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Design check for a telephone ringer

Discussion in 'Electronic Design' started by R C, Feb 10, 2005.

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  1. R C

    R C Guest

    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

    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
  2. JeffM

    JeffM Guest

  3. R C

    R C Guest

    A bit under 200 in this case, but:

    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
  4. Guest

    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
  5. Dave VanHorn

    Dave VanHorn Guest

    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.
  6. Dave VanHorn

    Dave VanHorn Guest

    What happens if the output (to phone) is shorted? Most likely you'll
    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

    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.
  7. Rich Grise

    Rich Grise Guest

    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!
  8. R C

    R C Guest

    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
    Good idea.. I'll add some weak pulldowns.
    5V.. threshold on the IR510's is 2-4V. Fully on.. possibly not.
    That's what I was wondering.
    Both FETs are off during IDLE cycle. The center tap is 12V, but the +/- 12
    legs are not conducting.
    It was the only small form factor non-mega AVR I hand on the bench..
    couldn't well use a '8515 or '8535.
    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

    R C
  9. R C

    R C Guest

    The director wants the phone to ring. In the past we have run it through the
    It is a real stage, but a fairly small stage. It's obvious the ringing is
    coming from the speakers, not the phone.
    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
  10. R C

    R C Guest

    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.
    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].
    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.
    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
    ([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.)
  11. R C

    R C Guest

    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.
    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
  12. Guest

    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... has info on how
    to work it out, if you'd like.
    1 A sounds like a lot for just a microprocessor sitting there, but then
    again I'm not that familiar with the Tiny26.
    Look up the October 1989 "Hardware Hacker" column by Don Lancaster
    (available in PDF at his Web site, , the last time I looked)
    for a neat way to work out power supply filter cap sizes.
    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.
    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.
    Agreed, or at least test it per the link above to know what it can do.

    Matt Roberds
  13. R C

    R C Guest

    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
    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.
    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.
    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.
    That's from my notes gathered from who knows where. It may be a bit dated.
    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
  14. R C

    R C Guest

    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
  15. Guest

    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.
    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
  16. Rich Grise

    Rich Grise Guest

    Here's a crazy idea. Does the theater have a phone of its own? Plug
    in a "Y", and run an extension to the stage, and put an actual phone
    there. Arrange for somebody at another line to hold the line open -
    i.e., call somebody on the theater phone sometime before the play starts,
    and just sit there. This will ensure that if anybody calls during the
    play, they'll get a busy signal. Then, at the right moment, the two people
    hang up, and the guy on the remote line immediately calls the theater
    again, and the phone on the stage rings (along with the extension, in the
    office or wherever. This time, the guy in the office doesn't answer!) They
    have their conversation, and if they don't hang up, the phone company will
    provide the noise for you. :)

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