I did the same thing when I first tested mine. I mounted the regulator and a pair of SMA jacks right to the board it came with using the big board copper as the heat sink for the regulator. I didn't want to build a driver for the LED so I wired it directly (with resistor, of course). Dim and logic low was way up at 2.3 volts.
I solved the problem using a high efficiency LED (P/N C503B-RCS-CW0Z0AA1 from Digi-Key). It's running at a bit over half a mA using a 6.2k resistor to the 5 V supply. Now the logic low level is at about 700 mV (still a bit high) and the PPS output works. The LED is bright enough to see from way across the lab. The current generation of LED chemistries is really impressive. -John -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of bob grant Sent: Thursday, February 02, 2012 2:46 PM To: time-nuts@febo.com Subject: [time-nuts] FE5680 missing PPS soln Some info... Its tempting to attach an LED to the /LOCK signal on the DB9. However this signal is very weak and the LED does not seem very bright and PPS signal does not pulse...Hmm Internally the /LOCK pin is connected the 74AC240 buffer, but with an LED helping to keep the signal voltage high (2.3V) a logic low is never asserted. This logic low is needed to enable one half of the 74AC240 buffer (pin 1) that gates the PPS signal. Don't directly drive LEDs from the LOCK signal on the DB9 and, voila, the PPS signal reappears. Bob -- bob grant bobgr...@fastmail.fm -- http://www.fastmail.fm - Choose from over 50 domains or use your own _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
