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.
  • How to make a Fake Telephone Ring Generator
    Duration: 2:08.


  • Press Herald
    Press Herald Don Trementozzi, president of Communications Workers of America (CWA) Local 1400, also said in the statement, “Our members and our customers have been through the with FairPoint over the last eight years, and our primary c
     5 December 2016
  • Now that the new collective bargaining agreement is set, major league baseball can turn to the real business of winter.
     5 December 2016
  • Emporia Gazette
    Emporia Gazette He's discovered a tubing system used to speak with other rooms in the house, an early system with a separate and even found the layout of a bathroom from the hole found in pieces of marble tile. With the goal of moving into
     3 December 2016
  • The Ringer (blog)
    The Ringer (blog) The owners of Village Voice Media gamed the online classified business with and made millions. But when it became a eeding ground for child ...
     2 December 2016
  • Reporter-Times
    Reporter-Times Sometime between 1:30 a.m. and 6 a.m. an individual or a group of vandals showed up at the business at 549 W. Mitchell Street, snipped the outside wires connected to the company's and alarm system, cut through a mesh window sc
     2 December 2016
  • Crossville Chronicle
    The following incidents have been documented by the Fairfield Glade Department of Public Safety for the week of Oct. 30—Nov. 5, 2016:
     16 November 2016
  • Holton Recorder
    Holton Recorder " marker stands out in Holton cemeteryVisitors at the Holton Cemetery west of town may have already noticed Campbell's future resting place, which includes a large silver bell with the words “Here Lies A Dead ”
     15 November 2016
  • Villages-News
    Villages-News Turn down your so no one outside can hear repeated rings, and never leave a message on your telephone answering machine stating you are out of town. Leave your window blinds as you normally would if you were home. Invite your&n
     13 November 2016
  • TravelersToday
    TravelersToday This is also good for your auto answering machines and . In case those who have bad intentions checks if someone will pick up the phone he will hear no phone rings. This will no give him the slightest clue if whether there's so
     7 November 2016