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Field Day


Two Battery Monitor Projects

A Communicator Reprise...

October 2015

John demonstrating his meter to the group

Last spring, SARC initiated a competition to see who could construct the most suitable and innovative 12 volt battery monitor for use at Field Day.  Here is what I came up with.
The following design criteria were used for my version of the monitor: a) it should provide an analog reading of voltage, accurate to 0.1 volt; b) it should have an alarm that would warn of critical low voltage at an adjustable level; c) the alarm should be prominent but not disruptive to other operators; and d) it should be cheap and easy to build.  Anderson power poles would be the connector of choice.
I prefer an analog display as it is easier to discern conditions at a glance without having to read a series of digits on a digital display which may be fluctuating rapidly.

The monitor was constructed around a low voltage FK915 alarm kit purchased on line for US$ 5.95 from  

For the voltage indicator, I found an old analog meter in my junk box, but I needed to change it from a 1 mA full-scale ammeter to a 9-16 volts voltmeter.   

I purchased locally a large red LED to substitute for the buzzer and a cast aluminum box to put it in.  Anderson power poles plus mounting blocks were obtained from QuickSilver Radio Products.   As will be described later, a few other small components were also required.

In order to change the 1 mA meter scale to read 9-16 volts, I calculated that a 16k resistor was needed in series with the meter (R=E/I = 16 volts/.001 amp = 16,000 ohms).  The resistance of the meter itself is not significant in this case.  To provide this resistance and allow calibration of the meter, a 10k ohm potentiometer was put in series with an 10k ohm fixed resistor.  I also added a 9 volt Zener diode so the scale would read 9-16 volts rather than 0-16 volts.
The meter, series resistor, potentiometer and Zener diode (all in series) were connected across the input of TR5 transistor and PZ buzzer (or LED in my case).  A potentiometer on the circuit board allows setting of the desired trigger voltage for the alarm.

A free scale drawing program called “Meter Basic” by Jim Tonne W4ENE (figure right) is available on the Internet.  A more sophisticated program simply called “Meter” is also available at a modest cost.  I found the former was adequate for my needs, and allowed me to change the appearance of the meter scale as shown in the figure.

It’s simple but it works.

~ John Brodie VA7XB


And another monitor...

Keenan VE7XEN also showed off his design for the voltage monitor at the September meeting and promptly walked away with first prize. It was a very impressive professionally produced board with surface mount components.

It reports both under and over voltage and provides both a visual and audible alarm when voltage deviates from the set parameters. Other features:
  •   Multiple Alarm Methods
  •   Voltage to better than ±0.1V; range 5-20V
  •   Parts cost $30 per unit - single supplier
  •   Safe, “field serviceable” input connection
  •   Small size
  •   Programmable Thresholds & Alarms
  •   “Mute” button
  •   Temperature Readout

See a video demo at URL: and Keenan’s slides at

Nice work Keenan!

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