Another PICAXE-Based Public Artwork


Senior Member
In the last year I have formed a business partnership with established visual artists Philip and Dawn Gamblen, who I have worked with on a subcontract basis for several years.

One of the public artwork projects that we completed in 2016 was for a local property developer.

The new residential development called for an artwork mounted on a street-facing wall of the small suburban apartment block to enhance the appeal of the development.

Our proposal for a damask-inspired design was accepted by the assessment panel. It features shapes of local wildflowers and uses UV-resistant artificial plants mounted on powder-coated, laser-cut 6mm sheet aluminium. The sheet aluminium of the artwork is mounted 120mm off the wall to maximise the day-time shadows cast onto the wall, and night-time LED lighting. In the evenings, the artwork is back-lit with 16 x 10 watt RGB LEDs. The IP67 low-profile lights are a custom design, where the COB LED chips are mounted directly onto the rear of the 6mm aluminium panels for cooling, resulting in a very low-profile.

The lighting is driven by a PICAXE-based lighting controller, using PWM-driven P-channel MOSFETs for each on the red, green and blue channels. The high-side MOSFET drivers are current-limited for safety should a short circuit to ground occur in the artwork. The PICAXE software includes random-number generated colours which slowly transition between hues. Colour intensity and mixing is managed through a 6-slope linear to pseudo-logarithmic algorithm to compensate for the non-linear response of the human eye. This gives the viewer an improved range of perceived colours.
(Photos by Philip Gamblen, Peter Gee)


Senior Member
Looks good.
Question though?, what does this mean? " a 6-slope linear to pseudo-logarithmic algorithm".
The colour of the LEDs is controlled by 3 PWM pins (RGB) on the 28X2, so over 1,000,000 colours are possible. The human eye has a logarithmic-like response to light intensity (similar to our ears and sound levels). The random number generator within the PICAXE is 'linear' Ie not weighted, and is used to create colour via the PWM outputs for Red, Green and Blue drivers. In order to get a better balance of colours, the mathematically linear values for Red, Green and Blue PWM are converted via the pseudo-logarithmic algorithm to a range of values that appear to be 'linear' to the human eye. This gives a more balanced and wider range of colours perceived by the viewer.

One option was to use a purely mathematical calculation for the PWM ratios for the colour. This would require a pretty heavy amount of processing from the PICAXE's integer-based mathematics. Instead, I used 6-slope approximation of a linear-to-logarithmic curve. Hopefully, the attached chart shows what I mean.
Lin-to-Log Chart.JPG
Very impressive !.

Do forum members get a discount if we want to move in ? :)
Sadly, not even the artists get a discount. I have viewed one of the apartments and they are very nicely finished. The car parking bays are a little small for my liking but I'm told they meet the current residential standards.