| Author |
Message |
R C
Guest
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 Posted:
Fri Feb 11, 2005 1:24 am Post subject:
Design check for a telephone ringer |
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Hi. I just built a small telephone ringer for the local community theatre
group and before I turn it loose on unsuspecting actors, I thought I'd ask
to see if anyone sees any major safety or reliability issues.
Design is rather simple: 12V unregulated comes in through a diode and power
switch. This goes on to the high voltage rail. This goes to the linear
regulator, 7805. There is a small ceramic cap across the 12V line here (68
pf). There is also a large electrolytic on the high rail (1000 uF 25V) to
reduce the sags from the mosfets.
The 5V feeds an attiny26, running off internal RC oscillator. There are both
a small (33 pf) and large (470 uF) bypass caps across the 5V rail. The
tiny26 drives two N-Channel mosfets (IR510). They alternately pull the two
sides of a small transformer (center is tied to the 12V rail) to ground,
generating the ring voltage. The primary of the transformer is then tied to
a standard telephone jack.
The entire unit is housed in an ABS box; control is handled by the power
switch. A small LED on the high rail indicates both on and ringing (it
flickers when the mosfets pull current from the high rail.) Duty cycle is
low.. 2 seconds on, 4 seconds off, and not likely to ring more than a minute
or two a night. The code waits ~250 ms from turn on to start the ringing to
give the power supply time to settle.
Thinking about it, the only part I'm really worried about is the inductive
kick from the transformer eating the MOSFETs. I haven't worked with them
much.
Thanks,
R C
(There were several schematics on the web for ringers; I borrowed ideas from
several and replaced various timers with the cheap micro. Much more
flexible.)
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JeffM
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R C
Guest
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| Posted:
Fri Feb 11, 2005 6:11 am Post subject:
Re: Design check for a telephone ringer |
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"JeffM" <jeffm_@email.com> wrote in message
news:1108075675.801263.105810@c13g2000cwb.googlegroups.com...
| Quote: | [Description of a circuit in words]
R C
A picture is worth a thousand words.
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A bit under 200 in this case, but:
http://home.comcast.net/~grythumn/ringer.png
I omitted the ISP header because it's not important, and I'm lazy. If
something is in the schematic backwards, I made a mistake in Eagle.. the
circuit works fine.
I had it running for about an hour at 12V (Ringer muted, of course.)
Heatsinks were a little warmer than ambient, but not painfully so.
R C
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Guest
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| Posted:
Fri Feb 11, 2005 9:21 pm Post subject:
Re: Design check for a telephone ringer |
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R C <yeah.right@example.com> wrote:
| Quote: | http://home.comcast.net/~grythumn/ringer.png
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In that schematic, the +12 V for the LED and transformer doesn't go
through the power switch. Probably just a mistake in the schematic.
I agree that the inductive spike from the transformer might be a
problem. I have observed spikes to over 100 V when switching 12 V
to a standard automotive relay. If you have a scope you can tell in
a second.
It's probably not a good idea to keep sending the ring signal after
the phone has gone off-hook. This can be addressed by more hardware,
or by user training.
What happens if the output (to phone) is shorted? Most likely you'll
bog down your 12 V supply. You might want a fuse in the 12 V line
against this possibility.
What kind of voltage does it end up generating? Spec is 86 V AC.
Under this (down to maybe 40-50 V) isn't a problem but too much over
might be bad.
Matt Roberds |
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Dave VanHorn
Guest
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| Posted:
Sat Feb 12, 2005 12:24 am Post subject:
Re: Design check for a telephone ringer |
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What state are the mosfets in, when the part is in reset?
Add turnoff resistors on the gates.
Is your output high enough to turn those fets fully on?
Do you need a snubber across drain and source?
Did you really mean to leave the transformer and fets powered when the logic
is off?
Why use a tiny-26 where a tiny-11 will do?
You need also, to stop the ringing current within a very short time of when
the handset goes offhook, by sensing the loop current. I see no provision
for that. |
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Dave VanHorn
Guest
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| Posted:
Sat Feb 12, 2005 12:27 am Post subject:
Re: Design check for a telephone ringer |
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| Quote: | What happens if the output (to phone) is shorted? Most likely you'll
bog down your 12 V supply. You might want a fuse in the 12 V line
against this possibility.
|
The ring voltage is added in series with the 48VDC as well, so there are
considerations for the transformer at 100mA loop current, and what happens
on the low voltage side of the transformer when you pulse dial, or go on/off
hook.
Also interesting how the impedance of the transformer will affect the line,
since it's in series with it.
There's a good book for this, "Understanding Telephone Electronics".
It used to be a $5 book at Radio Shack, red cover.
Now it's a hardback and much pricer. |
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Rich Grise
Guest
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| Posted:
Sun Feb 13, 2005 1:15 pm Post subject:
Re: Design check for a telephone ringer |
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On Thu, 10 Feb 2005 14:24:15 -0500, R C wrote:
| Quote: | The 5V feeds an attiny26, running off internal RC oscillator. There are
both a small (33 pf) and large (470 uF) bypass caps across the 5V rail.
The tiny26 drives two N-Channel mosfets (IR510). They alternately pull
the two sides of a small transformer (center is tied to the 12V rail) to
ground, generating the ring voltage. The primary of the transformer is
then tied to a standard telephone jack.
|
If it's a real stage, then having props that need live electricity on
stage is a Very Bad Idea. Every stage production I've ever participated
in, when they need the sound of a ringing phone, they play a recording of
a ringing phone through the PA.
Your theater does have a sound system, doesn't it?
But please, don't put electric stuff on a stage with live actors.
If worse comes to worst, have a stage hand turn a crank on a dynamo with
the ringer in the wings - unless you have Dolby Surround, the audience
won't know where the ringer is.
Break a Leg!
Rich |
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R C
Guest
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| Posted:
Mon Feb 14, 2005 6:11 am Post subject:
Re: Design check for a telephone ringer |
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"Dave VanHorn" <dvanhorn@dvanhorn.org> wrote in message
news:VN2dnYeDk_NiZZHfRVn-pw@comcast.com...
| Quote: | What happens if the output (to phone) is shorted? Most likely you'll
bog down your 12 V supply. You might want a fuse in the 12 V line
against this possibility.
|
Current tops out at 2A max drawn, shorted or not. A fuse wouldn't hurt..
might grab an inline automotive type around 2.5 or 3 amps.
| Quote: | The ring voltage is added in series with the 48VDC as well, so there are
considerations for the transformer at 100mA loop current, and what happens
on the low voltage side of the transformer when you pulse dial, or go
on/off hook.
|
There is no 48 VDC bias on the line. The only items connected are the
telephone and the ringer unit. There is no provision for talking on this
circuit.. it is strictly to ring the bell.
R C |
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R C
Guest
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| Posted:
Mon Feb 14, 2005 6:11 am Post subject:
Re: Design check for a telephone ringer |
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<mroberds@worldnet.att.net> wrote in message
news:444Pd.486$zs.51@okepread04...
| Quote: | R C <yeah.right@example.com> wrote:
http://home.comcast.net/~grythumn/ringer.png
In that schematic, the +12 V for the LED and transformer doesn't go
through the power switch. Probably just a mistake in the schematic.
|
Right.
| Quote: | I agree that the inductive spike from the transformer might be a
problem. I have observed spikes to over 100 V when switching 12 V
to a standard automotive relay. If you have a scope you can tell in
a second.
|
25 V max open circuit with the heavy supply; ~20 V open circuit with
standard supply. Less with a phone connected (22/17V) or shorted.
Transistors are rated to 100V.
| Quote: | It's probably not a good idea to keep sending the ring signal after
the phone has gone off-hook. This can be addressed by more hardware,
or by user training.
|
The users do turn it off after it is picked up; the ringing voltage does not
appear to do anything to the phone except some minor clicking in the
handset. I wouldn't hook a digital phone or modem up to it.
The transformer is not very beefy.. I'm not sure of the actual specs, but
it's less than half an amp[3].
| Quote: | What happens if the output (to phone) is shorted? Most likely you'll
bog down your 12 V supply. You might want a fuse in the 12 V line
against this possibility.
|
It draws a little more current, but not much. Max draw while ringing is 2A
(roughly 50% of the time.. the other 50% is at 1 amp.) Voltage sags pretty
far down during ringing.. all the way down to 7V with the weaker supply. If
I can dig up one or two more big caps it may help smooth it a bit more.
| Quote: | What kind of voltage does it end up generating? Spec is 86 V AC.
Under this (down to maybe 40-50 V) isn't a problem but too much over
might be bad.
|
Open circuit voltage is ~400 V PtP 12V beefy. 12V standard is 325 V. With a
0.6 REN load it's about 350/200V PtP. Note that the 70-90 V nominal AC is
the RMS value. In the field, you also see actual voltages of about 50-150 V
RMS. Rule of thumb conversion[1].. 0.707*(0.5x)=123/70.7 RMS. So if I go
with the standard pack I'm fine; also the actual phone is 0.9 REN. If I wish
to ring more than 1 phone, I'd probably need the beefy pack.
R C
([1]Yes, I know that's for sine waves.. but these aren't perfect square
waves either. I suspect the transformer is attenuating the higher frequency
components.)
([2] This is a random transformer I pulled from a broken cassette player; it
might be better off being replaced by a known 12-0-12/120 transformer.)
([3] DC resistance of each leg of the secondary is about 1.8 ohms.) |
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R C
Guest
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| Posted:
Mon Feb 14, 2005 6:11 am Post subject:
Re: Design check for a telephone ringer |
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"Dave VanHorn" <dvanhorn@dvanhorn.org> wrote in message
news:SIudnc1Nrd7PZZHfRVn-gA@comcast.com...
| Quote: |
What state are the mosfets in, when the part is in reset?
|
Whatever state they were in before the gates were tristated. The only way to
put the part into reset is turnon (reset for ~60ns, IIRC), or via the ISP
header.
| Quote: | Add turnoff resistors on the gates.
|
Good idea.. I'll add some weak pulldowns.
| Quote: | Is your output high enough to turn those fets fully on?
|
5V.. threshold on the IR510's is 2-4V. Fully on.. possibly not.
| Quote: | Do you need a snubber across drain and source?
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That's what I was wondering.
| Quote: | Did you really mean to leave the transformer and fets powered when the
logic is off?
|
Both FETs are off during IDLE cycle. The center tap is 12V, but the +/- 12
legs are not conducting.
| Quote: | Why use a tiny-26 where a tiny-11 will do?
|
It was the only small form factor non-mega AVR I hand on the bench..
couldn't well use a '8515 or '8535.
| Quote: | You need also, to stop the ringing current within a very short time of
when the handset goes offhook, by sensing the loop current. I see no
provision for that.
|
Even with an 18V 2A power supply, as soon as the phone is picked up all you
hear is a minor clicking in the handset. As I power it via a 12V 400 mA
supply now, it's not an issue. There is no provision to provide talk
voltage.
R C |
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R C
Guest
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| Posted:
Mon Feb 14, 2005 6:11 am Post subject:
Re: Design check for a telephone ringer |
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"Rich Grise" <richgrise@example.net> wrote in message
news:pan.2005.02.13.07.15.21.100640@example.net...
| Quote: | On Thu, 10 Feb 2005 14:24:15 -0500, R C wrote:
The 5V feeds an attiny26, running off internal RC oscillator. There are
both a small (33 pf) and large (470 uF) bypass caps across the 5V rail.
The tiny26 drives two N-Channel mosfets (IR510). They alternately pull
the two sides of a small transformer (center is tied to the 12V rail) to
ground, generating the ring voltage. The primary of the transformer is
then tied to a standard telephone jack.
If it's a real stage, then having props that need live electricity on
stage is a Very Bad Idea. Every stage production I've ever participated
in, when they need the sound of a ringing phone, they play a recording of
a ringing phone through the PA.
|
The director wants the phone to ring. In the past we have run it through the
PA.
It is a real stage, but a fairly small stage. It's obvious the ringing is
coming from the speakers, not the phone.
| Quote: | But please, don't put electric stuff on a stage with live actors.
If worse comes to worst, have a stage hand turn a crank on a dynamo with
the ringer in the wings - unless you have Dolby Surround, the audience
won't know where the ringer is.
|
They already have 2 lava lamps on set.
This won't put out any more current than a telephone dynamo, and is a heck
of a lot safer than the other idea they had (hook it to wall current).
R C |
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Guest
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| Posted:
Tue Feb 15, 2005 6:11 am Post subject:
Re: Design check for a telephone ringer |
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R C <yeah.right@example.com> wrote:
| Quote: | mroberds@worldnet.att.net> wrote in message
news:444Pd.486$zs.51@okepread04...
inductive spike
25 V max open circuit with the heavy supply; ~20 V open circuit with
standard supply. Less with a phone connected (22/17V) or shorted.
|
How'd you measure this? Scope or meter? If scope, OK, but if meter,
there might be higher voltages that the meter isn't catching...
| Quote: | The transformer is not very beefy.. I'm not sure of the actual specs, but
it's less than half an amp[3].
|
http://repairfaq.ece.drexel.edu/sam/aapsfaq.htm#adpsdpv has info on how
to work it out, if you'd like.
| Quote: | What happens if the output (to phone) is shorted?
It draws a little more current, but not much. Max draw while ringing is 2A
(roughly 50% of the time.. the other 50% is at 1 amp.)
|
1 A sounds like a lot for just a microprocessor sitting there, but then
again I'm not that familiar with the Tiny26.
| Quote: | Voltage sags pretty far down during ringing.. all the way down to 7V with
the weaker supply. If I can dig up one or two more big caps it may help
smooth it a bit more.
|
Look up the October 1989 "Hardware Hacker" column by Don Lancaster
(available in PDF at his Web site, tinaja.com , the last time I looked)
for a neat way to work out power supply filter cap sizes.
| Quote: | What kind of voltage does it end up generating? Spec is 86 V AC.
Under this (down to maybe 40-50 V) isn't a problem but too much over
might be bad.
Open circuit voltage is ~400 V PtP 12V beefy. 12V standard is 325 V. With
a 0.6 REN load it's about 350/200V PtP. Note that the 70-90 V nominal AC
is the RMS value.
|
Hmm... 86 V RMS is about 86 x 1.414 or 121 V PtP for a sine wave or
86 V PtP for a square wave with 50% duty cycle. Going by your loaded
value, you're delivering somewhere between 247 and 350 V RMS - this seems
high to me.
| Quote: | In the field, you also see actual voltages of about 50-150 V RMS.
|
I haven't stuck a meter on that many phone lines, but the ones I have
have been 90 V RMS or lower. But these were all CO-served lines; PBX
extensions might be higher.
| Quote: | ([2] This is a random transformer I pulled from a broken cassette player;
it might be better off being replaced by a known 12-0-12/120 transformer.)
|
Agreed, or at least test it per the link above to know what it can do.
Matt Roberds |
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R C
Guest
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| Posted:
Wed Feb 16, 2005 6:11 am Post subject:
Re: Design check for a telephone ringer |
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<mroberds@worldnet.att.net> wrote in message
news:8jgQd.6726$zs.5926@okepread04...
| Quote: | R C <yeah.right@example.com> wrote:
mroberds@worldnet.att.net> wrote in message
news:444Pd.486$zs.51@okepread04...
inductive spike
25 V max open circuit with the heavy supply; ~20 V open circuit with
standard supply. Less with a phone connected (22/17V) or shorted.
How'd you measure this? Scope or meter? If scope, OK, but if meter,
there might be higher voltages that the meter isn't catching...
|
Scope.
Describes how to measure the current ratings.. but not the ratio. However,
there are other sites that do.. I'll look into it, but I don't have a variac
handy.
| Quote: | What happens if the output (to phone) is shorted?
It draws a little more current, but not much. Max draw while ringing is 2A
(roughly 50% of the time.. the other 50% is at 1 amp.)
1 A sounds like a lot for just a microprocessor sitting there, but then
again I'm not that familiar with the Tiny26.
|
2/1 A is during ring cycle only. Total current during idle is roughly 20 mA
including the LED.. and I'm not using the sleep capabilities of the tiny26
at all. I suppose I could have rephrased that a bit.
The average draw while ringing is about 1.5 A, with a plateau at 2 and
valley at 1. The ring cycle is 2 seconds on, 4 seconds off, which gives
overall average of 500 mA.. which means I need to current limit the mosfets.
| Quote: | Voltage sags pretty far down during ringing.. all the way down to 7V with
the weaker supply. If I can dig up one or two more big caps it may help
smooth it a bit more.
Look up the October 1989 "Hardware Hacker" column by Don Lancaster
(available in PDF at his Web site, tinaja.com , the last time I looked)
for a neat way to work out power supply filter cap sizes.
|
Thanks, I'll look into it. To be honest, I may be better off
current-limiting the mosfets. The supplies are different designs.. the
'Beefy' one is 12V, regulated, and rated for 1A continuous. The standard one
is 400 mA, unregulated.
Hmm.. doesn't look like an exact match as I'm feeding off an already fixed
wall supply, but I'll read the whole thing. But all very interesting.. I'll
have to read some more tonight.
| Quote: | Open circuit voltage is ~400 V PtP 12V beefy. 12V standard is 325 V. With
a 0.6 REN load it's about 350/200V PtP. Note that the 70-90 V nominal AC
is the RMS value.
Hmm... 86 V RMS is about 86 x 1.414 or 121 V PtP for a sine wave or
86 V PtP for a square wave with 50% duty cycle. Going by your loaded
value, you're delivering somewhere between 247 and 350 V RMS - this seems
high to me.
|
It is a bit high.. esp. as I'm not testing it with the stage phone, which is
a lot higher draw (mechanical, not piezo). The 200V PtP value is the
important value, as it's the one with the weaker supply that I'm actually
using with the circuit.
Also, this is not a true square wave output.. it looks very much like a
square wave and a low-pass.
| Quote: | In the field, you also see actual voltages of about 50-150 V RMS.
I haven't stuck a meter on that many phone lines, but the ones I have
have been 90 V RMS or lower. But these were all CO-served lines; PBX
extensions might be higher.
|
That's from my notes gathered from who knows where. It may be a bit dated.
| Quote: | ([2] This is a random transformer I pulled from a broken cassette player;
it might be better off being replaced by a known 12-0-12/120 transformer.)
Agreed, or at least test it per the link above to know what it can do.
|
It may be a 6-0-6 or 9-0-9. I may try running the circuit at 9V if I
current-limit the mosfets to preserve the headroom on the 5V regulator.
Hmm.. probably a single beefy resistor on the center tap, or two smaller
ones between the mosfets and the transformer. Have to check the parts bin.
R C |
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R C
Guest
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| Posted:
Fri Feb 18, 2005 12:33 am Post subject:
Re: Design check for a telephone ringer |
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"R C" <yeah.right@example.com> wrote in message
news:2ZCdnS1o5pY6KI_fRVn-uw@comcast.com...
| Quote: |
mroberds@worldnet.att.net> wrote in message
news:8jgQd.6726$zs.5926@okepread04...
R C <yeah.right@example.com> wrote:
mroberds@worldnet.att.net> wrote in message
news:444Pd.486$zs.51@okepread04...
inductive spike
25 V max open circuit with the heavy supply; ~20 V open circuit with
standard supply. Less with a phone connected (22/17V) or shorted.
How'd you measure this? Scope or meter? If scope, OK, but if meter,
there might be higher voltages that the meter isn't catching...
Scope.
|
Quick update: Stage tested it; works fine when the actors _hang up the
phone_ properly when they're done... guess I have to add that off-hook
detect after all, and perhaps ring a separate phone after all..
R C |
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Guest
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| Posted:
Mon Feb 21, 2005 1:30 am Post subject:
Re: Design check for a telephone ringer |
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R C <yeah.right@example.com> wrote:
| Quote: | mroberds@worldnet.att.net> wrote in message
news:8jgQd.6726$zs.5926@okepread04...
http://repairfaq.ece.drexel.edu/sam/aapsfaq.htm#adpsdpv has info on how
to work it out, if you'd like.
Describes how to measure the current ratings.. but not the ratio. However,
there are other sites that do.. I'll look into it, but I don't have a
variac handy.
|
You shouldn't _need_ a Variac, just a way to load the output. For the
ratio, put a known AC voltage (from another transformer) into one
winding and measure what you get at the other windings.
| Quote: | 2/1 A is during ring cycle only. Total current during idle is roughly 20 mA
including the LED.. and I'm not using the sleep capabilities of the tiny26
at all.
|
OK, that sounds a lot more reasonable.
For the off-hook detect, a phone with a REN of 1.0 is supposed to have a
DC resistance of 50 megohms on hook. Off-hook, it drops to 200 ohms or
less. My initial thought would be something like a relay with SPDT
contacts on the "phone" side of the transfomer. One side of the phone
always goes to the transformer, and the other side goes to the contact
common. When you want to ring the phone, drive the relay such that the
phone is connected to the transformer. When you're not actively making
AC, drive the relay so that the phone is connected to a pullup to +5 V
(you'll have to ground the other side of the high-voltage winding to
DC ground) and run that into a port pin. When this value goes low, the
phone is off-hook.
I am almost 100% certain that the phone company doesn't do it this way.
I seem to remember reading that they have a small coil with two
windings. One determines the frequency of an oscillator, and the other
is connected to the DC loop to the phone. When the phone goes off-hook,
a few tens of milliamps of current flows through one winding, which
alters the characteristics of the coil enough that the oscillator
frequency changes notably.
If the actors are really bad about hanging up the phone, then after a
time-out, send that nasty loud staccato tone that the phone company
uses when you've left a phone off-hook.
Matt Roberds |
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