POTS landline neon ringing indicator - Page 1
Forum section: electronics / general

author: eti
author_rank: Super Contributor
timestamp: December 23, 2022, 03:14:56 am
content: As we know, the AC ringing current supplied on an average POTS line (UK/US) is around 70-90 VAC @ around 17-20Hz. Based on this, I recall that older phones had neon indicator lamps connected across the copper pair (48V tip and ring) - I just recreated this and the single neon lamp FLASHED LIKE CRAZY!Strangely, it didn't strobe with a "Brrr-brrr.... <pause>.... Brrr-brrr....<pause>" cadence, as per BT landlines, but basically strobed as long as there was any ringing current on the line - I don't currently own a scope capable of handling this voltage. I have 10 of these lamps and I want to parallel them all up together across the line, and knowing they draw a gnat's fart of current, would this affect the "REN" (Ringer Equivalence Number) of a BT landline, do you reckon? In fact, if anything, since they are a type of gas discharge tube, surely they would also aid in surge suppression.In the 90s I'd not have the net and would wing it and work it out myself, but I'd like to know if anyone has some insight. I know, it's a VERY trivial circuit, but I do not want to interrupt our broadband or landline calls.PS: Not fussed about BT's ridiculous "BABT" rules - this is a theoretical circuit for now.Cheers me dears, and if we don't speak again, a very blessed and peaceful Christmas to you all!

author: octillion
author_rank: Contributor
timestamp: December 23, 2022, 04:09:12 am
content: You need to add a series limiting resistor, as the resistance of the neon lamp is very low when ionized (you don't want it acting as a transient suppressor, just a ring indicator). An old phone I have that uses neon ring indicators has 220k resistors on each lamp. This will present negligible loading on the line, so no effect on REN.

author: eti
author_rank: Super Contributor
timestamp: December 23, 2022, 05:32:01 am
content: Quote from: octillion on December 23, 2022, 04:09:12 amYes I had a feeling that might be the case. Thank you ever so much for the help. Ciao.

author: Gregg
author_rank: Super Contributor
timestamp: December 25, 2022, 03:20:15 am
content: In the not so good old days of relay switched telephone mega-monopolies the local ring generator could be switched off by a quick short across the lines leaving the line connected if the receiving end was capacitive coupled it would appear like it wasn’t answered. After the ring generator disconnected, the line voltage dropped back to 50 to 54 volts DC below the threshold of the neon lamp; however the higher voltage of the ring signal would jump the gap in the neon lamp which usually had the desired results. That was the basis of the infamous black box that started Jobs and Wozniak on their way to fame and fortune.

author: WA1ICI
author_rank: Contributor
timestamp: December 25, 2022, 04:49:44 am
content: Here is the Western Electric version (used in the AT&T system):This contains a Western Electric 358A neon bulb.

author: WA1ICI
author_rank: Contributor
timestamp: December 25, 2022, 04:52:16 am
content: Here is the interior and a description from Bernie Magers' "75 Years of Western Electric Tube Manufacturing" (1994). Note the 4.3K series resistor.