bigger batteries

Here is some information on a prototype powered case for the REX. The following comes from Mark, with photos and a circuit diagram. Thanks, Mark!

history

The first Rex I owned was the Rex Pro (now known as the 5000.) I used it continuously until I dropped it into a bucket full of salt water. I was bending over the bucket and it slid out of my shirt pocket before I could catch it. Even though I grabbed it out of the water as quickly as possible the screen was already flickering. That should not have happened since the Rex was turned off, so it was a pretty strong indication thatthe unit was already ruined. I removed the batteries as quickly as I could and then let it dry out for two days. When I put the batteries back in, it started normally and even allowed data entry. I thought I had saved it. But after a few minutes the screen would start flickering and the thing would lock up. Removing and replacing the batteries several times indicated that the power circuits were the problem, so I decided to keep it. I couldn't use it but I thought I could get a look inside.

After about a week I went on the Internet to look for another Rex Pro, only to find out that the Rex Pro was now a full blown Palm sized PDA. After searching around I realized that they had renamed the old Rex Pro to Rex 5000 and had come out with a new unit which they called the Rex 6000. And it had a touch screen! I couldn't believe the features they had added. So I ordered one around the beginning of December 2000 and had to wait till the end of January before they started shipping again. After receiving the new Rex 6000 I used it so often that I was going through batteries every five days. Reading E-Books takes a lot out of it and I suppose I shouldn't have hit the delete button so many times. About that time I dreamed about an add-on battery pack that would let me use my Rex for extended periods of time. But, a few months later, I lost my new 6000 when I left it sitting in a public place on board a Navy ship. It was never returned so again I went to the Net to find another one.

Now after less than a year and working on my third unit, I found myself with three serial docking stations just sitting around. I use a laptop with the PC Card slot for synchronization and never used the docking stations even once. So I just threw one of them away. Why not the others, too? I don't really know. Then, about a week later, on December 1st 2001 I came across a request on Greg Searle's web site for any information that would allow the Rex to operate from four AAA batteries. It reminded me that this was something I had wanted to do and so I began hacking.

where to start

The first thing I had to do was figure out where the Rex was getting its power from within the docking cradle. I disassembled one of the cradles and put the batteries in without the Rex being in the slot. After checking each pin and finding voltage on a single pin (pin 1), it seemed that hacking together an external power supply would be very simple.

Number one big mistake: I didn't check the voltages with the Rex inserted. What I did do was wire up 6 volts directly to pin 1 of the Rex and then connected the return. Whoa! I'm glad I had a meter on the batteries, because the voltage dropped to zero! Allrighty then, I dropped back a step and this time I tested all the pins with the Rex inserted into the cradle. Pin 1 did drop to zero and I found quite a few other pins that had jumped to 5 volts. After tracing the circuit back from pin 1, I found that this initial voltage was being fed through a 1 Meg resistor, and was there only to sense when the Rex was inserted into the cradle. After that discovery, I spent some time taking readings on the circuit with the card inserted and then removed. What I found was that pin 1 fed into the base of a transistor, which would act like a power switch when the Rex was inserted and removed. A very simple technique that ensures the batteries are not being drained when the Rex is not in the cradle. A bit more testing and it was obvious how the power circuit was operating so now I was on to...

building the prototype

After creating a hand-drawn schematic of the power circuit for the cradle, I bread-boarded a working circuit for testing. Part of the testing included finding out how much current was being used with the Rex both inserted and removed from the circuit. I used a 7805 +5 voltage regulator in the test circuit and found that the regulator was drawing as much current as the Rex. What that meant is that the battery life would be reduced by half. Comparing that with the current draw from the cradle's power circuit, I decided to use the components from the cradle. Thankfully all the power supply components were situated close together on the circuit board. For the fastest protyping, I could just cut that section out of the circuit board and use it in my project. All the components are surface mount devices, and although I can solder individual pieces when new, it is much harder to remove and reuse these devices. The pictures clearly show which section of the circuit board I removed and used on the prototype.

Since I had all the material on hand, I decided to etch a circuit board for this project. It was a very simple layout, since most of the traces would simply interface to the section of circuit board I was using from the cradle. Later on I plan on finding the surface mount devices in single quantities (I hope), and then I will have a complete schematic and component layout diagram made up.

schematic

SW1 is not an actual switch. It is drawn this way to show that upon inserting the Rex into the cradle, the circuit is completed to ground through pins 68 and 35. This causes Q1 to conduct which then powers up the Rex.

prototype photos

things I would do differently

As you look at the pictures, you will notice the new circuit board is extending about 3/4" beyond the back end of the Rex card slot. This additional material can be removed by building the power circuit onto the battery case. I can probably still modify this one to be more compact by making that change.

Greg (as mentioned above) pointed out how thin the new AAAA batteries are. These would make the unit lighter and smaller. He also mentioned rechargable NiMH batteries as an option. Both of those suggestions brought up the idea of a built-in rechargable battery pack. I believe all of that could be included in a smaller package than the prototype you are seeing here.

but it doesn't work with the 6000

Holy smokes! After all that work and testing I finally get up the nerve to test the unit with my Rex 6000, the one I use daily. Upon inserting the 6000 into a cradle it wants to synchronize. But with most of the other pins missing out of the dock, it just flashes the Pin Entry screen for a moment and then shuts off, with no way to turn it back on. I thought that it was the end of this project. But I wrote to Greg anyway and told him that it was at least useful for people who still use the 5000 and any earlier units. Much to my surprise, he wrote back with a web address that has a firmware upgrade that allows the 6000 to be used in the cradle! Update: DockPatch will only display "please connect..." on the REX 6000 with this setup.

All information and photos released by Mark under the General Public License.