I'm not sure why I first made one of these, but they've been a big favourite with my friends and work colleagues. Basically speaking the device is a tiny NiMh rechargeable battery pack with a series resistor to limit current through an LED. The connector on the battery pack is used both for charging the battery and for connecting to the LED.

The whole concept is that an LED can have it's legs pushed through a piece of clothing and into the battery packs connector. With one of the leads bent over to keep it attached to the battery pack, the LED will light on the front of the clothing like a tiny pixel of light. For favourite clothing, a male connector can be soldered to the LED to make it a permanent part of the garment.
The device has it's own micro charger based on a standard easily available plug in neon nightlight glow-plug. The one shown here was bought from ASDA in the UK for less than a pound. Because the charging current is only about one milliamp, the circuit was made super simple by deriving the charging current directly from the mains via a couple of limiting resistors and a rectifier.
The design of the charger is such that it is possible to get a little tingle from the charging port in much the same way as you can get a tingle from a neon test screwdriver. As such, the device is only really suitable for techno dudes who put simplicity and coolness above safety. Those who dissaprove of this charger can build an ugly one based on a plug in 9V mains adapter with a resistor in series.

These are the basic components required to build the battery pack and the charger. The battery pack is simply a 3.6V 12mAh NiMh rechargeable pack with a 470R resistor and a two pin socket. The charger consists of two 100K resistors from the Live and Neutral terminals of the plug and a bridge rectifier to convert the current limited supply to DC.
The DC is fed to the charging port and will either flow through the "charging" LED and the battery pack, or if a battery is not plugged in to charge the current will flow through the "standby" LED via a 12V zener diode that stops it lighting if a battery is connected.

As you can see the battery pack is pretty simple. Plenty of heatshrink is used to ensure that all exposed metal is covered to make the unit more finger friendly when using the compact charger. A blue LED is very attractive in this unit and will emit light continuously for over a day on a single battery charge due to it's very low current requirement.

The lens of the LED can be ground off with a file to make the LED look visually sharper and give the maximum viewing angle.

This is what the neon nightlight looks like before modification.

After modification it is somewhat busier inside, but will still glow orange due to the inclusion of a new neon lamp and resistor (well sleeved with heatshrink).

Here's the charger in use with the charging LED lit.

With no battery connected the units standby LED lights up.

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