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  Adding a Separate Control Receiver to a System That Doesn't Have a Spare Controller Port
By Mike Morris WA6ILQ
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Many years ago (in the days long before cellphones) I helped a group that had a co-sited 2m and UHF repeater on a homebrew 2-port controller. The UHF repeater was used as an intercom and autopatch for the control operators and their families. Since the DTMF decoder was hard wired to the UHF repeater all controlling of both the 2m repeater and the UHF repeater was done from the UHF receiver. We had a 2m troublemaker discover the UHF channel and PL, and he had enough ERP on his base station to capture out some of the legitimate control operators, especially when they were mobile. This circuit was developed to allow us to control the system when he had a capturing signal on the main UHF channel.

The 2m repeater is not shown in the diagram below. The added secret control receiver is shown as a 220 receiver. That's not where it was.

All COR and PL decode interfacing signals were active low (i.e. ground when active).

                                                      !                           !
                                                      ! 440 MHz repeater receiver !
                                                      !                           !
                                     O----------------! Audio                     !
                                    /                 !                           !
                 To repeater       /           +------! COR                       !
                 controller       /!           !       !                           !
                 audio input ----O ! O-------+ ! +----! PL decoder                !
                                   !         ! ! !    !                           !
                                   !         ! ! !    +---------------------------+
                                   !         ! ! !
                                   ! O-------!-+ !    +---------------------------+
                                   !/        !   !    !                           !
                                   /         !   !    ! 220 MHz control receiver  !
                 To repeater      /!         !   !    !                           !
                 controller -----O ! O--GND  +--------! Audio                     !
                 COR input         !             !    !                           !
                                   !             ! +--! PL decoder                !
                                   !             ! !  !                           !
                                   ! O-----------+ !  +---------------------------+
                                   !/              !
                                   /               !
                 To repeater      /!               !
                 controller------O ! O--GND        !
                 PL decode         !               !
                 input              ---------------+
                                   ( 3 pole double throw 24v DC relay
                                    ----- to +24vDC
Yes, you could do it with a 2 pole relay, but we had the UHF repeater receiver running in carrier squelch for simple repeat as the control operator intercom, but it took PL to enable the DTMF decoder (i.e. to enable commanding). This was a workaround that resolved the issue where the voice of one control operators wife triggered the DTMF decoder. That issue was resolved when we built a better DTMF decoder (one designed by Allan Burgstahler WA6AWD - it used 88mh toroids instead of NE-567 chips).

When the control receiver hears a signal with the proper PL tone the PL decode pin goes to ground and pulls in the relay. This disconnects the controller from the main channel receiver and replaces it with the control receiver, while forcing the COR input and the PL decode input active. The control receiver audio replaces the main channel audio for the duration of the PL decode time. The control operator can enter DTMF commands without having to fight to capture the UHF repeater receiver.

Update: Dec 2005... I see that the fine folks at Computer Automation Technology have reinvented this wheel as their CAT Note 20.

Acknowledgements and Credits:

Idea suggested by Neil McKie WA6KLA.

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.

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Article text, diagrams, and hand-coded HTML © Copyright January 2005 and date of last update by Mike Morris WA6ILQ.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.