Stew’s Spot

I’ve got a 1998 HP 200LX palmtop. Wikipedia describes it as:

a personal digital assistant introduced by in 1994. It was often called a palmtop computer, and it was notable that it was, with some minor exceptions, a MS-DOS-compatible computer in a palmtop format, complete with a monochrome graphic display, qwerty keyboard, serial port, and PCMCIA expansion slot.

What I wanted was a single “dongle” that will allow me to connect to the palmtop over USB as well as power/charge it. Unfortunately it require 12V DC to charge or run the plamtop so I’m going to need to use a DC/DC boost converter.

Some links for those that want know more about the HP 200LX

• Wikipedia
• The Palmtop Network
• The Palmtop Paper

Design

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I just finished a little project that need a DVM (digital volt meter) panel meter. You can buy these cheapies for R60.00. The one I got was a 3 ½ digit, 7-segment LCD with ±0.5% accuracy.

You get two types, one has a common GND connection, the other needs the supply and measurement GNDs to be isolated. I can’t find the common GND variety in South Africa (well RS has them but they are R300.00). If you connect the GNDs together the meter just displays the “-1″ overload indication. What to do?

Bellow is a simple circuit that will isolated the DVMs power supply and will allow you to use it in your project. Here’s a quick run down of how it works.

Click for PDF

The 555 timer outputs a 70kHz square wave, which is rectified by the two BAT85 Schottky diodes. Capacitors C4, C5 provide the isolation between VCC, GND and the DVM supply. C6 is just a smoothing capacitor.

As you can see it’s very easy to build on a bit of vero-board and could be made much smaller with SMDs.

I bought a Lego Mindstorms kit last month to mess with, I’ve got some ideas for robot localisation that I want to try out. Anyway, the kit comes with a sonar range finder but I wanted a little more accuracy and smaller detection area. I’ve got a bunch of Sharp rangefinders from another project and they would work perfectly for what I wanted. A couple of companies sell after market sensors for the NXT, Mindsensors and HiTechnic but it’s much more fun to design and build your own.

For a little background on how the Sharp sensors work, Acroname Robotics has a good article on them.

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I’ve been making good progress with the main PCB. Just got a couple of power tracks to lay down and then I’m sure there will be some optimisations to do.

I’ve also found a suitable case, 4X20 LCD and various other part for the project. As you can see things are taking shape.

Just a quick update on the Function Generator project.

Starting with the front panel…

• Schematic design.
• Prototype parts.
• Panel design. (The image above shows a first attempt to lay it out, it still needs text and alignment)
• PCB Layout.

then the Main PCB:

Work in Progress

• Schematic design.
• Simulation.
• Prototyping. (I’ve done some prototyping, the last thing to test is the actual DDS IC and Low-Pass filters.)
• PCB Layout.

finally the Power supply:

• Schematic design.
• PCB Layout.
• Test and prove.

I’ve not had time to finish the Irrigate Project page so in the mean time I’ve posted some Schematics and Photos of the PCBs.

I’ll add more on the software/firmware side of things soon.

Dave Jones runs a videoblog for anyone interested in electronics. He’s just released a “Lecture length” one on how to design a product from start to finish. Have a look, he’s got some good advice.

EEVblog #72 – Let’s Design a Product

I’m not sure about the rest of the world, but here in SA Easter weekend is a 4 day weekend. What to do, what to do? Some design work that’s what!

Above was a first attempt to get my ideas and design written down. There are some fundamental problems with the above design, but it is as a starting point and has the major elements laid out. Read More…

Just a quick update on the DDS Function generator project. I’ve just finished designing and laying out the power supply module. It’s a very basic design that uses 4 linear regulators to output 15V, 5V, GND, -5V, -15V. It’s not the most efficient design but because the system will not be using huge amounts of power, I think it’ll serve quite well, and because it’s a module if heat does becomes a issue it can always be replaced with a more efficient switch-mode supply.

Above is a quick 3D rendering of what it looks like at the moment. For scale, it’ll easily fit in the palm or your hand.

I’ve finally finished the Dual Battery Switcher page. Let me know if there are any glaring errors or spelling/grammar problems.