Hi all,
for measuring light data in a lake I want to build a device. I put a schematic here.
First sensor is a presurre sensor (I2C) and a photodiode in a pressure proof container. The data from the photodiode and the pressure sensor are converted to ASCII characters via a Arduino mini and transmitted via cable as serial data. On the boat the serial line (on a rotating cable drum) is connected to an Arduino BT to transmit the signal via Bluetooth.
The second sensor is principally the same one as the first, but without the pressure part.
Now I need a device to collect the data from the two sources. They should be written on a USB solid drive in csv format.
The first sensor is connected via Bluetooth (because of the rotating drum) and the second one on a serial input.
Does it make sense to use a Raspberry Pi for that? Or is there also a solution with an Arduino?
The Raspberry Pi is more power hungry and if that and the Pi's Linux environment isn't a problem for you then it would probably be the easiest way to go. If you want to stick with Arduinos then you could write your data to an SD card, not a USB stick, but writing in CSV format is not at all difficult.
Wireless (Bluetooth) transmission through water is questionable. The 2.4GHz frequency doesn't pass through water well. You'd be much better off with a wired transmission; TTL serial will work for a couple meters or you can jump to RS485 if you need tens of meters of distance.
Linux isn't a problem - I'm writing this on a Linux computer by the way. Power consumption is more serious. SD would do, too.
There is no BT connection through water. I'll have a rotating cable drum on board of the vessel. To transmit data from this cable to the logging device (which will not be mounted on the cable drum) I'll use BT, through air and less than 1 meter.
I assumed serial TTL will cover mthe whole 50 m. I thought I could avoid RS485, that would mean additional hardware (bus driver) and it would be more complicated. There is a cheap module (MAX485) for Arduino, I'll have to find out if this fits into the housing of the underwater device.
Richard
r_a_mueller:
I just see, the MAX485 needs 4 lines - I have only three (Vcc, Tx, GND). That makes this solution impossible.
Use another cable. Cheap CAT5 network cable may do the trick and has 4 pairs of conductors so you could also supply power for the MAX485 underwater.
Another option would be to battery power the underwater sensors and then you only need the 3 wires (A, B, GND)
I have limited space in the underwater case. And the watertight junction needs a cable with a certain diameter and rubber insulation, a cheap CAT5 cable won't help. 50 m is 5 bar!
But what about a MAX232? It needs 5 V (existing), GND (existing), a TTL Tx (existing) and makes a + - 12V Tx. And it is small enough as SMD version! The other contacts of the IC (Rx, CTS, RTS) can be left alone? (My device only sends data, it does not receive anything!)
A widely used rule of thumb indicates that cables more than 50 feet (15 m) long will have too much capacitance, unless special cables are used. By using low-capacitance cables, full speed communication can be maintained over larger distances up to about 1,000 feet (300 m).
I make measurements every second. I think of using 4800 baud, as in NMEA 0183 standard. I think with this low baud rate 50 m of no low capacitance cable (H07RN-F3x1.5) will make no problem. Baud rate could be lower with this frequency of measurement. If that does not work I can switch to 2400 baud.