacksaw, grinder, and file. Selenium is HARD!
It seems to work quite well, but I'll have to wait until tomorrow to see what happens in full daylight. Let's hear it for low tech.
By the way is it "Highway 61" because you're a Dylan fan, or are you also from Minnesota?
Cheers,
Dez
Back to Selenium
"Why didn't you decide to look for another Se cell ? Any cell from a still working handheld lightmeter would do it given that you resize it to the size of the original one."
I tried my Silicon Blue cell, but the response of the Silicon cell is wildly different than that of the Selenium, and no matter what I did, I could not manage to get it anywhere near accurate over the range. So, I took a photocell out of a thoroughly defunct prewar Contax in which the meter was miraculously still good, and went at it with hacksaw, grinder, and file. Selenium is HARD!
It seems to work quite well, but I'll have to wait until tomorrow to see what happens in full daylight. Let's hear it for low tech.
By the way is it "Highway 61" because you're a Dylan fan, or are you also from Minnesota?
Cheers,
Dez
A straight Silicon Cell would need an IR cut filter placed in front of it to eliminate the IR sensitivity.
A Silicon cell can be used as a voltage source, without a battery.
A Silicon cell can be used as a voltage source, without a battery.