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This site was last updated 4/13/2002.

There have been visitors to my site since it began on August 25, 2000.

  

Build a QRP Wattmeter using an LM3914

Here is a wattmeter that I designed. It has two scales, 1W and 3 1/2W. It is not linear, however. It uses a 10 LED bargraph display available from from Radio Shack, and an LM3914 bargraph driver. I built mine into a Pomona Electronics box (0.9"x2.25"x1.1"). It is a tight fit, but it works. This is built to be used with a 50 ohm load, so if your antenna is not about 50 ohms the indicated power will be different.

Here is the schematic:

This table shows the voltage and power when each LED begins to light (V measured P-P):
HIGH PWRLOW PWR
Supply Voltage -->127.56127.56
1st LED -- Volts53332.52.5
1st LED -- Watts60mW20mW20mW20mW20mW20mW
2nd LED -- Volts96.564.53.753
2nd LED -- Watts200mW110mW90mW50mW40mW20mW
3rd LED -- Volts13107.5654
3rd LED -- Watts420mW250mW140mW90mW60mW40mW
4th LED -- Volts16129.5865
4th LED -- Watts640mW360mW230mW160mW90mW60mW
5th LED -- Volts1915121086
5th LED -- Watts90mW560mW360mW250mW160mW90mW
6th LED -- Volts211712.511.597
6th LED -- Watts1.1W720mW390mW330mW200mW120mW
7th LED -- Volts24191413108
7th LED -- Watts1.44W900mW490mW420mW250mW160mW
8th LED -- Volts26201514.5129
8th LED -- Watts1.69W1W560mW530mW360mW200mW
9th LED -- Volts282318161310
9th LED -- Watts2W1.32W810mW640mW420mW250mW
10th LED -- Volts252017.51511
10th LED -- Watts1.56W1W770mW560mW300mW

The wattmeter draws about 6mA on receive, but about 125mA with all LEDs lit! However, it can be changed to dot mode by simply leaving pin 9 disconnected. It does not even have to be tied to ground. I don't recommend running this on a 9V battery in bar mode, because it draws a lot of current in this circuit. When the battery starts to die, the indicated power changes quite a bit.

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This page last updated 4/13/2002.
© 2002 N8MX.