Post by thread:Light sensors, 555

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.

> 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. I do this in one of my projects. Works okay, but the system seems to be annoyingly sensitive to how long I discharge the cap before letting it rise (if less than about 1ms). I'm using a 0.01uf cap, a photo-darlington, and a 1K series (protection) resistor in my application.