Adding a TOSLINK compatible output to the SPEED-LINK UltraPortable Audio Card

Some months ago I bought the SPEED-LINK UltraPortable Audio Card from Amazon. It worked quite fine and it was easy to open. I wondered what kind of chip they use, so I scratched the "OK" Sticker away. It was a CM108. Of course I checked the datasheet and I found this nice SPDIFO pin. The drivers delivered with the stick supported already the SPDIF... Coaxial SPDIF is nice, but having it optical is much better... :-) So what we need? A TOSLINK module? No - way too expensive and too big for the stick. Why not take a 5mm Ultrabright LED and drill a hole? Some filing was needed too, otherwise the LED would not fit between the MIC and the headphone plugs... Now this stick is operating at the stereo system in the living room of a good friend. He tested it for me and said that it started already working 1.5cm away from the toslink plug of his amp :-)

Pictures:

Schematic:

Description:
The 1nF cap parallel to the 1k resistor is used to limit the effects of the parasitic capacitances in and around the transistor.
The schottky diode is used to shorten the switching off time of the transistor (schottky-desaturation).
The +5V and GND are just taken directly from the USB plug, because there is enough space to solder :-)

Parts needed:

1x	SPEED-LINK UltraPortable Audio Card	Get it from Amazon.
1x	5mm LED Red Ultrabright
1x	Transistor NPN (eg. BC547)
1x	Resistor 100Ohm
1x	SMD-Resistor 1kOhm
1x	SMD-Capacitor 1nF
1x	Shottky Diode (eg. BAT48)
	Some shrinking tube and a bit of hot glue

Tools needed:

	Soldering iron and soldering skills	The CM108 is quite small
	Some thin enamelled wire	To connect the SPDIFO pin of the CM108
	A flat file	To mod the LED and the case
	A drill	To drill a hole in the LED

Christian Auer
website@auer-elektrotechnik.com
Last update: 17.11.2007