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
