Remote Voltage Monitoring
When you are using an observatory under remote control there are a number of things that can go 
wrong and ruin your evening. You need to keep an eye on a lot of stuff.  Things like temperature 
wind, clouds and the scope's position should be monitored.  
While the majority of the permanent observatory setups are powered by 120 volt AC. Some, like in my 
case are either fully solar powered or partially solar powered. For those observatories the user needs to 
be able the monitor the solar panel output and battery voltage remotely. To do this I use a Velleman 4 
channel data logger shown in Figure 1. The logger connects to the observatory PC via a USB 1.1 interface and 
allows 4 separate channels from 0 to 30 Vdc to be monitored and recorded.
		Figure 1.
The data logger has 2 modes: Digital Volt Meter (DVM) mode and  a analog Strip Chart mode.  Each mode has its 
useful advantages.  The DVM mode shown in Figure 2 can be  used get an immediate easy to read status of voltages. 
Those  voltages are as follows:
Channel 1 - 100 amp hour deep cycle batteries (2)
Channel 2 - Solar powered LED interior lights (2)
Channel 3 - 120 watt solar panel
Channel 4 - 12 volt (unregulated) power supply

					Figure 2.

As you can see from the figure above there are a number of settings that can be selected from the DVM 
mode screen. The Voltage Range selection allows the user to select a range that will encompass the highest 
voltage output that the channel will see.  You can also select either the Max or Min box under each one of 
the voltage displays. Selecting the Max box will hold the highest voltage measured and the Min will hold 
the lowest.  The Time/Div buttons are not used in the DVM display mode. The "Run" button will start the 
logger continuously recording until the button is pressed again. The Single button will display and record about 
15 seconds of data. When the logger is in record mode the red LED on the front of the logger will illuminate.

The other mode is Strip Chart mode. This mode allows the user to trend the voltage of a specified period of time. That time is set by the Time/Div buttons on the right side of the display. In Strip Chart mode the individual channels are displayed as different colored lines on a grid.
In the display above the 5 second per division button was selected. Since there are 17 horizontal divisions on 
the grid that means the trace represents 85 seconds (1 minute 25 seconds) of data. If the "Run" button was 
used to start recording, when the trace reaches the end of the grid on the right side, the screen is cleared and 
the trace restarts from the left side. This will continue until the "Run" button is pressed to stop recording. If 
the "Single" button is pressed the trace will make one single pass across the grid. Again the time of the trace is 
determined by the Time/Div selection.
For each channel the value of each vertical box is equal to the voltage range divided by 8 (which is the number 
of vertical boxes). That means in case of channels 1, 3 and 4 above each division is 3.75 volts. For channel 2, 
each division is 0.75 volts. 
During remote operation monitoring of certain voltage telemetry will give the user specific info about what is 
happening with the system.  Take a look at the telemetry from channel 3 above.  Notice the white line has a 
saw-tooth type of pattern. This is a result of the solar charge controller regulating the battery charge by sensing 
the battery voltage under a small load. It then modulates the battery charging so the batteries are not over 
charged.  When the controller  disconnects the battery load from the solar panel the voltage goes up to 17.66 
volts where it levels off.  As soon as the batteries are reconnected the voltage drops to 13.88 volts which is also 
the high voltage value of the flooded lead acid batteries.  You can see from the saw-tooth the cycle is constant 
at about one cycle per second.  So if I was using the system remotely for solar observation, the telemetry from
channel #1 and channel #3  would tell me the observatory's batteries are in a fully charged state.
The other thing I could tell is that one set of LED lights inside the observatory (Figure 4) had turned on about 
45 seconds ago. I would know this because the voltage on channel #3 (green line in figure 3) had dropped about 
one volt.  If both sets of LED had turned on the voltage drop would have been greater (closer to 2 volts).  The 
LED lights will automatic turn on when it gets dark. The purpose for this is so the observer does not enter a dark 
observatory and risk hitting their head on the counter weights (trust me that hurts). There are 2 sets of LED lights
in the observatory. A white light set and a set with a rubylith cover. The lights can be manually turned OFF by use
of rocker switches seen below the light in figure 4. Which, both or any of the lights can be configured to turn on.
For the purposes of this article darkness was simulated by placing a black cover of the small solar panel for these 
lights. When the solar panel voltage drops to zero (which happens when it gets dark) the lights turn on.

Figure 4. White LED light
If a device stopped working correctly a check of the voltage telemetry might clue the remote user as to what the 
problem was. For example a sudden increase in voltage in the battery or power supply telemetry could indicate a 
blown fuse on a device using that supply.  Low battery voltage might explain a non-responsive camera or indicate a
shorted battery which could cause of a fire.

So as you can see there are other things that might need to be monitored besides just weather and physical position
of the telescope in a remote controlled observatory.

Go to the JATObservatory Home page

Updated 10/08/2006- Please report broken links