Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A)

[Dave Wise]

Fix a couple of typos in the writeup, and upload its LibreOffice Writer source.

Meanwhile, I'm still goofing around with individual photocells... probably reproducing bits of 1950s-era research.  The time it takes for a cell to recover to ten times its lit resistance, T10 (and by extension T5 which I was using) turns out to have an essentially linear relationship with R0, the lit resistance itself.  Dim -> long.  But the slope is much less than 1, so going dim is good (dR/dt gets faster and faster) until you run into cell/load voltage divider losses.  Intercepts are all over the place but always positive.

Most cells have a slight upward curve on their R/t graph, but one cell has a distinct curve the other way which suggests a parasitic conductance alongside the light-sensitive one.

[Dave Wise]

Both 740B's are looking good with my hybrid choppers.

I updated the writeup to version 1.1 .  Most of the change is a simpler board insulator, LDPE film instead of semirigid PET.  The LDPE is easy to cut with a craft knife and you can use the board itself as a template.

Just for fun I measured the generic CdS photocells in my junkbox.  Their T5 runs the gamut from five times the HP cells, to 25 times.  Chopper cells are super fast.  According to a Journal article (I think), their active material is a mix of CdS and CdSe, the latter being fast but unstable.  A couple of niche scientific companies make PbS and PbSe cells but they are crazy-expensive.

I also used a tight beam to probe some HP cells looking for fast and slow spots.  Nope, each cell is uniform across the whole active area.  The slow cells were coated with a bad batch, not an insufficiently-mixed batch.

I put the fastest eight cells in my two 740B's.  Here are my notes for the other cells.

PHOTOCELL DATA

Current for 20K, T(20K-100K)

1            3uA       8ms       NOT USABLE
2            20uA     3.7ms    USABLE BESIDE A FAST CELL
3            50uA     13ms     NOT USABLE
4            20uA     3.2ms    USABLE BESIDE A FAST CELL
5            45uA     3.2ms    MAYBE USABLE BESIDE A FAST CELL (1)
6            15uA     2.8ms    USABLE
7            65uA     2.4ms    GOOD
8            20uA     2.1ms    GOOD
205         350uA   1.8ms    GOOD (2)
PC5191    7uA      4.4ms    USABLE BESIDE A FAST CELL (3)

(1) Cell resistance rises to a relatively low value asymptotically.
(2) Was A9V1, marked PC5191-205
(3) Was A9V1, marked PC5191

Most cells R vs t curves upward a bit, so T(20K-100K) < 0.5 * T(10K-100K).

T10   10K-100K   20K-200K   30K-300K   40K-400K
2            5.3ms      7.8ms      >10ms      >10ms
4            4.6ms      5.5ms      TBD          9.3ms
5            5.4ms      8.6ms      >10ms      >10ms
6            3.8ms      5.1ms      6.0ms      7.0ms
7            3.2ms      3.9ms      4.5ms      5.3ms
8            2.6ms      3.5ms      4.2ms      4.9ms
205        2.3ms      3.5ms      4.8ms      6.2ms
PC5191  5.3ms      6.8ms      8.0ms      9.4ms

[Kleinstein]

The speed of the LDRs also effects the sensitivity.  So an LDR that is faster is naturally also less sensitive.  In the LDRs the photogenrated carriers or one sort are caught by trapping centers and as long as they are trapped the counterpart is free to contribute to the conductivity.  Long life trapping centers give more conductivity (the same effect for a longer time), but of cause also a slow decay.
The correlation can also be seen in the data for the old cells.

The chopper cells are specially fast, but for this reason also not super sensitive. As such they arather special product.

The PbS / PbSe cells are used because of the IR sensitity (AFAIR  up to some 3500 nm) - the high price is likely because of low numbers and more precision ones for a measurement as opposed to the CdS cells made for low cost uses for a simple trigger level. Because of the IR sensitity I would expect them to be rather temperature sensitive.

AFAIK there are a few rather special, fast photoresistors made from direct bandgap materials (e.g. InP, GaAs or such) - here fast means really fast with response in the ns range. It could be tricky to find them and they may be to low in resistance to get good high frequency performance.

AFAIK the CdS/CdSe mix was also used in cells for exposure meters. The composition effects the response to different wavelengths a little.

[Dave Wise]

I'm still happily using my rehabilitated 740B's.  I just noticed that I forgot to make my pcb design shareable at oshpark.  I'm "davetheresurrector" there.

In post #66 (July 26, 2022, 05:54:35 pm), Kleinstein wonders if photomos i.e. Solid-State Relay is a viable demodulator.  I suppose so, but their timing delays are a nontrivial fraction of the sampling period so you'd have to drive them before the corresponding modulator edge.  And I worry that the delays drift with temperature or time.  I could add this photomos advance timing to my microcontroller program but I won't, because I'm completely satisfied with the H11F1's.