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Calculator blog

Musings and comments about our common interest


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Arrived more Spice battery holders

We have received another batch of Spice battery holders, of which we had run out of shortly after its introduction.

Important: battery polarity needs to be respected! See picture:



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Testing card reader side locks

We have received the card reader side locks samples and we will try to test them during this weekend.

It is likely that we have to reduce the "spring" made in nylon - it looks too rigid for the purpose:


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Battery adaptor for Spice series calculators (HP31E, HP32E, HP33R/C, HP34C, HP37E, HP38E/C)

The Spice series calculators used a battery pack that had two cells with a total of around 2.4 volts. The cells were very similar to AA batteries, just a couple of milimiters short. So, when the battery pack ended its life, many users had the temptation of putting two AA batteries. As these were a couple of milimiteres too long, the case cracked and the calculator contacts were broken.

You could find ready-made packages in eBay, but Ignacio Sánchez had a better idea: why not create an adaptor, so that normal (or rechargeable) AAA batteries can be used? As they are shorter, an adapter can be created where pressure to the contacts is moderate, and the battery cover can be closed without difficulty. So we went to the drawing board (actually Autodesk Fusion 3D) and off to the 3D printing company!

You can find the product here, and some pictures below:




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The TVM formula

I have a son studying engineering and who is now having some finance subjects as part of his major. Today he asked me about a finance problem, which involved, at first sight, two Time Value of Money steps. He was getting a wrong result and he asked me how to do it.

I solved it with a HP12c quite fast - he was surprised how a such small calculator could do it so simply. Then I remembered that the HP Prime that my son has, there is a Finance app! I guided him through it and he was surprised that it could be so fast and practical. I am still interested on his delving deep into the formulae and understanding the math behind - and not just use the solver (which solved his problem in two steps).


n   number of periods
i   interest rate	
PMT periodic payment
PV  present value
FV  future value
Solve equation:

where (i1-1) = e1^(n*ln1(i))
and i1 = e^(n*ln1(i))
and ln1(x) = ln(1+x)
and e1(x) = e^x-1
p=0 for END, p=1 for BEGIN

The use of ln1+x and e^x-1 prevents the loss of significant digits for small values of i. Consider these extreme examples for accuracy checking:
Example 1:

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Some more pictures of the other colors for Voyager cases

Here are some pictures of the other colors for voyager cases. The ambient light makes the Bordeaux much lighter than it really is. The brown is actually very similar in color to the original vinyl cases of 40 years ago. Here they are in comparison with the standard black leather case. BTW: the more I use it, the more I like it and enjoy the feeling.



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Tomcat battery prototype

image0-38We have received the first two halves of the Tomcat battery bay replacement. The initial tests with the Sub-C batteries show that it is a little tight in all directions for the batteries. We'll enlarge it . Both halves clipping together, on the other hand, works well as expected. Now we need to solder the terminals and it should be ready! anyone interested?

The prototype is in white but most likely the final units will be in black. Problem is, the parts are very expensive and I don't see it being below 50€ VAT included (20% cheaper if outside EU). On the other hand, the slimming that is needed (remember the little tightness inside?) can cut somehow the cost. In any case it is much better than the products I have seen in eBay.

This battery will work with HP92, HP97 calculators and diverse HP41c printers: HP82143A, HP 82162A IL printer, etc.

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Three new products in the making

Upon request of some customers, we have developed three new products:

  • The first one, we have already discussed about: the tomcat battery module: the one used in the HP92/97 and the HP41c printers. It is bound to be expensive, because the 3D printing process makes the piece expensive. I will show pictures later when I have both pieces and assemble them.
  • A Spice battery holder: there is always a problem with the AA batteries, since they are jusr slightly too long to enter properly. So Ignacio Sánchez had the idea of using AAA batteries, and creating some adaptor with springs that allows it to fit. you can see a picture here. Some samples will arrive soon.
  • Spice_-_4
  • The last one is a couple of side locks for the card reader. These get broken from time to time, and it is a real pain to find others! There is also the added problem of having the metal springs well attached. It is quite difficult to get them in place when reassembly the machine - thus we have made them integral to the part. Let's hope that the flexibility of the part is good enough for the task. In my opinion, this was not the brightest design moment in HP's history! For the sake of manufacturability, we produce 5 pair in the same "tree":
  • sidelocks_card_reader
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Voyager cases - how the colors really look


I have here some pictures of the Voyager cases (valid for the HP10c, HP11c, HP12c, HP12c Platinum, HP15c, HP15c LE and HP16c). The new colors Bordeaux and brown are shown here. You can see that the brown color is a very dark one - actually the closest to the original dark brown vinyl case of the original calculators!!

In the first picture, the order is brown - Bordeaux - black, while in the second the order is inverted: black - Bordeaux - brown

voyager cases


















Voyager cases

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Battery pack for HP printers and HP92, HP97 machines


By now, all original batteries made for Tomcat machines and HP41c printers cannot hold a charge anymore. There are several replacements in the market, all of them using the ubiquitous Sub C batteries, in different energy capacities. These batteries are cheap, and most of the packs are held in place by some rubber foam corners and adhesive film. You can find them in eBay mainly. 

I have tried to break apart two one of the original batteries that I have, to refurbish the original cells for new, current cells, but it was impossible without doing a lot of damage. This cannot be the right way to do more than one of them, and forget about series production! So we decided to make our own case

We have designed a two-halves case for these batteries, with precise holes for the connectors, to be 3D printed. Here the main challlenge is the cost of the parts in any 3D provider: it is far too expensive and it eats most of the cost. The parts are clipped one onto the other - and like the original, it is a single part used for both sides. We have not decided yet whether to offer it as a kit or send them fully assembled - I am quite frightened with batteries as opposed to passive, non-energy storing parts. There would be some soldering involved, in order to connect the cells among them and to the terminals.

There may be still some work to do in optimizing the part to reduce material - I can see a couple of areas where material can be taken out. However is the sheer size of the piece that determines most of the cost, I think.

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First run of Voyager cases finished!

Apparently there was demand for a true leather Voyager case. All the initial production run is gone. We have ordered a second run, which includes brown and Bourdeaux colors. 

image6The new colors are made of the same leather quality - just a different tint. 

If this is again successful, we will try with other models. The first that comes to mind is the Pioneer line: very similar design (but we need to take into account the slight wedge shape) and similar size. 

Other models are more complex - compared with the Pioneer or Voyager. If you look at one of them, it has several separate pieces, zippers and other parts that make them considerably more difficult to produce - and substantially more costly too. Probably it will not make too much sense to sell it if the price is too high. However, second hand cases like HP41c's command extremely high prices - like 40 dollars in some cases!

As we have explained, we also have ordered new sponges for the HP41 case

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