A Communicator Reprise...
|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.
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 www.Qkits.com.
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
Nice work Keenan!