Phone Ring Detector

When a phone line rings, there's 90 V RMS AC at 20 HZ on the line. It's enough to give you a jolt you won't soon forget. Thus, it's hard to miss! My favorite detection scheme looks like this. This is off the top of my head so you may have to twiddle component values a bit. Also, this is for driving logic circuits. I'll treat your specific problem in a bit.

Mostly, there is only DC or small signal AC (audio) on the phone line. C1 blocks the DC, and the R3-R2 voltage divider prevents the low level AC from having any effect. When the ring signal comes along (90V RMS) enough voltage is developed at the juncture of R3 and R2 that some charge is pumped into C2 via CR3. Protective diodes CR1 and CR2 ensure that the output of this circuit will not grossly exceed the power supply levels and thus damage the logic circuits it may drive. C2 and R1 have a time constant of 1 second, so one second after each ring ends, the output will fall to a logic zero again. This circuit could easily drive a counter, to count rings. A timer with a longer period could be used to reset the counter if no rings have come in within say 10 seconds. I haven't built this, but here's my theory: C1 blocks DC, R1 limits the current that the ring voltage could cause. The ring voltage is rectified by CR2, filtered by C2, and limited in amplitude by zener CR1. Then the charge stored in C2 is slowly used to light LED1. As long as C2 is large enough (I'd start with 10 uF and experiment from there) to keep the LED on between rings, and small enough that the LED goes off within a reasonable amount of time after the last ring, you're set. For the specific problem in the post I'm answering, (` LED should stay ON while the phone rings`) you would want to change the design somewhat. Here's how I would make a ring indicator light that stays on until the phone acutally stops ringing:

Recommended videos

  • Phone Ringer Circuit
    Duration: 1:18.
  • Telephone intercom circuit with ringer signal generator
    Duration: 1:21.
  • Telephone ring detector circuit test
    Duration: 0:36.


  • The Ringer (blog)
    The Ringer (blog) illustrationCenter Mitch Morse explained the game of : Smith gives the quarterbacks a show to watch, and then the quarterbacks tell others until the reco works its way through the roster. It's good for bonding: Morse an
     27 September 2016
  • Asheville Citizen-Times
    Asheville Citizen-Times For reasons unknown, conversations with Dr. Owens, literary doorkeepers and a growing circle of famous admirers — all conducted in the androgynous voice of Terminator, whose full name becomes Jeremy Terminator &
     22 September 2016
  • September 2016 | Manatt, Phelps & Phillips, LLP ...
    JD Supra (press release)
    SPECIAL FOCUS: Responses to Retail Webinar Attendee Questions - During our hugely successful “Avoiding TCPA Pitfalls: Essential Guidance for Retailers” ...
     21 September 2016
  • Lexology (registration)
    In a resounding victory for a Consumer Protection Act defendant, a California court held that a plaintiff was required to demonstrate she suffered a concrete injury for each individual call that she alleged violated the statute to establish
     21 September 2016
  • Observer
    Observer If any budding filmmakers out there want a lesson in mining high-tension from very little, listen to the way Sam Esmail uses a beeping timer. In fact, you could play the illiantly-acted, meticulously paced conversation between Angela
     15 September 2016
  • The Ringer (blog)
    The Ringer (blog) (He stays tethered to his wife, Lorraine, played by Laura Linney, by .) It's the nightmares too, of course, and what they suggest of Sully's state of mind as they eak the movie up and push it every which way, backward and fo
     9 September 2016
  • Juneau Empire (subscription)
    Juneau Empire (subscription) Rie Muñoz, the well-known Alaskan artist, died on April 6, 2015. Although she's gone, her experiences are not lost, from her first memories in Holland and assisting with the Allies' WWII efforts, to journey
     4 September 2016
  • The Ringer (blog)
    The Ringer (blog) Olsen's songs have the vivid, hallucinatory feel of dreams, and she writes like someone with a crisp line connected directly to her subconscious. Listening to her new album, My Woman, is like entering a whole other dimensio
     2 September 2016
  • Jewish Link of New Jersey
    Jewish Link of New Jersey He especially mentioned that putting a on viate or turning the off is not enough. Phones needed to be turned off. When davening to Hashem, concentrate only on your conversation with Him, is what he emphasized. Do n
     1 September 2016