Opto isolator

It happens that Dave Plowman (News) formulated :

Have you looked at the opto in an old rubber ball type mouse?

I often use a quad opto isolator ILQ74 (CPC SC07729), and I feed the output into darlington driver such as a ULN2804A.

This enables me to easily drive something like 50mA relay coils from a 1mA input signal to the opto isolator. If you try driving the relays directly from the opto isolator, you need to feed in about the same current you switch, which requires more power (a concern if battery powered) and the components run hotter.

In message , "Dave Plowman (News)" writes

Just look at the CTR figure for the opto in question. CTR = current transfer ratio, i.e. the current gain of the opto. Most are 25% to 400%, but split into narrower gain bands. Look for one from the higher end of the range, or as you say a darlington one would do. Default opto choice for me would be CNY17 series (mains isolation rated,

Thanks for that. I wasn't clear if you needed a minimum drive current for the internal LED to work - the data sheets just seem to give a max value.

The isolation needed is only between a 5 and 12 volt rail - so hardly demanding.