Electronic – CE certification: battery-powered LED lights

For Problem 1:
A string of LEDs hooked up in series with a current regulator like SuperTex CL220 would do the trick: The component is a simple a 2-terminal device (like a diode) and needs no additional components or configuration. It allows 20 mA (+/-10%) current to pass through as long as there is sufficient voltage headroom: 5 volts above the total forward voltage of the LEDs is sufficient. This current regulation is stable up to 160 volts, enough for your purposes.

Note that LEDs are current-dependent rather than voltage dependent devices. They glow at essentially the same brightness as long as the current is constant.

For your application, if the LEDs need to start glowing from around 60 Volts, half the 120 Volts your students generate, then a string of 25-30 standard 5-mm red LEDs would be optimal. They would glow without intensity change till your maximum voltage.

Too many LEDs = they won't glow till a higher voltage.
Too few LEDs = the CL220 device would overheat in dissipating the surplus voltage.

For Problem 2: The extent of energy storage required to keep a string of around 25 LEDs (from above section) glowing for even half a minute, is pretty high. Capacitors would not be the way to go, unless you have access to big power-line capacitors through surplus channels.

• Your capacitor bank would need to provide 20 mA at a minimum of 60 volts (again from above section), for "a few" seconds.
• Capacitor needs to be rated for a voltage higher than the highest the generator could conceivably generate.
• Though "supercapacitor" is a popular term these days, typical supercaps are rated for 5.5 Volts or 12 Volts, not hundreds of Volts.
• Adding in buck/boost generator trickery to make this work would result in complexity far beyond using a battery and off-the-shelf charger.

I hope this helped.

I can only speak about CE marking. It's no big-deal on this simple product to CE mark (and if it is too small then the packaging needs to be CE marked). It's electrical and my understanding is that it will need CE marking.

Don't despair, for something like this it isn't a minefield of legislation. You need to look-up the list of directives. Here's one I prepared earlier: -

Then you decide which ones may apply - ignore the rest but be concientious and not cynical about this. Liklihood is that several may apply and you prepare a document that justifies why they don't apply OR, if some do you justify why only certain sections of a directive apply. This is called the self certification route and there's a realm of good info here.

This covers you but like i say, don't be cynical about stuff - low voltage directive applies to anything where the voltage is above a certain level and although you are powered from a cell-phone, give a little more than lip-service to why you won't produce voltages that mean you have to comply.

EMC directive might be applicable and maybe you have to throw in some sentences that justifies you won't be interfering with the radio signal in any way - difficult to prove but best have answers documented.

You'll have several days (14 I think) if someone challenges your "right" to CE mark your product so it's best to have the info on-hand in the form of a "technical file".