How to Add ' Glow in the Dark' LED Eyes to a Plaster of Paris Ornament

This 'Instructable' makes use of the fact that modern white light emitting diodes can emit quite usable amounts of light when run at very low current, even as low as 5 to 10 micro-amps. Such a low power requirement means that batteries can be very small and have a very long operating life. In this case the battery is made from redundant alkaline button cells that might otherwise have been thrown away and the battery part of the project could find application elsewhere.

The project uses an ornament made of plaster of Paris which, being a very soft material, can be easily drilled and worked. Models made from other material will be possible.

There are a couple of examples of similar projects on this site but it is hoped that this one is sufficiently different, see:

https://www.instructables.com/id/Adding-Led-eyes-to...

https://www.instructables.com/id/Lego-Octopus-LED-L...

You will need:

Some very basic electronic skills and certainly a soldering iron, solder, cutters and wire.

The wherewithal to drill the eye-holes in your model.

A junior hack saw to cut small pads of the printed circuit board.

In the UK Maplin can supply the AA battery box, resistors and single sided printed circuit board.

The 3mm diffused white LED came from www.bitsbox.co.uk

Step 1: Find a Suitable Ornament

Our guinea pig for this Instructable is a Plaster of Paris black cat won in a fair in the 1950's and it has been lurking in the back of a drawer ever since. Looking rather battered and bewildered perhaps but this moggie is a remarkable survivor. Sadly there is no picture of it in the untouched state. Note that this model has a flat rear side on which we can mount and conceal the modest electronics.

For a small model like this one normal 5 mm LED's are too big and I used ready diffused 3 mm ones. These may take a little hunting down and I got mine from www.bitsbox.co.uk. See the first picture above.

The first step is to drill out the eyes to take the two 3 mm LED's which was done with a PCB drill and various drill bits starting small and working up to the size that allows the LED to be inserted with some slight clearance. This can be done with a hand drill but you will have to be very gentle with a steady hand as plaster of Paris is so soft and vulnerable.

The two LED's can now have a little epoxy resin applied around the circumference towards the rear of the plastic body and then be pulled through the eye holes by the wires from the rear and then the epoxy resin should be allowed to set, ideally overnight.. The second picture above shows the result with just the right amount of epoxy. The third picture above shows how the wires protrude at the rear and have been bent downwards. The fourth picture above shows how four small squares of single sided printed circuit board have been stuck to the model at the top using glue and I used MS polymer flexible glue. These are mounted at the top as the battery will take up most of the space below.

The next steps will show the battery construction.

Step 2: The Battery--1

This section may have relevance outside this article.

The first picture above will strike a chord with many readers. A number of cards bearing an assortment of button cells have been purchased from our local 'pound shop' and, after the AG1 cells have been taken for watch duty, we are left with a motley selection that builds up in number until the decision is reluctantly made to throw them away. Here is one way of using the ones that are 10 mm in diameter, i.e. AG 10, 12 and 13.

In the second picture above we see a single AA cell battery box, some AG13 cells, a piece of 0.5 inch wooden dowelling, some aluminium foil and a lump of Blu Tack. This is our extremely flexible battery factory!

The next picture above shows how 9 AG13 cells can be placed in series in the battery box. Note that the all metal side of a button cell is positive and this positive side of the cell is placed against the curly spring connection of the battery box which is normally the negative connection. Hence the positive side of the battery is what is normally negative when the battery box is used with a single AA cell--this doesn't matter for this application but it could be important in others. Note that the cells are a rather slack fit in the battery box and the sausage of blue Blu Tack also in the picture can be massaged into place to stop the cells from moving around.

Step 3: The Battery--2

The battery box will hold 9 AG13 cells which is too many for many applications, and even more of the thinner AG10 and AG12 can be fitted in. This is where the dowel and aluminium foil comes into play.

The first picture above shows a piece of dowel sawn to length with a junior hack-saw together with rectangle of aluminium foil. The second picture shows the foil being wound on to the dowel and in the third picture we see the completed item with the ends squeezed into shape.

One important proviso is that the cells should all be of the same type i.e. do not mix AG 10 and AG13 together for instance. Some cards have cells in groups of five and using five of one type at a time may be good practice.

Step 4: The Battery--3

This shows what can be done. The first picture shows the battery box full with nine AG13 cells and the voltage output is nearly 14 Volts. In the second picture with five cells and aluminium covered dowel to pad out the space we have 7.69 Volts. Bear in mind the capacity of these button cells when planning their use. The AG13, when new, should have a capacity of 150 mA hours and the thinner cells will have proportionally less and these may well be old cells packaged to be sold off.

Step 5: The Circuit

There are two possible circuit configurations shown above. We have two eyes/LED's and they can be wired either in series or parallel. My experience is that a current of 4 micro-Amps through a modern white LED gives sufficient light for our purpose and this is achieved in the chosen configuration with nine cells in the battery box and a ballast resistor of 2.2 meg Ohm. This equates to a projected battery life of just over four years but these are probably old cells and if we achieve half of this life then it will be creditable. If you choose the parallel configuration for the LED's, (which doubles the current,) and also use fewer cells in the battery then you will need a smaller ballast resistor and battery life will be reduced.

Ideally, try your components out on a solder-less breadboard and measure the current by measuring the voltage across the ballast resistor and using Ohm's Law:

Current = Voltage/Resistance

See Step 7 for an example of breadboard use.

Step 6: Assembly

The left hand half of the picture above shows how everything can be brought together. I opted to fill the battery box with AG13 cells resulting in in a battery output of around 14 volts. Note the sausage of Blu Tack that holds the cells in place. The LED's are wired in series via the pads of single sided printed circuit board and the ballast resistor is 2.2 meg Ohm. There is no switch--I didn't think it necessary but the constructor might like to add one rescued from an old solar light or toy.

It has to be admitted that a 'kludge' was needed in the form of a piece of dowel glued to the low end of the ornament to make it stand steadily as the battery weight has made the assembly slightly top heavy.

The right hand side of the picture shows the view from the front with all of the electronics nicely hidden and the eyes glowing. The ornament can now be used as a micro-power night light as the light output is considerable when allied with the night vision of the observer and some will be happy with this. Others may feel that the whole effect could be enhanced if the eyes of the cat were green or another colour and a way of achieving this is shown in the next step.

Step 7: Adding Green Eyes---1

When considering green eyes for our model many will ask 'Why not just use green LEDs?'

The first picture above should go some way towards answering that question. It shows a white LED of the type used in our project and a 3 mm green one mounted on a breadboard. Each is fed via its own 100k Ohm resistor from a 9 Volt PP3 battery, (out of shot.) Apologies for the poor quality of the picture--the white LED has dominated the focus--but it does illustrate the point with the green one barely lighting up at around 80 micro-Amps.

It may be that there are modern green LED's that are more efficient but I suspect that most of the coloured LED's that are sold in 'grab bags' like the one above are not. If you want to explore this route with state of the art components then test them out first using a cheap solder-less breadboard such as the one illustrated. Mine came from www.bitsbox.co.uk but they should be widely available. This approach also allows you to experiment with the two LED's wired in parallel or series and thus set the ballast resistor value with various battery voltages.

Step 8: Adding Green Eyes--2

The problem was overcome by placing a green filter over the white LED's. Many items suggest themselves for this duty such as half beads made from clear green glass or plastic, multi-layer clear green sweet wrapper etc. etc. but I was helped by the fact that a local health food store has taken to packaging its wares in clear dark green plastic bottles--see the first picture above.

To make a lens of the right shape an area on the bottom of the jar was softened using a gas lighter flame and a small punch forced into the softened area--see the second picture above. The bottom was then sawn off using a junior hacksaw and the the green lenses carefully sawn off horizontally and then lightly glued on the the LED's using MS polymer glue as shown in the third picture above.

Step 9: Some Last Thoughts

Modern white LED's have developed at a fantastic rate and very large amounts of light can be generated from single LED's and arrays of them. However, it may come as a surprise to some that useful amounts of light can be generated at very low currents, even as low as 5 micro-Amps thus enabling physically small batteries to have a long life in applications. One of our neglected senses--night vision comes into play here as well.

The model used was a Plaster of Paris example and this material is still a staple of the craft scene today. The technique could be applied to other types of model, wood, plastic or even ceramics indeed any application where the batteries can be catered for.

The way of using redundant button cells could be particularly helpful as vast numbers of these must get thrown away.