Of or pertaining to cake

In my research group there is something of a tradition to make novelty cakes for the farewell gatherings when someone leaves. I’m not going to lie to you: this is a tradition invented and driven by two of us and tolerated by the others.

(This site has moved to http://www.flashingleds.net, go there if you have comments or questions)

The madness began with a multimeter cake for a departing lab technician:

Sadly as an engineer he was unimpressed with a measely 3 digit meter

Over time things started to get out of control, with subsequent efforts over the years encompassing entirely edible renditions of:

along with a few others. Sadly I don’t have a full set of photos to show you. Our most recent effort returned to the theme of measurement equipment with the venerable SR830 lock-in amplifier:

>100dB of dynamic reserve and >3 sticks of butter

At some point along the way I got it into my head that the pinnacle of cake achievement would be one of either:

  • A cake with (edible) moving parts
  • A cake with (edible) electronic functionality

In terms of electronics expertise I am a pretend embedded engineer in my spare time; in terms of mechanical expertise I am an unmitigated disaster who thinks all solutions begin and end with a hot glue gun. I recognize that my best hope of success lies with edible electronics.

You will probably not be surprised to learn that very few electrical components can be rendered in an edible form. Even the most basic element, low resistance wiring, is difficult. The first (edible) thing that comes to mind is something based on salt, but there are two problems with this:

  1. Nobody wants to eat a cake decorated with some kind of gross salt paste, with the possible exception of the Dutch (see: Salty Liquorice).  I am all about putting gross salt paste on my toast, but not my cake.
  2. Ionic conductivity is not very good. Wikipedia tells me that seawater has a typical resistivity of 20 Ωcm, which means a 10cm long, 5mm diameter tube of it would be about 1kΩ.  This is not outrageous, but suggests that you would need to use something very salty to get reasonable wires.

The breakthrough came when I discovered that edible silver foil is a real thing, that you can buy. It’s popular in Indian sweets, and provided it’s relatively pure it is generally regarded as safe to eat – the foil is only a few microns thick so the quantities involved are low. (As always, you will find dissenting opinions on the internet; certainly go and do some reading if you’re concerned).

In truth it was The Hungry Scientist Handbook that put me onto this, and furthermore they had even gone ahead and done exactly what I had in mind! Their recipe (see here) used silver foil wrapped around liquorice strips to illuminate a chain of LEDs on a cake. (Obviously the LEDs are not edible, but hey, you have to start somewhere)

Clearly it would be necessary to raise the bar. Instead of a chain of static, battery driven LEDs on a cake,  I want … a scrolling LED sign cake.

Ok, so maybe we relaxed our typically stringent aesthetic standards this time…

And that’s exactly what we did. Because writing something in icing is last century my friends. By coating only one side of a liquorice strip with the foil, you can cross wires without them shorting to each other. Then you just have to lay out a matrix and stick your carefully cleaned LEDs into the straps. A microcontroller is driving the LEDs to scroll out a message. For the curious, the source code is here. It’s written for an ATMega8 running on the 8MHz internal oscillator, but will also compile on the Mega48…328 line. It should be easy to employ for any 5×8 LED matrix, cake or not.

It all sounds very easy, and I have so far been describing it in the manner of somebody who has realized absolute success in their venture. Sadly that is not quite the case.  The truth is that the cake was something of a failure, saved only by the temerity of its vision (and also its deliciousness). It turns out that silver foil is extremely difficult to work with. In the final product we ended up with a lot of hairline cracks in the foil which resulted in a number of broken connections. A few LEDs still flashed in time, but not enough to carry the effect.

So the cake ended up being salty anyway. Salty from my tears of shame.

Here is the sign executed on a more conventional LED matrix (halfway through my camera gets out of sync so it looks a little funky). If you’re reading this Mark, here’s what I was trying to say:

I’ve put this post up for two reasons:

1) Despite failing in the end, it was still awesome.

2) Hopefully some kindred spirit out there takes what I’ve learned and has another, more competent attempt at it. Please do, and please let me know how it goes!

P.S. I’ve since seen edible silver paint being used which looks much easier to work with: http://www.youtube.com/watch?v=pdAqI8IAxCE

About Craig
Craig is getting towards the end of a PhD in experimental nanotechnology. Arguably he might be finished by now if it weren't for all the crap described on this blog. Queries/comments to flashingleds@gmail.com

One Response to Of or pertaining to cake

  1. DeadlyDad says:

    Gold leaf would also work as a conductor. FYI, graphite is non-toxic; perhaps you can find some ideas to work with in this instructable. I also wonder if using zinc and silver pastes to plug the ends of a licorice stick filled with gelled lemon-juice would work as a battery cell.

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