A LED power meter for roving operations

 

It is always nice to get some visual feedback that one is actually generating RF when transmitting. At home seeing the measured power output on the bird meter is always comforting. However when roving a more robust and compact technique is necessary. Absolute accuracy is not a necessity, just a relative indication. Previous attempts with plastic cased analogue meters show that they do not survive long in rover operation. My 3456 and 5760MHz transverters got through 3 plastic covers between them before alternative action was considered.

 

With these thoughts in mind 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 the Qualcom 1W PA

 

The 12V supply for the module is obtained from the 12V on transmit supply to the input of the PA voltage regulator. The rectified sample of the RF output is obtained from the green lead coming out of the Qualcom unit. Note that there is already a preset inside the amplifier, accessible from the round hole in the amplifier cover (normally covered with conductive foil), which I adjusted so that 9 of the 10 LEDs are lit when the amplifier is producing 1.1W. In this application VR1 could be omitted. In my 10GHz transverter system I mounted the display by filing a hole in the box front panel (made of 1/8” aluminum plate) just big enough for the led display which is then glued into place. There are probably more elegant methods!  A photograph is shown in Fig 4.

 

 

Fig 1. Circuit

Fig 2: PCB Layout                                                        Fig 3: Overlay

 

                            

 

 

Table 1. Components for display

 

Component	Value
IC1	LM3914
VR1	10k vertical mount preset
R1	3k3
BAR1	10 LED bar display
C1, C3	0.01u ceramic
C2	10uF 25V electrolytic