LED Bargraph Meter
for the BITX20A
The combination of the LED 5 segment display and it's driver IC (LB1413N) create a simple AC/DC peak reading meter that is used to indicate relative signal strength on receive and minimum SWR on transmit. The circuit is very straightforward as can be seen from the diagram ( click images to enlarge). Note that the relay and contacts shown on the circuit are there for information only and will actually be on an auxillary board.
Construction: The circuit was constructed on a piece of copper strip board ('Verobaord') 50 x 35 mm (2 in x 1.375 in) with the strips running along the 50 mm length. The LED bargraph is soldered on the 35 mm end (each pin going it's own strip). My intention is to use epoxy resin to affix the LED display into the front panel but there is room on the board to include a screw fixing if necessary. Wiring up the display is pretty straight forward and the layout more or less follows the circuit.
Operation: The LED bargraph operated as expected after finding the bad joint at the 10K resistor on pin 5 of the driver IC! Current consumption with all LEDs on was 35 ma for a 13.8 supply, which is well within the specs of the LED display and the driver. The driver has it's own internal supply stabiliser and is therefore fairly independent of supply voltage. Driving the display from a DC source I found that 210 mv input was required for all 5 LEDS to be illuminated and 60 mv to light the first LED, there being a 'jump' of approximately 40 mv per segment.
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