A LED power meter for DEMI Transverters

 

For a long time now I have considered that the only thing missing from the DEMI range of transverters is some indication of whether the transverter is actually putting out a signal. Using four Bird 43 to monitor just the VHF/UHF transverters is a very expensive method so a cheaper solution was sought.

Thoughts turned to LED indicators, in particular the 10 LED bar displays. I remembered an IC that could drive the displays directly, the LM3914; and I still had some in the junk box from using them as part of the G3WCY SSTV converter many years.

 

The circuit shown in Fig 1 was devised. The device specification shows that it may be run from 9-24V. R1 sets the current through each LED in the display, being set by the formula

 

I= 1.25 /R1

 

and can be adjusted to individual preferences as necessary

 

Construction

 

A small PCB was designed as shown in Fig 2; the component overlay is shown in Fig 3.  Postscript files are available for the board on request. VR1, C2, C3 are mounted on the track side of the board, both can be SMT types if available. IC1, BAR1 and R1 are mounted on the component side of the board.

 

Interfacing with a Down East Transverter

 

Currently no DEMI transverter comes with a power detector at the output. Therefore the crude detector circuit shown in fig 4 was built dead bug style near the output filter and the position of the components adjusted to give the minimum required voltage to illuminate all 10 segments of around 1.25V. Purists may want to devise a coupler/detector and place it inline between the antenna relay and the Antenna connector. I assumed that if I detected RF at the filter, it would get to the antenna as the antenna relay is also switching the bias to the amplifier module; if it was not working no RF would be detected! A photograph of the detector installed on the front panel of  my DEMI50/28 is shown in Fig 5, the internal view of the mounting method is shown in Fig 6.

 

 

Fig 1. Circuit

Fig 2: PCB Layout                                                    Fig 3: Overlay

 

                            

 

 

Fig 4. Power detector

 

Fig 5. Front panel view of Indicator mounted on Front Panel of DEMI 50/28 Transverter

 

Fig 6. Internal view of Indicator mounted in DEMI 50/28 Transverter

 

 

 

 

Table 1. Components for display