A ferrite core is prefered. A metal nut doesn’t really work. Magnetic property is just not the same.

More turns equal more Henry value of the coil.

But given the size of the hole, how to calculate roughly how many times a wire of given size can pass through it?

So first, we need to convert each round wire into a hexagon in order to fill up the gaps in between them.

Each hexagon will be composed of six equilateral triangles.

Each equilateral triangle will have a height of W/2.

The area of each equilateral triangle is then calculated.

Multiply the area of equilateral triangle by six will be the area of each hexagon.

The area of the hole H will can be obtained easily.

Divide the area of the hole with the area of one hexagon will give you a rough idea of how many wire can pass through said hole.

The thickness and the height of the cylindrical core will tell you the length of each winding. Multiply this with the number of times the wire can pass through will produce the total length of the wire needed.

But with more winding, the length of the wire each turn will increase. So you may need to take this into consideration if there is a very big hole to be filled.

Again, this is just a rough calculation, accuracy is more than enough for small projects.

Switching the AA battery with a D-cell. I have changed one of the 100nF ceramic capacitor to a 10nF version. Thus the frequency is now 10Hz instead of 1Hz. So, it can still blink for 10 years. This Energizer Max D-cell has a shelf life of 10 years.

Need to find a housing for it.

The 4 screws holding the circuit board to the battery holder are 1.6mm glasses/watch screws.

I have also pulled away the LED so that it may be afixed somewhere else in its future housing.

The same circuit on breadboard. This enables me to swap components with different values to test out different configurations. For example, to make the LED blink faster.

Don’t worry, I have tidied it up later, replacing those naked copper connections with proper jumper wires.

And the four jumper wires in the photo connects to a toroid elsewhere.

The problem with this circuit board is the gaps were too narrow. Even with a little moisture, where will be leaking current somewhere. However, with thorough cleaning (with dish washing paste) and drying (with a hot air gun), it worked without a problem. Hot components will make the LED blink faster.

So I made another one:

Still need to wash and dry to get rid of leaking current.

I have also made a templates out of 2mm acrylic sheet. This will let me make more circuit board easily.

The obvious difference is the lack of a diode 1N5819 in the later.

This circuit is supposed to light an LED bulb for 1/1000th of a second (1mS), every second, for 10 years, using an AA battery.

If you switch the AA battery with a D type battery, which carries 10 times the charge, it may blink up to 100 years, provided that the battery doesn’t deteriorate sooner.