Fifty years ago, Hewlett-Packard introduced the first handheld scientific calculator, the HP-35. It was quite the engineering feat, since equivalent machines of the day were bulky desktop affairs, if not rack-mounted. [Rob Weinstein] has long been a fan of HP calculators, and used an HP-41C for many years until it wore out. Since then he gradually developed a curiosity about these old calculators and what made them tick. The more he read, the more engrossed he became. [Rob] eventually decided to embark on a three year long reverse-engineer journey that culminated a recreation of the original design on a protoboard that operates exactly like the original from 1972 (although not quite pocket-sized). In this presentation he walks us through the history of the calculator design and his efforts in understanding and eventually replicating it using modern FPGAs.

The HP patent ( US Patent 4,001,569 ) contains an extremely detailed explanation of the calculator in nearly every aspect. There are many novel concepts in the design, and [Rob] delves into two of them in his presentation. Early LED devices were a drain on batteries, and HP engineers came up with a clever solution. In a complex orchestra of multiplexed switches, they steered current through inductors and LED segments, storing energy temporarily and eliminating the need for inefficient dropping resistors. But even more complicated is the serial processor architecture of the calculator. The first microprocessors were not available when HP started this design, so the entire processor was done at the gate level. Everything operates on 56-bit registers which are constantly circulating around in circular shift registers. [Rob] has really done his homework here, carefully studying each section of the design in great depth, drawing upon old documents and books when available, and making his own material when not. For example, in the course of figuring everything out, [Rob] prepared 338 pages of timing charts in addition to those in the patent.

One section called the “Micro-Programmed Controller” is presented as just a black-box in the patent. This is the heart of the systems, and is essential to the calculator’s operation. However, all the other parts that talk to the controller were so well-described in the patent that [Rob] was able to back out the details. The controller, and all sections of the calculator, was implemented in Verilog, and tested on an instrumented workbench he built to test each module.

Once everything was working in the simulations, [Rob] set out to build a working model. TInyFPGA models were used, one for each custom chip. A few understandable departures were made from the original design. An 18650 lithium ion cell powers the board, kept topped off by a modern battery charging controller. The board is larger than the original, and yes, he’s using the Hackaday-obligatory 555 chip in the power-on circuit. In this short demonstration video, you can see the final prototype being put through its paces side by side with an original HP-35, working through examples from the owner’s manual.

This is an incredibly researched and thoroughly documented project. [Rob] has made the design open source and is sharing it on the project’s GitLab repository. [Rob]’s slides for Remoticon are not only a great overview of the project, but have some good references included. Its clear he has a real passion for these old calculators and has done a fantastic job exploring the HP-35. But even after three years, there’s more to come. He’s thinking about making a PCB version, and a discrete implementation using individual logic gates may be in the works.

We wrote about the history of the HP-35 before. And if you like hacking into these old calculators, check out our writeup of a similar dive into the Sinclair scientific calculator.

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Emer Gillespie's house renovation has been a labour of love, and the process from start to finish required resourcefulness, creativity, and perseverance.

Emer is the founder of lighting company Spark & Bell, and she lives with her husband Brad Guin, a product developer, their children, aged seven, and 21, and French bulldog Duke. When they first moved into a five-bedroom Victorian terraced house in Hove, East Sussex, there was work to do, but there's no doubt as to whether it was worth it.

We take a closer look at their gorgeous home, and find out all the details of how it came to be.

1. 'We created a cosy diner'

Emer's vision of a stylish but homely dining area came together beautifully thanks to the natural textures and green elements she incorporated.

'In this dining space, our banquette seating is upholstered in dark green cord, a hard-wearing material with a texture that echoes the fluted wood in our kitchen,' Emer says. 'The artwork was created by Hove artist Shyama Ruffell, who happens to live on our street; our large collection of plants was her inspiration.'

And kitchen lighting ideas provided the finishing touches. 'The 3.8-metre high ceiling makes this the perfect spot for the trio of beautiful pendant lights made from spalted beech.’

2. 'We used local firms for a bespoke look'

'As our kitchen had a tricky floor plan, we decided to go bespoke. Working with local small businesses allowed us to be more involved in the design process – we could visit the workshop to pick out materials and oversee the build.’

Bespoke joinery is ideal for unique spaces that don't follow a typical kitchen layout. Going bespoke with her culinary space allowed Emer to get creative in others ways too.

'My lighting business was actually started at my kitchen table – this is a special area, where creativity often strikes.'

Ridged materials bring interesting texture to a functional space. Look out for surfaces, fabrics and even lighting with a fluted finish.

3. 'Earthy hues add drama'

'As this is a north-facing living room, I had to be mindful when picking paint colours – I spent hours deliberating over a million samples,' Emer says. 'In the end, I chose a soft, earthy pink and a deep green. These highly pigmented paints change colour during the day, so the room never feels flat and dull.'

Living room paint colours should be chosen to complement the space, including how much light the room gets. For overlooked rooms, shutters are a great way to retain privacy while still allowing light to filter into your space.

'The finishing touch is the rug, which picks out all the colours of the room,' she adds.

4. 'Artwork reflects our personality'

Emer's inspiration for her gallery wall is rooted in her previous vocation. ‘I was a photographer for years and I love collecting prints,' she explains. 'We found this box set of six prints by Dan Hillier in a Brighton gallery. I love the way the graphic colours and gold leaf detail add character to our blank walls.'

Underneath the drama-filled gallery wall is a contrastingly simple console table, which houses a select few decorative items. 'The 1970s sideboard is from Brighton’s North Laine. We invest in reclaimed furniture because it is always well made and durable and I look out for mid-century pieces in warmer woods.'

Emer adds, 'The mix of retro furniture within our Victorian house brings an edge to our decor, styled with colourful accessories.’

5. 'We filled the room with light'

‘One of the first changes we made to our home was to knock down the wall between the two sitting rooms. Previously this was a guest bedroom but it had become a dumping ground.'

Emer transformed this overcrowded space into a perfectly cosy sitting area, with dark green walls, houseplants, and fluffy textures. She made use of the space with older pieces of furniture, which tend to have smaller proportions so they can slot into small alcoves or dead spaces.

'Now the large south-facing window floods the open-plan space with sunlight. For harmony, I painted the walls in the same colours as the living room – deep green and soft pink. Above the mantelpiece hangs a limited-edition print from the David Bowie photographic exhibition held in Brighton. We love to fill our walls with work by local artists or pieces that have a special connection.’

6. 'Smart ideas maximise a small room'

Emer was clever when working with small bedroom ideas, carefully placing furniture and creating multifunctional pieces to maximise space. The custom-made headboard incorporates storage, with shelving and cupboards at the sides.

'The bed was placed centrally to allow the room to flow,' she says. 'As space was limited, I had a headboard made with shelving, integrated plug sockets and overhead lighting. I painted the ceiling and walls in the same tone to blur the distinction, which makes the room look larger.'

7. 'I love natural materials in the bathroom'

The gorgeous bathroom is both eclectic and modern. Emer introduced a bold pattern to a small en suite with on-trend terrazzo tiles.

'The en suite was kitted out with ornate terrazzo tiles. The basin is concrete, and of course we had to have some fluting on the oak vanity unit! I made the light fitting that matches the basin colour.’

8. 'Wallpapers with heavy patterns feel indulgent'

Emer created a showstopper guest bedroom with bold wallpaper and colourful accents. ‘This woodland wallpaper dictated the colour scheme in our guest bedroom. I brought in the curtains from my daughter’s old bedroom, then accessorised with touchy-feely fabrics on the bed.’

You can make a bedroom feel sumptuous with textiles that are soft to the touch such as velvet, silk and cotton.

'The mix of retro furniture within our Victorian house gives our decor an edge, especially when styled with colourful accessories,' Emer adds.

9. 'Small bathrooms have to work much harder'

‘We inherited a small bathroom that felt cluttered with sanitaryware so we had to start from scratch,' says Emer. She used small bathroom tile ideas and pops of colour on the sink and bathtub to create a simply stunning space.

'Our new bath and metal French-style basin were painted in Orange Aurora by Little Greene. The pattern for the white wall tiles was inspired by a Victorian dado rail, with the penny tiles appearing halfway up the wall.

With winter fast approaching and households up and down the country now worrying about their energy bills once again, some may be looking for ways to save money.