The K3020P - Optocoupler - strange effect

Discussion in 'The Projects Forum' started by blackcapsteve, Apr 24, 2012.

  1. blackcapsteve

    Thread Starter New Member

    Apr 8, 2012
    Hi All
    I want to build a theatre phone ringer. (UK 400ms On 200ms Off 400ms On 2000 ms Off)
    I have found a lovely circuit (555/556 based) which does the job, and buzzes a buzzer. I actually want the timer to not buzz, but to instead drive a relay which sends pulses of 120V down the line to the phone on stage.
    I bought a goody bag from Maplin and set about cataloging the bits I received, and in the bag was a K3020P, which I found a datasheet for. I thought that this was perfect, because it is a low voltage timer circuit switching a high level ring circuit, BUT, the K3020P switches AC. (see attached datasheet). I am a bit unfamiliar with AC, so I thought I would see what happened with low level DC at both ends. I built a test circuit (layout attached) and something starnge happens. If I turn on the switch, both LED's light, but If I turn off the switch LED2 stays alight. It will only go out if I break the circuit. When I rejoin the circuit the LED stays off.

    1) What is happening?
    2) If the pin 4 6 part of the circuit was DC would the circuit turn off when the 1 2 circuit turns off?
    3) Is there a similar part which will switch high DC using low DC?

    Sorry if this question is a bit niave

    regards steve
  2. blackcapsteve

    Thread Starter New Member

    Apr 8, 2012
    Sorry Q2 should have said "if the 4 6 circuit was AC"
  3. crutschow


    Mar 14, 2008
    As the datasheet states, the K3020P had a Triac output. If you look up Triac you will see that once a Triac is triggered it stays on until the current is reduced to zero, as you found. That's why a Triac is normally used with AC circuits since the voltage (and thus current) goes to zero twice each cycle, turning off the Triac.

    What you want is a DC solid-state relay.

    Does the phone require 120Vdc or 120Vac?
    blackcapsteve likes this.
  4. blackcapsteve

    Thread Starter New Member

    Apr 8, 2012
    Thanks - I'm a bit new to this..

    it requires DC for the ringer, which is why I tested to see if DC worked on what was defined as an AC output from the pins. (using a bit I had rather than a bit I sourced)

    Triac?.. learned something else. The idea of using a solid state relay is fantastic so you dont get a mix of power to fry the 555/6