Be watchful about the black epoxy. It may just be a dyed (otherwise clear) epoxy, and some of the black dyes in common use pass near IR quite readily. This would probably be most troublesome if the ambient light source were of the incandescent persuasion.
Also, small diodes in clear packages can also make photodetectors. I once, in desperation at a customer's site, successfully kluged a slightly sub-nanosecond detector out of a 1N914 (or one of those types like that) and a 9V battery. Dana On Sun, Aug 19, 2018 at 8:15 AM, Didier Juges <shali...@gmail.com> wrote: > That's a very good point. I have used LEDs as photocells many times, the > first time was in 1974 when it was easier for this university student to > get LEDs than photocells. > > I have also made a bi-directional optically isolated data link using a > single fiber optic cable and two fiber optic transmitters (LEDs) in the > HFBR-500 series. > > A couple coats of black paint, or a dab of black epoxy covering the LED > should work. Include the back side of the PWB for good measure, this is > time-nuts :) > > Didier KO4BB > > > On Sun, Aug 19, 2018, 3:38 AM Poul-Henning Kamp <p...@phk.freebsd.dk> > wrote: > > > -------- > > In message <1818735266.3837388.1534666949...@webmail.xtra.co.nz>, Bruce > > Griffit > > hs writes: > > > > >It exploits the fact that for a RED LED at least the difference > > >between the LED forward voltage and the transistor Vbe is ~ 1V and > > >has a fairly low tempco and has low noise (at least for RED LEDs). > > > > ... in darkness. > > > > When using LEDs this sort of way, they should always be totally > > shielded from all external light. > > > > -- > > Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 > > p...@freebsd.org | TCP/IP since RFC 956 > > FreeBSD committer | BSD since 4.3-tahoe > > Never attribute to malice what can adequately be explained by > incompetence. > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to http://lists.febo.com/mailman/ > listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
