Some simple experiments using VELA

Whilst I've been writing the software to interact with the VELA for my BBC Micro, I've had the need to build a few data sets with some simple experiments that I can verify using other tools or established fact. To that end, I've currently performed a handful of experiments to ensure that the VELA is working and my software is interpreting the data correctly.

VELA - Vodka chemistry sketch

The image above was taken from the handbook "Get to know your VELA - for chemists" which I humbly received as a gift from Dr Ashley Clarke. The artist was Steve Bloomer. I suspect they're using the two VELA's pictured to monitor the Vodka making process!

Experiment one - To measure a variable DC voltage

My first experiment using the VELA after writing my simple graphing application was to connect a 12V DC supply to an old PC case fan speed controller and measure the voltage changes as I altered the position of the speed controller.

The PC fan speed controller used to alter the DC voltage being monitored by the VELA

I hooked up the controller to a multimeter and the VELA so I could monitor the effects of changing the variable resistor whilst the VELA was logging the voltage.

The graph of the DC voltage changing over time using the PC fan speed controller

The VELA stores 4085 data points and the BBC Micro is outputting all of those data points on an X-axis of only 1023 pixels which creates the multi-line effect as shown on the graph. By using smoothing or averaging techniques, the graph could be smoothed out.

Why only 4085 data points?

The VELA uses 4Kb of RAM divided between the four input channels creating four banks of 1Kb memory. When using program 1, the VELA uses all four banks of RAM to provide 4Kb of data storage for the channel 1 inputs. The VELA however only stores 1023 points per memory bank reducing the data storage capability to 4092 bytes. A further 7 bytes of RAM from the top of bank 4 are used to store the program parameters that are used by the program to run which reduces the maximum number of data points to 4085.

When the data is transferred to the computer, a total of 1023 bytes per bank are transferred so this must be taken into account when creating the graphs.

By storing the program parameters in the top of bank 4, the program parameters are inherent to the 4Kb data block that can be written out to disc and used when re-loaded in order to configure the correct scales on the graphing application.

It's interesting to note that these 7 bytes of data that store the program parameters are not the same format as the 7 byte data pre-amble that is sent in advance of the 1023 byte data bursts. Instead, the 7 bytes store the program parameter (2 bytes - comprising a hi and lo byte value), the program number (1 byte) and finally, the channel gain values (4 bytes - 1 byte per channel).

Experiment two - To measure the voltage of a 9V AC power supply

After the success of Experiment one, a suggestion was made to try monitoring an AC power supply to attempt to create a graph of the mains AC waveform. Using a 9VAC supply (which my multimeter measured at 11.3VAC)  connected to the VELA, the following graph was produced when the AC supply was monitored for 20 seconds.

The graph produced by connecting a 9VAC power supply to the VELA

Following on from the 20 second analysis of the AC waveform, it was clear that a faster sampling rate was needed to see the pure AC waveform correctly with the VELA so the supply was re-sampled for a period of 0.2 seconds which produced the following waveform.

A 50Hz AC power supply monitored for 0.2 seconds with a VELA

As you can see from the graph above, the graph shows exactly 10 peaks and 10 troughs as you would expect when monitoring a 50Hz AC power supply for 0.2 seconds.

Experiment three - Sampling one seconds worth of an audio signal

As a change from measuring AC waveforms whilst writing my software application, I used the VELA as a sound sampler and recorded one seconds worth of audio from a DAB radio. The resulting waveform can be seen represented on the graph below.

A one second audio sample as recorded on the VELA and drawn on the BBC Micro

The observant amongst you will notice that the screenshot of the audio waveform was taken from Beeb-Em, an emulator for the BBC Micro with which I've been testing the non-VELA dependent parts of my software application. You'll also note the extra features compared to earlier images showing function key options for manipulating the graph and an indication of whether the graph is interpolating the data or not.

The sample rate used for this experiment was 4KHz using the VELA's 250uS sampling period (Program 1 - Parameter 5). Faster sampling rates of up to 29KHz are available by setting the VELA's parameters accordingly. For instance, the "flat-out" mode using a 34uS sampling period (Program 1 - Parameter 0) would result in a maximum sample length of 0.13 seconds recorded at 29KHz due to the VELA's 4KB memory limit.

Experiments with Capacitance - Monitoring the charge and discharge of a capacitor

As part of the Beeb@30 birthday celebrations, I created a couple of simple experiments that would have been taught using the VELA at O-level and GCSE level in the mid 1980's and early 1990's to show the charge and discharge curves of a capacitor. I created a couple of explanatory pages to describe the experiments and here are a couple of screen shots of the resulting curves which illustrate the nature of the charge and discharge curves.

Charging a capacitor with a 9V battery

Monitoring a capacitor being charged to 9V with the VELA

Discharging a capacitor after being charged with a 9V battery

Discharging a capacitor charged to 9V

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If you've come across this page because you've found a dusty old VELA or any of the peripherals I've mentioned on the site in the store cupboard of the science department and you're wondering what you can do with it... You could always consider donating it to a good home such as mine where it would be reunited with a BBC Micro, archived for the future and used!

Although I now have many VELA related items, I am still missing many more bits and pieces so if you happen to have a VELA, any VELA software, manuals or related VELA handbooks, then I'd still be very interested in hearing from you so please do get in touch!

Of particular interest are any VELA SAM4 compatible modules and software for the RM380Z, Apple II and IBM-PC's of the day which currently are lost to the mists of time with the prospects of viable floppy discs containing the software being more remote as the years go by.

For a full list of VELA related items I'm looking for, please check out my VELA - WANTED, DEAD or ALIVE! page.

VELA Technical Downloads


The following ROM images are Mk I EPROMS and are incompatible with the ISL1* ROM from the VELA Mk II.  You can use the Mk I ROMs in a VELA Mk II or VELA PLUS but there are several differences in the data formats they produce when exporting from the VELA so when sending data to a microcomputer, it should be told it is connected to a Mk I VELA.

The following ROM images are for the VELA Mk II as indicated by a red *.

The following ROM images are for the VELA PLUS as indicated by a green *.

The following ROM image is for the SAM4 Sensor Adaptor Module

  • SAM* ROM

This final ROM is the RS Components ROM image that replaces ISL1* for their re-branded Multi-function Intelligent Data Logger.

IC Datasheets

The following datasheets cover all of the IC chips that are used in the VELA Mk. II data logger.

Other VELA Software

The VELA manuals contain software listings for the transfer of data to several computers, the link below contains an SSD disc image containing the source for the BBC Micro transfer code.


I'd like to thank Jon Neville-Green of The Discovery Academy School in Stoke-On-Trent who kindly arranged for me to obtain their VELA which had been gathering dust in exchange for a donation to the Science department at the Academy. Thank you very much.

I'd also like to thank Cathy Herriman of Sherborne School for providing me with a full surplus set of manuals and supporting books and documents for the VELA in exchange for a donation to the School funds. Thank you.

Finally, I'd like to thank Mr Robert Boyd of Glossop High School in Adelaide and Dr. Ashley Clarke (one of the original creators of VELA) who have both contributed information in various e-mails that has allowed me to build up and present a wider picture of the VELA data logger and what it could do.