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Two way radio simplex repeater control circuit using ISD1820 voice record Chip?

Discussion in 'General Electronics Discussion' started by TechieTom, Nov 28, 2013.

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  1. TechieTom


    Nov 28, 2013
    Hi everyone,
    I am planning on building a 'simplex parrot' repeater for my two way radios. Basically it takes the audio received, records it then transmits it again.

    I have ordered one of these
    to record and play the recording but need help with the control circuit.

    I need a control circuit to trigger the chip into record mode as soon as it senses audio from the line level audio output of the radio, and then trigger the play input of the isd1820 as soon as the audio from the radio stops.

    I'm thinking this should be possible with a few transistors, resistors and capacitors but not quite sure how to wire it!?

    Any help would be greatly appreciated!

  2. davenn

    davenn Moderator

    Sep 5, 2009
    hi Tom
    welcome to the forums :)

    your link doesn't work

  3. TechieTom


    Nov 28, 2013
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Tom and welcome to Electronics Point :)

    So, you want to monitor a receiver and detect when it is receiving. When it is receiving, you want to start recording the received audio with the ISD1820. Then when the reception stops, you want to enable the transmitter, replay the recorded audio from the ISD1820, and at the end of the recorded audio, disable the transmitter. Is that right?

    I think you would be best to use a microcontroller to control things. I implemented a repeater controller for two radio transceivers and the logic was a bit too complicated to easily implement with discrete circuitry. It is possible; just quite messy.

    The Microchip PIC series of microcontrollers would be well suited to this task. My repeater controller used an Atmel AVR device; these are similar to the PIC. You would need to learn a bit of programming, but there is plenty of support out there for people learning to program the PIC, and for the AVR through the Arduino projects.

    Embedded programming is a very interesting subject. It opens up a whole world of possibilities.

    That said, if you don't want to get involved, I can probably design a circuit to do what you want, once you have defined it clearly.

    I would use the carrier detect signal on the receiver to determine when to start and stop recording. If you don't do that, you have to implement some sort of delay to detect a period of silence so you know the transmission has ended. Do you have access to the carrier detect signal from the receiver?
  5. Centar2001


    Feb 1, 2016
    If anyone is interested I have successfully interfaced an ISD1820PY chip to a POFUNG BF-888S as a simplex repeater using only 1 transistor, 1 Diode and a couple of caps and resistors...It is set for 10 secs record time and has good enough sound quality for not only voice but music to be repeated quite nicely. I believe 10 secs is enough time to say what you need to without jabbering on and an and thus makes a simples repeater more usable. The ones on the market boasting up to 120 secs and beyond are really just too annoying to use when people yack on and short, a long record time on a simplex repeater is self defeating and counter productive. With one more transistor a "say again" feature can be added where you "ker-chunk" the repeater to relisten to the last recording but in practice it adds too much delay before recording to be of much desire...also it can be built to not repeat if someone runs past the 10 secs (this is the default and how mine works)...helps discourage long messages and makes sender resend (and make shorter) their message when they don't hear it repeated (missing the last part over 10 secs)...I can provide the schematic (simple as it is) if anyone is interested in doing the same...It does require a radio that has some sort of COR or COS output (easy to find on BF-888S as pin 1 on audio amp ic)
    k3rnell and 7gtb like this.
  6. 7gtb


    Jun 22, 2017
    Just came across your post from last year, could you send more info on your project, I would like to try this
  7. TechieTom


    Nov 28, 2013
    Hi all! I completely forgot i even made this post! It was a few years ago!

    KrisBlueNZ , Your suggestion led my to start programming with a Pic, eventually i started playing around with Arduinos, and now nearly all my projects contain a Micro controller! Many thanks for suggesting it, it really does open up so many possibilities!

    Centar2001 , I'd be interested in the schematic? It sounds nice and simple and could be made into a very small board which i'd like, How do you trigger the PTT on the radio? Are you using Vox?
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