Digital HF Power-SWR Meter

2009-11-17T17:42:00.000-08:00

This power-SWR meter is intended to 5 to 100W ranges but you can build heavier measurement bridge and scale instrument for whatever range you want. Current version of software provides few basic displays. Scaling and other feature setup parameters are in software code (no setup menus).

Features:
Digital Power / SWR meter for HF use

Power range 5 to 100W
Frequency range 1 to 30 MHz
Peak hold display operation for SSB operation.
Few basic displays showing Forward, Reverse, net outgoing power and SWR
Dual bar graph display for forward and reverse power
Large and clear 2*16 character LCD display
Simple two button interface
Hi SWR warning and alarm LEDs
Separate display and SWR bridge units. No antenna cable routing to meter.

Building the meter
You can build and test SWR bridge and meter part separately. SWR Bridge can be tested with volt meter and display unit with 0 to 10V psu. Remember that high power RF radiation softens your head. No ready-made PCBs are available. Beware faulty Microchip 16F877A parts. You can read more from Microchip errata pages.

SWR bridge
Critical component is transformer L1. L1 is 10 turns bifilar wound on Amidon FT50-43 core. First twist two 0,4mm wires together and then wind 10 evenly distributed turns on core. Primary (TX to antenna) is just wire going through the core. Wire can be just straight wire or thin coax other end of the braid grounded. You can buy Amidon cores here http://www.srat.fi . For safety reasons and for better performance it is wise to install SWR bridge board into metal enclosure. You can find SWR bridge theory from ARRL handbook.

Picture of SWR bridge prototypes and component side

SWR bridge pc board
SWR bridge schematics
SWR bridge layout

Display unit
Display unit is build around Microchip 16F877A processor and Solomon LM1125SYLU1 LCD display. LCD displays you can buy from Matti oh2mh@sral.fi . Microchip 16F877 (non A version) is also supported but without LCD backlight control. Only critical components on display board are resistors R2,R3,R5,R6 and IC2. Display unit PCB is designed so that you can separate push button switches and SWR warning LEDs if your enclosure selection requires this. Back light current generator fet Q2 may require additional cooling if operating voltage exceeds 10 VDC.

LCD contrast trimmer is missing in this picture

Display unit schematic
Display unit layout top side and bottom side. Note few jumper wires required.
Display unit pc board

Software
Project is done with Hi-Tech PICC compiler. Not strictly ANSI C but with some work project can be transferred to other environment. This first test version does not include any user setup functions. All setup and calibration variables are in software code. Displays are not carefully planned they just show needed basic information. Things may improve in the future.

Software source code can be found here
Ready to load HEX code for 16F877A
Ready to load HEX code for 16F877

Picture of Basic Power and SWR display - Forward/Reflected Power Bar Display

Source: Digital Power and SWR Meter

Audio Level Meter

2009-09-22T23:20:00.000-07:00

This audio meter project for measuring audio levels. If you listen to most MPEG, AVI and WMV files, you will find the audio levels all over the place. This level meter is the tool able to use as audio test equipment in the multimedia audio.

How The Audio Level Meter Circuit Works
The circuit given below is a simple audio-amplifier powered from a 9v battery. The first two transistors are a voltage amplifier, and the right-hand pair form a complementary output pair. The two diodes between the bases of the output transistors give the required bias, but if the output devices ever draw too much current (or get even slightly warm) then increase the 1K0 resistor to another value until they do not get in the slightest bit warm.

The gain of the amplifier is controlled by the negative feedback, which in turn is controlled by the 470 Ohm resistor (R4) and the 4K7 (5K) pot. If the pot is set to about 100 Ohms then the voltage gain of the amplifier is about 5. But, that gain only occurs when the 4-diode bridge rectifier is conducting. If the point of the waveform is less than the forward conducting voltage of any diode, then the gain is probably around 250,000. When the diodes conduct, they rectify the signal and pass the current through the meter. In this way the diode conduction / amplifier transfer characteristic has become more linear. The value of the POT determines how much current actually flows. This circuit works well with a low sensitivity 20mA FSD meter, but works equally well with those cheap 100uA meters you get from most component shops.

If you do not need accuracy, for example, a simple audio level meter application, then a good source of cheap meters is to rob one from a battery tester.

The project needs no special tools, no setting up (other than a single sensitivity adjustment) and almost all components have an extremely wide range. With low sensitivity meters then the 5K Pot could be replaced with a 500-Ohm pot, or replaced with a 25K pot if you used a 30uA meter. The potentiometer (pot) can be any type of device, from a small pre-set pot glued to the board, to a normal 1950's type volume- control mounted in a hole drilled in the board. Download Project Documentation.

Source: Audio Level Meter - Dual Stereo Vu Meter Kit

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Digital HF Power-SWR Meter

Audio Level Meter