Adding a control receiver to a system that doesn't have a spare controller port

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Back to Home How to Add a Separate Control Receiver to a System That Doesn't Have an Extra Controller Port
Written, HTML'd and Maintained by Mike Morris WA6ILQ

Many years ago (in the days long before cellphones) I helped a group that had a co-sited open 2 meter repeater and private UHF repeater on a homebrew 2-port controller (TRCDR logic based.... Transistor, Resistor, Capacitor, Diode, Relay logic). The design of the controller had one DTMF decoder that was hardwired to the UHF port, hence all controlling of both the 2m repeater and the UHF repeater was done from the UHF receiver. In addition to the repeater control functions the UHF repeater was used as an intercom and autopatch for the owner, his family, and the control operators and their families (everyone was licensed).

We had a 2m troublemaker discover the UHF channel and PL tone, and his base station signal was strong enough to capture out some of the legitimate control operators, especially when they were mobile. The homebrew repeater controller design was such that there was no easy way to expand it, so this circuit was developed to allow us to maintain control of the system.

The 2m repeater is not shown in the diagram below. The added secret control receiver is shown as a 220 MHz 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 (to ground)           !
                 controller       /!           !      !                           !
                 audio input ----O ! O-------+ ! +----! PL decoder (to ground)    !
                 (DTMF input)      !         ! ! !    !                           !
                                   !         ! ! !    +---------------------------+
                                   !         ! ! !
                                   ! O-------!-+ !    +---------------------------+
                                   !/        !   !    !                           !
                                   /         !   !    ! 220 MHz control receiver  !
                 To repeater      /!         !   !    !                           !
                 controller -----O ! O--+    +--------! Audio                     !
                 COR input         !    !        !    !                           !
                                   !   GND       ! +--! PL decoder (to ground)    !
                                   !             ! !  !                           !
                                   ! O-----------+ !  +---------------------------+
                                   !/              !
                                   /               !
                 To repeater      /!               !
                 controller------O ! O--+          !
                 PL decode         !    !          !
                 input             !   GND         !
                                   !               !
                                                   !
                                    ---------------+
                                   (
                                   (
                                   ( 3 pole double throw 24v DC relay
                                   (
                                   (
                                    ----- to +24vDC

Yes, you could do it with a 2 pole relay, but there were more 3-pole relays in the junk box than 2-pole relays. We also had the UHF repeater receiver running in carrier squelch for simple repeat as the control operator intercom, but it took a PL tone to enable the DTMF decoder (i.e. to enable commanding). This was a workaround that resolved an issue where the voice of one control operators wife triggered (falsed) the DTMF decoder. That issue was resolved when we replaced the DTMF decoder with a better one (designed by Allan Burgstahler WA6AWD for the Cactus Radio System (Allan was later N7BF, now SK) - it was a totally different design that used 88mh toroids in one parallel tuned circuit per tone instead of one NE-567 chip per tone).

The control receiver we were using was carrier squelch and didn't have a COR output. In the intended use we honestly didn't need one. We added an early design Com-Spec PL decoder that had a SPDT relay output.

One pole of the 3-pole relay disconnected the audio DTMF input from the main channel receiver and replaced it with the control receiver, while forcing the COR input and the PL decode input lines active. The control receiver audio replaced the main channel audio for the duration of the PL decode time. The control operator could enter DTMF commands without having to fight to capture the UHF repeater receiver.

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

Acknowledgements and Credits:

Idea suggested by Neil McKie WA6KLA (SK). He used it on a VHF repeater in the late 1970s / early 1980s.

Contact Information:

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

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This page originally posted on 02-Jan-2005

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.