Well....the client was really hooked on hall effect sensing. So I hooked up a demo circuit, and got some thin strip magnets (have to be thin) and found that based on where the circuit board is mounted, the magnets were too far away and didnt have enough flux density to them, unless I used a thicker magnet. That of course, didn't play into the overall design, as the magnet would end up being too large so.... As Mr Rolf pointed out...there are other choices.... The biggest issue with using ambient light to a PV type sensor is that it would be easy to be covered or not have enough light and give a false reading. But, what about doing something with reflective IR? in that....if there is a hole above an IR LED, nothing would reflect. But I would need something that would emit and detect...are there such devices that are also inexpensive? I like the idea of the coils, did a search for chess board piece sensing, came up empty as far as circuits. I'd like to look into this....any pointers to where some information and design data might be? Robert Rolf wrote: Your sensor choices seem to be magnetic, optical, mechanical. If the number of pages is small, then 11 optical sensors along one of edges to detect page tabs seems to be a clean solution. Use LED carrier modulation and the 11th sensor to remove ambient light concerns. I don't think you can do this for $3 but I'm not into mass manufacturing so it may be doable. You could also look at coils that are HF excited, and have metallic disks on the pages that would change the coil Q or L. This would make the lumps in the pages much less noticeable. You could binary code the sensors because the more iron you throw above a coil, the more the L/Q shifts. And using different metals would give you different detuning as well. An analog mux and oscillator are all you'd need to read your coil array. Look for 'chess board' 'piece' 'sensing' for existing ccts to do this. --------------------------------- Yahoo! FareChase - Search multiple travel sites in one click. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist