l bought an optoisolator from Maplins last week but l don't fully understand the spec l'd like to know what sort of power there is available please. l plan to drive 3 or 4 standard 5mm green leds Many thanks
Max current isn't enough to drive a 3 or 4 LEDs - you'll need a second transistor to drive your LEDs.
SteveyG's right, in fact it will only just run 1 led without a boost transistor, full datasheet here (pick the one to match your brand) will give you the chip pinout and a graph of diode current against transistor current. Important value is CTR (current transfer ratio), 15% means 100mA through the diode gives 15mA through the transistor, not a lot. A suitable booster circuit here, lots of small NPN transistors will do (2N3904, BC337, etc.)
Thanks very much for the info, l'll see what transistors l might have in my 'box of bits'. BTW The chip in question turns out to be a Seimens ILQ74
