Search found 4 matches

by JohnDay
Tue May 16, 2006 10:56 am
Forum: General Discussion
Topic: People detecting
Replies: 7
Views: 2613

Backin the 70's I designed some ultrasonic motion detectors for the security industry. They are probably still the most reliable way of detecting people- particularly as you can (if you want) readily remove the effects of things like curtains blowing and pets running around. The 40kHz sensors used b...
by JohnDay
Tue May 16, 2006 10:51 am
Forum: General Discussion
Topic: ID this RFID
Replies: 10
Views: 2462

If they could only read a few inches then the store is in trouble! TH eopening is generally nearer three feet and they have to be able to virtually gaurantee reading anything that goes within about 3 feet of the door sensor. For 125kHz tags with a good sized antenna - 3 feet is common.
by JohnDay
Tue May 16, 2006 10:49 am
Forum: General Discussion
Topic: What kind of wire?
Replies: 7
Views: 1986

Another good source is old printer cables and serial cables. The conductor is often pretty lightweight, but it is generally pretty good for 'in the box' type wiring and you can pick them up at flea markets for next to nothing.
by JohnDay
Tue May 16, 2006 8:12 am
Forum: General Discussion
Topic: The dBu re-examined...
Replies: 12
Views: 2684

Well, the dBu, like the dBm, dBV and so on are all defined in ANSI T1.523-2001. The dBm is a power ratio referred to 1mW in 600 ohms, for audio, or in 50 ohms usually for RF, dBu and dBV are voltage ratios. The dBu is grandfathered in on the basis that 0.775V RMS is the standard reference voltage fo...