Help with artificial star project

[IanL]

Sorry i should have posted this in the DIY/Projects area. If any of the admin see this could you move this over thanks...Ian

Hi all
I am building the artificial star project that Barry Waters kindly posted but with plumbing pipe instead of board.
So after a trip to Bunnings I got some pipe and fittings and sprayed them with blackboard paint.
I then got the fibre to attach to this.
My problem is when I went to the local electronics store to get the LED and battery pack. I was told by staff that I would need to put a resistor or capacitor in between them.
Can someone that has some electronics expertise tell me if this is correct?
I would not know I thought that I could just attach the LED to the wires coming from the battery pack.
I have attached some photos of the pipe. I went for pipe as I could house the parts inside and easy access to them
Cheers
Ian

[Kal (Andrew)]

You will need a resister, and there are some web pages like this one which will help you calculate what size resister you will need

[MrB (Simon)]

Yes you will need a current limiting resistor to stop the LED from frying.

to calculate:
Resistor(ohms) = (Battery voltage - LED voltage) / LED current (in amps)

example: if using a 12v battery and a 2.5v 20mA led:
(12-2.5)/ 0.02A = 475 ohms.... the closest resistor value in the E12 series will be 470 ohms.

The E12 series(DSE have them) have multiples of the following:
1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2

eg, 0.18(0R18), 1.8(1R8), 18, 180, 1800(1k8), 18k, 180k etc etc (R= Ohms, k = kiloOhms)

At 12v supply and 20mA, the resistor will be pretty close to it's power limit (1/4 Watt),
Power is voltage (dropped thru resistor) x current used, eg (12-2.5) * 0.02A = 0.2Watts

Two resistors in parallel will share the load, so two 1k 1/4W resistors in parallel will handle 0.5watts(1/4W * 2)and will have a resistance of 500 Ohms(1k / 2)

Hope that helps, just ask for more info if it's as clear as mud

[miki63au]

It's may be not relevant for your project, but I had the idea for a artificial star for a long time. Too many projects, no time to make it...
Here how I would do it;
the size is important, so get a piece of fiberglass. Put it in a box folded
back from a collector to a black piece of sheet. The collector could be a lens or a mirror at 45 deg. The contraption can be very small.
The benefit of this is that it's passive. You just place it 20m away and put your laser pointer at the scope finder bracket. Aim at the collector and the
fiber glass will guide the light to a small point of light. You can live the passive box there, not need to go and switch on and off...
what you think?

[Roger Davis]

Depends upon the size of the fibreglass. You'd be better off with a piece of fibre optic of known numerical aperture. Holes in aluminium foil are cheaper to produce!

[Omaroo (Chris Malikoff)]

Originally Posted by Roger Davis

Depends upon the size of the fibreglass. You'd be better off with a piece of fibre optic of known numerical aperture. Holes in aluminium foil are cheaper to produce!

Absolutely!!! A single STRAND of fibre optic cable is what you want because it has a very round end. Our artificial star, with a single F/O strand looks like a planet through a telescope from 50 metres away at night. I've seen people try and use a bare LED glued through a hole in the side of a box, but I can't imagine how large a whole LED would appear... it'd be unusable IMHO. I've also tried the old pinprick in the aluminium foil trick and it was very successful.

[gbeal]

I too used a normal LED, and pin=pricked the alloy foil, experimentation is required.
No resistor for me, simple LED and battery (2xAA).
Gary

[citivolus (Ric)]

Another option is a cheap ~10mm eyepiece located 30cm or so from the LED. I used plastic irrigation pipe to build one like this quite successfully, with the LED at one end and the eyepiece at the other. A Meade 9.4mm Plossl that otherwise never sees use fits snugly on the end, giving an effective aperture size well under the resolving limit for my telescope at 50m. I threw a few baffles in the tube to clean up the internal reflections.

Regards,
Eric