This page details the design and build process of the Primrose Solar Power monitor This was an interesting project and involves the use of a Raspberry Pi computer to control all the components. The brief was for me to make a 1930's styled power monitor for Primrose Solar that will display on large Nixie tubes, the total power output in Megawatt Hours of a specific Solar Farm. The power data is located on a database, and the monitor needs to be able to use Primroses IT network to access this and then display it on the Nixie Tubes, along with other information that is displayed on old analogue panel meters.

I made a couple of mock-up's for Primrose, and they decided on this one. It is a 1930's combined volt / Amp meter made by Vickers of Manchester. The size of the casing looks just right for 9 large IN-18 Nixie tubes. The original connections for the meter are still visible top right. It also has 2 very large analogue panel meters that can be utilised.

The next step is to completely remove all the original internal parts, take off anything bolted to the wood and literally get a wooden 'chassis' to work with. Once done, I noted there were a lot of holes on the top of the instrument - may well get in the way of mounting the Nixie tubes, so I turned the whole casing upside down, and used the bottom as the new top as it was relatively free of any holes. The big wooden box behind the dial contained the shunt resistors.

Probably the most delicate and time consuming stage, you only get 1 chance to drill the holes! I can't simply go and get another meter if I make a mistake as it is a real vintage piece used. You can see all the marking out still on the wood, I use a pillar drill to make sure the holes are perfectly vertical otherwise the tubes may lean. This took about 4 hours just to drill the holes in the top - but I like to get it right.

Once drilled, I temporarily installed the boards that support the IN-18 Nixie Tubes. This is so I can mount the tubes in place to check all the spacings and proportions. I can also cut all the ribbon cables to length that connect the tubes to the Nixie controller board.

With the front back on I can see that the tubes fit perfectly and the proportions just as I wanted them to be. The casing is nice and symmetrical, so the front panel still lines up and fits, even though the rear casing is upside down. There is also a hairline crack in one of the glass windows that I will have to get a new piece cut for.

Back inside, I have mounted the driver circuit board and cabled up the first 7 nixie tubes. The last 2 tubes are permanently lit with zero's, as the numbers indicated are in the magnitude of millions. The electronics only drive 7 digits. I have also re-instated the original dial plates and mountings, although the movements have yet to be put in.

Image courtesy of PV electronics

The Nixie decoder comprises a PIC micro that has 28 digital IO, that get grouped into 7 BCD outputs. The PIC also has an on-board UART and its sole task is to read in a serial string into an internal array, and then echo the contents of the array to the 7 BCD outputs. Each of these outputs drives a K5515D Nixie driver chip and that drives the Nixie.

Image courtesy of PV electronics

Here's the schematic for each of the BCD drivers. The tubes are direct driven and not multiplexed, the number is displayed continuously until a new serial string is received. The only other electronics is a 170v Power supply - that is the little red PCB you can see sitting on top of the driver board. The Raspberry Pi then drives the Nixie Decoder via RS232.

During the initial talks with Primrose, it was thought to be a good idea to incorporate RGB Led's under the Nixie tubes. A lot of my clocks have under lit Nixies, but in this application it can be used to indicate efficiency, or even weather conditions i.e. red lighting means a bit cloudy so the solar production could be lower. I constructed an array of RGB leds that position themselves directly under each tube.

The RGB Leds have a small FET to switch them, and they are driven via a PWM output from the Raspberry Pi. I used a 3rd party add on board to give 16 independent PWM channels as the RGB requires 3 separate channels. Here the composite image shows each colour in turn, and them all on to give a white light effect.

Some attention to the casing and detailing. I've removed all the electronics and sanded the wooden casing and front using various grades of paper, and then applied several thin coats of varnish to get a lovely soft gloss effect. I also had some brass bezels made for the tubes in groups of 3, engraved in a suitable 1930's typeface to finish of the top.

Final test of the Nixie decoder, I've just hooked a laptop up and using a terminal emulator program to talk directly to the Com port on the Laptop. There is a USB - to TTL voltage converter to allow comms to the Nixie decoder. You can just make out the window on the laptop containing the string displayed on the Nixie tubes.

Here you can see the Raspberry Pi mounted on the old Dial chassis, and sitting on top of the Pi is the 16 channel PWM card supplied by AB Electronics that I use to drive the RGB Leds, and also the 2 large dial pointers. The Pi has a USB WiFi dongle installed and is operated via a remote desktop session, so there is no need to hook up a monitor / mouse and keyboard. The Pi is programmed with Python.

Installed last are the 2 servo motors that drive the pointers on the 2 dials. Completely under the control of the Raspberry Pi, they can be programmed to move to any position at will, and a lot less delicate than traditional moving coil meters. In this picture are some temporary pointers used to gauge size and allowed me to put the servo's in the right position.

Here's a more detailed view of the later servo arrangement. The pointer is actually a second hand for a clock but suits this application perfectly. It is a friction fit into the servo arm and easily replaceable should it get bent or damaged.

This shows the wiring plugged into the Pi's add on board. The two servo motors and the RGB drive. The only remaining connection is the RS232 from the Nixie decoder board, and these just plug onto the Pi's multi-way header. The Pi power connection is also via the add on board, and will be the final connection, once the Pi is programmed.

I've polished up the original Label, and decided to put it back on the front of the instrument, very nice 1930's style script and looks like it should be there. Also to be fitted on the top between the two dials is a suitably styled version of Primrose Solar's Logo.

Finally the completed power monitor, sitting on soak test for a week to thoroughly check the HV power supply, the Nixie tubes and the internal mains power supply. All tubes set to zero waiting for and input and all the RGB Leds switched on to pull maximum power for testing.