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29/9/2012 at 3:33pm
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Outfit: Coachman Pastiche
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Yes and no, Birder. What I am looking to do is take the analogue signals from the various sensors and feed them back to a micro controller. Some of these signals, eg battery voltage are too high for the micro controller and hence need to be converted to an analogue voltage of 0 to 5 volts. This is a simple enough task to do, requiring only 3 components (actually it can be done with 2 but offers no over voltage protection this way)
Once the signals are received by the micro controller, they can be used however we want, be that stored locally, displayed on an LCD display, used for comparison and action taken if required for example heating an aqua roll of the temperature is below freezing or passed to a PC or other code reader via a serial or USB port or indeed any combination of the above.
While the automotive industry use standardised protocols of which CAN is just one of a hand full for sending these details to a PC or hand held code reader, obviously a standard protocol doesn't exist for caravans, nor is it something there has ever been a need for.
Just because there is no standard protocol however, doesn't mean that it cannot be done, it simply means a new protocol needs to be created along with new software for communicating with the processor. Creating a new protocol is a simple task, for a new development it is simpler than trying to use an existing system. Standard protocols all started off as a development protocol at some point and if a standard protocol for caravans did arrive in the future, there is no reason why a firmware update couldn't install it to my system although any hand held reader would also need flashing and any PC software would need updating in order to communicate to the new firmware.
This then brings things on a little further by highlighting that just like automotive systems, a built in display is not needed as a hand held reader can be plugged in or it can be connected to a hand held reader. Thus there is no power demand for a display that is not connected.
Obviously the information will be different with a caravan than it is for a motor vehicle, but the data transmission is basically the same but uses different protocols.
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30/9/2012 at 5:59pm
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Hi Chalkie. Can I ask if your PV panel has a regulator attached? if not, I assume it is a small panel, of under 10 watts as anything larger would need a regulator to prevent damaging the battery.
There are digital ammeters that can be connected between the PV panel and the battery which will give an analogue signal of 0 - 5 volts dependent on the current being passed through them, which could then be interfaced to a micro controller.
The LED display you have is in essence a very crude volt meter which only has 3 possible readings thus not really giving you much information at all.
A simple way of improving on the 3 LEDs would be to but a simple volt meter across the battery rather than the display board you have which consists of three LEDs and some voltage shunts.
There are several ways of measuring temperature, using various thermistors but Microchip make one that is designed to interface directly with their micro processors giving a 10 bit digital reading which has a range of -50 to +450 degrees in steps of or 0.5 degrees or a range of -25 to +245 degrees in steps of 0.26 degrees celcius depending on the thermistor used.
Obviously, no one is ever going to be caravanning in the South Pole or on Venus so these limits are not an issue at all and the resolution is also plenty fine enough.
An aquarium heater to heat the aqua roll is not a bad idea at all although I wouldn't want to use a second hand one.
 This also has the added backup that it has a built in thermostat, although this is far to warm to be relied on without the micro processor switching it off earlier as water at this temperature would become a breeding ground for bacteria.
The processor would easily be capable of switching the heater off at 1 or 2 degrees celcius and back on at 0 or 1 degrees.
However, I do see a conflict of interests here. Anyone using a solar panel is likely to be doing so because they are not on EHU. If this is the case, I question where they are going to be taking the power for the aquarium heater? From an inverter powered by the leisure battery?
The smallest aquarium heater I can find on the internet is 50 watts. Assuming an inverter is 100% efficient, this is going to draw 4.2 amps from the battery. In the winter a solar panel is not going to return much of this to the battery and in extreme conditions, an 85 aH battery would be flat in little over 20 hours. In reality the inverter would have tripped out long before this.
While it could be argued that the heater is not going to be on constantly, this is going to depend on the ambient temperature. Also the inverter is not going to be 100% efficient, at best it might be 80% efficient. This would certainly render the system impractical unless EHU was available.
This then brings me back to the question of do I design one system around low current usage for those that have no EHU and another system for those that do?
It is also possible to detect if an EHU is connected and behave accordingly. Both of these have there upside and downsides. Maybe a third option would be a better bet for those that use a mixture of EHU and non EHU sites.
A universal system would have greater limits put upon further development and also goes against my ethos that a system tailored around the individual is better than a generic one size fits all system that is generally available, provided the cost is not greater than a generic off the shelf item.
A low power system could pump a small amount of hot water back into the aqua roll, using very little power other than opening a solenoid valve, whereas a high power system could switch on an aquarium heater for example. A universal system would need the option of both of these plus an input to detect if the EHU is connected, and of course the temperature sensor. This would use 4 of the micro controller ports whereas a system tailored specifically to low or high power usage would only need 2 ports, the temperature sensor and the heating output.
Using ports unnecessarily can seriously preclude future developments or at least make them more costly if they involve adding additional hardware rather than a simple firmware update.
Post last edited on 30/09/2012 18:05:38
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