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Calculator blog

Musings and comments about our common interest


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more pictures of the leather case for the HP Pioneer series.

Here are some more pictures of the new Pioneer case. I have checked that it also works well for the HP17bII+ silver (there was an uglier HP17bII+ in gold, with a slighlty curved body):


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The HP Prime Finance app

Initially, the HP Prime finance app only had the TVM solver. It was a good solver since it included everything, from number of payments per year. Good to calculate mortgage payments, but with the right knowledge, useful for many other financial calculations. Here is a screenshot of how it looked like (and continues looking since this part has not changed)


But now there are many more options that turn your prime into a complete financial calculator. To get them, you need to update your firmware to the last onem through your HP connectivity kit application.(choose the one for your operating system and then have it upgraded to the latest version by running the "look for upgrades" option in the help menu. Then connect your calculator and update it. Do not disconnect your calculator during the process)


(By the way: you can test all these features at no cost by downloading the HP Prime app in, with all alternatives for different operating systems)

Let's take a look at the first menu: interest conversion with different composition periods:


The following one is more interesting when calculating investments: the date calculation app: serves to find which date is, for example, 90 days from now. It has also a provision for 360 days per year calculations, as it is common in the banking industry to calculate interest over 360 and not 365,25 days.


The following one is the cashflow app. It has two main screens: the data entry and the results. It allows both risk-free interest rate and investment interest rate, and calculates all the typical figures off an investment.


Then there is the depreciation portion of the app. Depreciation is one of the most boring parts of finance for me (as a finance manager). Well, here we have the complete palette of possibilities:


Now, you have a short app for break even analysis. (all of these calculations - and this is valid for the rest of the apps - are usually done in Excel or similar; but these are extremely useful when you are in a board room or checking someone else's hypothesis)


Then we have the percent calculations. It is surprising how many people do not understand how percentages work. ("I raise the price a 20% and then I offer a 20% discount" type of weird logic). These two pages help us. When analyzing financial statements, the one with % change and % of total is a must for quick calculations. Both numbers are calculated each time, making comparisons faster)


There are two more left. One is bonds (as I am in a production company, this is something I have never used - but if you are in an investment bank or fund, this is something you want to have in your calculator)


And the other is the Black and Scholes formula for valuation of financial products at risk:


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Card reader available!

image2-38We have received a working card reader - ready to be shipped after our tests!

Price is 130€ plus shipping, including English manual.

(comes without calculator)


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second attempt to card reader side lock

I have received the second prototype of latch for the card reader. The first had not exactly the right dimensions. This second, while OK in the dimension side, does not provide enough spring action to have the latch "lock" in place. We need to angle more the leg (just like the original) so that it provides enough torque so that it latches into the calculator.

On the aspect side, it is good enoug. It will be sold always in pairs, so that no one will be able to see the difference between an original one and ours. Please email to if you are interested on the product!


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HP Prime

I have now several HP Prime machines. One for each of my sons (one of them using it at full power for engineering at University of Michigan), and several G1 models. I have just opened a G2 unit and updated it to the latest firmware version (20.12.2021)

I will spend some time with it, trying to learn how to program it in Python. I have take a look at the Finance app (remember that I created several finance programs, among them net present value and internal rate of return), and while in the previous firmware there was a single TVM solver, now there is a wealth of applications, including the Time Value of Money of before, but also many others, including clashflow, breakeven, black&scholes and others. You will see some of my explorations in the coming blogs too. (Note: once you click in finance and select what you want to do, you need to go to the Num part, the third key in the second column of the upper part. Then you see all the fillable fields and calculate the ones you have not filled in.)


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More about: HP prime, Python
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HP1CL is over!

tall_keys_-_5We have sold the last unit of HP41CL. So far there are no more made and no more planned - and it will be difficult to produce more since there are many parts that are out-of -life. The board would require a deep redesign which current demand does not warrant - so it is a pity for all of us!!

One of the last units sold was a 1937A serial number unit - one of the very first. I have only seen an earlier unit, a 1932A. It is a nice closing to the story, 43 years after its inception in 1979. I was there then and I remember the lust for that powerful machine, far from me as primary student. My father bought instead a HP33c for me - which hooked me into RPN - a hook that I have not been able to unleash so far.

With the so-far demise of the HP35s, there are now the HP12C, the HP12C platinum (which has an RPN mode), the HP17bII+ and the Prime as RPN machines. So far in Europe the HP12C has been absent for some time, but I have been promised that it is returned - so far that I have been able to place an order for them. I expect the order to arrive before month end.

We are missing a RPN scientific (not graphical), preferably of small size, not competing with bigger student units. I have been given hope by the current calculator management - let's see!


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More about: HP41cl, hp15c
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Battery adaptors for other calculators

Spice_back_-_1Next on the agenda is to prepare battery bays to use AAA calculators in place of AA-sized cells, and also to prepare back covers for a number of calculators. For example, I remember that I had a cracked back door cover in my HP33c when I was student. There was no 3D printing at that time (gosh - the computer I had access to was an HP85!), so I had to make do with tape.

Here is what we want to do:we have already the black silicone rubber feet, we just need to design the cover (which is similar in shape to others in HP, but with a different size). This is not a big seller but surely will help some of you! My friend Ignacio was very happy to be able to use again his Spices - which always failed on the battery side.


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Tomcat battery prototype

We have designed a set of modified parts for the Tomcat battery - something more solid than what you can buy in eBay 

There are two halves that assemble together to create a body that houses the subC cells: just imagine one half on top of the other. There are holes for the metallic contacts. Some soldering will be required, I think


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Card reader side locks prototypes

I have had the time to test the initial prototypes for a replacemento for the often broken card reader side locks. Still a couple of angles to fix, and also add some texture to the figer pressing areas - and it will work! It does not require metal legs to work, but it uses the relative flexibility of a the P12 nylon stud to act like the metal band. It is integral to the piece so it will not disconnect or fall during assembly - as we are tired of suffering when reassembling card readers! If anyone else is interested, please drop me a line at





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Considerations on leather cases: the HP41c


Coconutfundas_-_6We have lately commissioned a couple of calculator cases for Voyager and Pioneer calculators. These cover many of the most used calculators, and we plan to extend it to other calculator ranges. Clearly now we need to address the HP41c range. Old vinyl cases are quite expensive nowadays - around 35€ for the whole length case.

When discussing with the leather technicians, I learned that the original HP41c vinyl case is a quite complicated case to build. As it has a relatively short zipper, it is quite complicated to assemble and requires a technique that was well known 50 years ago, but that fewer and fewer artisans master now: it is called "wood block". It is very similar to assemble luxuty leather shoes - but it explains also why later cases were shorter and had wider zipper mouths. It also has black ribbons in the corners - which makes it also more expensive to makeAnyway, the price for a complete leather HP41c long case was prohibitive. 

If we release the short zipper limitation then other techniques may be used and the price is substantially lower. Also the perimetral ribbon makes it more expensive - but let's see what the final cost is. 

The second question is whether it makes sense to produce the long case (for which we already have the foam sponge!) or go directly to the smaller, HP41c sized units. Probably we will end up with both. Color will be dark brown to start with. 

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More about: HP41c case
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