Of course you can use a 555 astable multivib. and a LDR to produce a light to
frequency convertor. The beauty of the TI parts (TL220, TL230) is that you
get a *huge* dynamic range, well specified fast response, and the price of
the part is less than when you construct your own L-F convertor from
discretes. Having said that, for a one-off nothing beats the price of bits
from the junkbox.

Would it not be possible to use a LDR and cap directly on an I/O pin. This
app was described in one of the handbooks. I think it went like this: You tie
the cap to ground, the ldr to Vcc, and the junction of cap and LDR to an
input pin. Make the output low to discharge the capacitor, then make the pin
an input and measure how long it takes for the input to go 'high'.
Since for room lighting levels the sampling does not have to be fast (in fact
probably *has* to be slow) this might be an very cheap option.

TI used to give free samples of the '220 and '230, including data sheet and
application information.

Wouter 'hurrah the full PIC development platform arrived' Suverkropp.