-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 >Practical considerations for type (3) devices: >It is useful to feed the IR LED with a constant current source so power supply >voltage fluctuations do not affect operation. A variable constant current source >is useful so you can adjust for skin thickness, etc. I did something like this for an IR densitometer. I used an op-amp with NPN follower to drive the LED, and took the feedback from the photocurrent into the op-amp's inverting input. The non-inverting input was tied to a reference voltage, and I took my output from a current sense resistor in series with the follower and LED. The advantage of this is that it drives the led as hard as needed to get everything in linear range, and you can AC couple for changes, or DC couple for absolute transmissivity. It worked over a huge range, since I set the max current to the IRLED at 100mA. In air, I measured only a few uA, since air is mostly transparent to IR. I also discovered that some white ABS plastic we were buying was essentially frosted glass to IR, which was a BAD thing. What I specified was opaque, but what we got from the vendor was something rather different. - -- Are you an ISP? Tired of spam? www.spamwhack.com A pre-emptive strike against spam! Where's Dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 6.5.2 for non-commercial use iQA/AwUBOR8WDoFlGDz1l6VWEQJ32ACgvXqVqEGbUa+G+iBGQVrjgvXtQb8Anj5g 7RhxDvvEZYQBGtsVXcwLxRcQ =mHvr -----END PGP SIGNATURE-----