Running Light Controller

My friend Brian Boot is a modeller.  He's not just any old modeller who assembles a few Airfix kits in his spare time though, he's absolutely dedicated to model making at the highest level, adding additional details and working features wherever possible.  If you visit his house, you have to be careful moving around, because almost all horizontal space has been taken over by models and work-in-progress building more models.  His work has won medals at competitions and some are displayed in museums, I believe you can see some at the Shoreham aircraft museum. He also has galleries on the Farnborough and West Middlesex club websites.

One thing that sets Brian apart from many other model makers is his desire to add lights and motors where possible.  One of his works-in-progress is a Bandai 1/16 scale Showman's Engine (link shows another finished example) but it had reached a state of impasse due to his desire to illuminate the canopy with some chasing lights and motorise the mechanism.  I set about designing a circuit for the lights, so he could get on and finish it.

There are a lot of circuits for this type of thing using a microcontroller, but really that isn't necessary, the basic circuit is almost a classic from the 1980s, a 555 timer driving a 4017 decade counter.  The hard part (as far as I was concerned) was to find a good way of driving the LEDs properly using constant-current source.

If the LEDs were simply wired in parallel, each with its own resistor, the overall current required would be multiplied by the number of LEDs, the wiring would become more of a mess with the resistors needing to be embedded up in the roof of the model, it wasn't going to be ideal.  I needed to connect the LEDs in series so that the current for each set of lights could be set correctly,  this would equalise the brightness, avoid over-driving them, and keep the circuit simpler with reduced wiring.

The constant-current driver design was taken from an excellent Instructable "High Power LED Driver Circuits" using the final circuit at step 8 with the zener diode.  Each LED requires a forward voltage of between 2 and 3 volts (depending on colour) so a DC-DC boost converter (from ebay) provides the 30v necessary for 10 LEDs in series.

Circuit diagram:

 Stripboard layout:

Prototype: