The nub of the issue is that to keep the circuit cheap, most designers at the time relied on the fact that a mercury cell gives a nearly constant voltage all its life before dying suddenly, so the meter could rely on the battery to give a fixed known output. The electronics could then consist of nothing but battery, LDR (light dependent resistor - the light-sensitive CdS or CdSe cell), meter and one resistor, all wired in series. It’s that simple!
So what other cells give the constant voltage we need and are still available? Several, is the answer, but easily the best for us is a silver oxide cell which gives a nearly constant 1.55V. You can put one of these directly in a meter but it is likely to read too high by several stops, and be non-linear. We need to reduce the battery voltage by 0.2V, and the way to do that is a suitable diode (a Schottky diode) which creates a voltage drop of 0.2V, that itself is also constant; a resistor would give a voltage drop proportional to the current flowing, which would not do at all. The diode is wired in series with the battery.
This is the conclusion reached by Frans de Gruijter, and is the basis of his conversion kit – which I have tried and can recommend. But maybe we can slim it down even more? Here goes.
The slimline adaptor is a paper disc with a conducting layer each side, with the diode soldered to the edge of each layer. This slips under the battery, with the diode near the edge of the compartment, where the shape of the battery leaves a little space. The pad itself is less than 0.5mm thick and should not cause any clearance problems in the instrument. If your device needs two batteries, you also need two adaptors.
Most of the job can be done with readily available hand tools – a craft knife and scissors are useful, tweezers, small snipe-nose pliers, and a fine-tipped soldering iron. The trickiest part is probably the punching out of neat discs of paper and backed copper foil, best done with wad punches. These can be bought individually, or as a set of punches with a common handle. You need two sizes – either 3/8” and 7/16” or 10mm and 11mm would do. Expect to spend around £20 if you need to buy these. I happen to have a small hand-press which does a good job of punching out these materials. But usually you would use a block of wood and a hammer, and you need to give the punch a surprisingly meaty thump even when just cutting paper.
Cut a piece of the red card to approximately the width of the foil tape, and stick down a length of the copper foil, to cover one side.
Using the larger punch, punch out as many discs as you like. A piece of material 8”x1” should make 20-odd discs. Each disc should be completely covered with copper on one side, and bare card on the other.
Using the smaller punch, punch out the same number of discs directly from the copper foil plus its backing paper. This should give you a set of discs with the copper on one side and still on backing paper.
Using the tweezers, lift the copper from these smaller discs one by one, and stick them to the bare card side of the larger discs, trying to keep an even margin of red showing all the way round. I call this side of the card the positive side. You need to be sure not to get a short-circuit between the copper pads on the two sides of the card.
Now for each disc:
With the soldering iron, make a little dot of solder just at the edge of the positive pad.
Take a diode, and note the black ring round one end. Trim that lead to about 6mm long. Hold the lead with fine-pointed pliers, butted up to the diode, and bend it about 70°. The reason for the pliers is not to do the bend at the actual end of the diode body, which is fragile.
Hold the diode by the other lead, so that it is positioned on the edge of the disc, over the red rim, roughly parallel to the edge, and with the end of the short lead over the little solder dot you made. Solder lead to dot (only a couple of seconds contact at most, so the diode doesn’t get overheated).
Using the thin pliers as before to protect the diode body, bend the second lead up and around the edge of the disc, so it lies flat and in close contact with the edge of the negative pad. Trim off the excess lead and solder in place, rapidly as before.
That’s it done.
Back row, negative side up - front row, positive side (red ring) up
An occasional and irregular blog, mostly of photographic experimentation and photographic history.