Ringer Equivalence is a measure of the power.

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The cable company that provides my home internet service assured me that the package that came with the new modem would be cheaper than what I'd been paying with my old modem.That was true for internet service.The internet connection worked fine even though I had to switch my laptop's wireless connection to dhclient.I found a problem quickly.

Ronald Reagan and Mikhail Gorbachev used to talk to each other on the Hotline, which is why I use a traditional touch-tone landline phone.The standard set is from the Cortelco 2500-series.One thing I like about this phone is that it has a classic ringer with real analog bells, instead of the silly little digital feeper you get in more recent telephone instruments.It sounds great!In the past I've plugged this phone into my phone service and it's always worked.The electrical interface for telephone instruments hasn't changed in decades, and not a lot that can go wrong with this level of technology.On an incoming call, this is what I got when I plugged into the new cable modem.

The bell was barely audible with the new cable modem driving the phone.The phone worked well.

The "Ringer Equivalence Number" is used to rate telephone ringers.One REN unit is supposed to be the equivalent of one traditional phone instrument's ringer, which is one of several definitions.The standard phone service should be able to ring 5 REN.

My first thought was that my old phone was presenting too much of a load to the cable modem, and the modem wasn't sending enough power down the line to ring the gongs.The label on the phone said its rating was "0.5A", but the cable modem's documentation claimed it was good for the full load of standard phone service.More on that "A" in a moment.The modem should be able to drive ten phones.

I did some more testing.I have an electronic telephone instrument that I don't like to use because it's kind of how-you-doin', but I tried plugging it in and calling my home line.The phone rang.The sticker had a rating of REN 0.8B.In that case, the cable modem should be able to drive a phone with a rating of only 0.5.Right?

I did more research after I started to wonder what those "A" and "B" words meant.Tomi's discussion of the issues was the best I could find.The ringing signal on a phone line is supposed to be 90V AC and peak to peak, such as we often use in the synthesizer business.In practice, the ringing voltage can drop a bit in wiring, and phones are expected to ring at lower voltages, typically as low as 60VAC.

The frequencies may be an issue.The phones with "A-type" ringers that are indicated by an " A" in the REN number, like my 2500, expect the frequencies to be very close.It takes a certain amount of time for the clapper to bounce off the gong and be ready for another cycle, so it can't just be sped up by feeding it a higher frequency of electricity and still work.The phones with "B-type" ringers, which are more common in this era of electronic feepers, can accept anything from 15hertz to 68hertz because they just recognize the signal and then power an electronic sound generator, and frequencies higher than 20hertz are often observed on the phone

The new cable modem must be sending enough power, but on a higher Frequency than 20hertz.That would ring the newer phone, not the older one.I tried plugging both phones into the cable modem and getting a splitter.There was a total of 1.3 REN of load.The old phone rang weakly on the incoming call, while the new one rang strongly.The new cable modem couldn't ring the old phone because it was putting out too much power, but the wrong frequencies, so it seemed confirmed.

What should we do about it?At this point, it was probably the best course of action to give up on the old phone.If I didn't want to use the new phone, I could just leave the old one plugged in.I briefly looked into getting a commercial ring-boosting device, but they cost more than I wanted to pay, and I wasn't sure if they would help.The most popular device in the market seems to be the Viking Electronics RG-10A, which with shipping and so on would end up setting me back about Ca$200 - and one of its advertised features is that it does not change the frequency of the ringing signal!Compatibility with some specialized switchboard systems that use different frequencies of ring signal to signal different kinds of calls is supposed to be a positive feature.I would be paying $200 for a device that would leave me no better off if that was true.It's probably time to give up and join the 21st century.

It seems like I shouldn't have to give up when I run an electronics manufacturing business.How hard can it be to detect an incoming ringing signal, shoot 20Hz AC down the phone line, and mumble whatever else may be needed to interface to a phone?I decided to think about building my own substitute for the Viking box, but one that would force the Frequency to 20Hz so it would work with my phone.

To get a feel for what the input to my circuit would look like, I hooked the phone line up to the scope and captured the ringing signal.It was a bit of a surprise.

The wave shape is peculiar, but phones don't normally care about it, and it is within the scope's precision.I was wrong in my diagnosis of the problem.Why wasn't my phone ringing?I wanted to check the voltage.

It's 88V, where 60V is supposed to be good enough.The signal should work, but it doesn't.What's going on?What is wrong?

Look again.Is it really 88V?The 88V peak to peak signal is not the right way to measure it.There is a ratio of 2.818 between the two ring signals.The signal from the cable modem is about one third of what it should be, because the ratio only applies to sine waves.The phone couldn't ring.

I took a careful look inside the phone after setting aside the circuit sketches I'd been drawing.I found a service manual with a wiring diagram for the exact model of phone that I was working on, but by that point I had already reverse-engineering nearly everything I needed to know about it.

The phone's main connection board has a 0.47F capacitor built in that connects the single coil across the line.Unlike other 2500-style phones, it doesn't have a mechanical adjustment for "bias" that might allow it to ring at a lower voltage without any electronic interference.The clapper that strikes the gongs is attracted to a permanent magnet because there is no spring inside the ringer.The positions of the gongs and the magnet are not changeable.

If I broke into the phone line between the modem and phone, I would have to do a lot of logic to figure out the various states of the system.If I were to blow up the cable modem's output stage with my 90V supply, it would be a fun thing to explain to the technicians.When the phone went off-hook, it would be necessary to cut off the ringing signal, deal with any spike that might be created by the coil, and then patch it through to the other side.I wouldn't get out of it for much less than the price of a commercial booster device, even though none of this stuff is really difficult.

I broke in between the phone's connection board and the ringer.I wouldn't have to deal with the hook switch.I could amplify the AC voltage that would be sent to the ringer by a factor of 3 because I already had the right frequencies coming from the cable modem.In the on-hook state, there would be nothing but DC on the line, which my amplifier wouldn't see, and the other party would not be able to talk.Because they have a mass-production discount but are solving a harder problem, digging inside the phone turned out to be the key to keeping the cost below that of the commercial product.

I designed and built a simple amplifier that could boost a 30V 20Hz signal to 90V.The schematic is what I ended up with.I feed the output of the Class B amplifier through a transformer that might be used to produce 5VAC power from European mains, because it's pretty straightforward to drive a push-pull amplifier which reduces the input voltage significantly but with large current gain.The output is the amount of power needed to drive the coil.The "ground" symbol on the schematic only represents an 0V reference and I'm not connecting anything to ground myself.

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