Man-over-board alarmsystem

Hi! I'm new to this forum and im hoping for help. We are sig guys who are going sailing for a year. I want to build a system that sets of an alarm if anyone was to fall overboard. Im new to working with microcontrollers etc. but i have tried to read up on the subject and i think ive come up with a working idea. I would very much appreciate any help or ideas you can give me. I can program C fairly well.

The system has to be extremely low-cost as we are on a very tight budget. I'm thinking that everyone onboard will wear a radio transmitter that constantly transmits a "code" to a base unit. If the base unit hasnt received one of the six individual codes in 15 seconds it sets of an alarm.

Im planning to do it this way, with everyone wearing a module that transmits to a base. If you have abetter idea of how to do it, im all ears.

The module

Ill get this microcontroller as it is cheap and has a low energymode that seems suitable for my needs. There are both cheaper and more eneryeffecient uC ive found, but the atemel programmers are so much cheaper that they get a lower total anyhow.

The uC will be connected to this radio transmitter. Ive found that the transmitter would require an antenna of about 30cm length. Can the antenna be any normal piece of copper wire? can i have it coiled? i want to wear the final unit as you would a wrist watch.

I will power the components with a buttoncell battery at 3v and 1Ah. Hopefully this will get me about a year of batterylife i think. I guess. I hope?

Would i need a crystal to get the uC runnig? would i need a voltage regulator? Im thinking that it doesnt matter very much that the unit transmits at perfectly regular intervals, nor that the voltage is stable.

Fynally im going to encapsulate it all in epoxy to make it waterproof and resistant to shocks. Could this get me overheating-issues? The battery will be outside of the epoxi and sealed in some other, unknown, way.

The base unit would consist of the same kind of uC and this radio receiver. It would also be connected to this MOSFET. The MOSFET will be connected to the boats 12v system and a beeper of some kind. I picked a P-gated MOSFET to have it sound the alarm if the base malfunctinos for some reason.

I was thinking that the base needs a crystal, and as i dont have any powerissues here it might aswell be a 20MHZ one. I havent quite figured out how to run the uC and receiver from the boats 12v system yet. Ideas?

Are there any flaws in this design that you can see? Ive heard that crystals need to be paired with capacitors, is that correct?


I posted this question in a swedish forum but i ended up getting barrated about why harnesses should be worn. So ill say raight away that we have plenty of methods for staying onboard, and weve discussed this through and through. We will ofcourse be using harnesses aswell. What i really need help with is the electronics.


Thanks in advance,
Joakim.

 

Thank you for your answer. I think I understand most of what your saying. That i wont need a uC for the modules are great news, as that will cut the cost of the project by almost 20%. Im not sure how i would go about modulating the signal nor setting the time between transmisssions randomly without using a uC though. what component should i use to control the transmitter?

The signal doesnt have to go further than at maximum fifteen meters, so hopefully an antenna cioled around the wrist will do. The boat is made froom steel though, do you think that would completely block the signal? i could mount the receiver outside, but it would be annoying to have it sound the alarm every time somebody goes below deck. Maybe if i put to antennas on the receiever, one outside and one inside?

I want to make the transmitter modules waterproof so that they dont get destroyed by spray or rain, hopefully this wont block the signal?

 

Hmm, im a little bit at a loss here now. On the upside, while googling on the 555 chip, i found a transceiver which i think will work better for my project. its also cheaper.

http://imall.iteadstudio.com/wireless/im120606002.html

Using a transceiver instead of a transmitter doesnt make much difference if i have to use a uC anyhow, but it does save a lot of noise as i can have them transmitting one at the time, at the order of the main.

Are there any microcontrollers except for the AVRs that can be pogrammed using a 15 dollar programmer?

If i have the base asking for responses i should be able to have the uCs sleeping until they are activated by the transceiver to save power, no?

So were im at now is a module consisting of a transceiver, a microcontroller and a battery. No need for crystals, voltage regulators or anything else?

 

would you care to elaborate? What you are describing would be useless ofcourse. Do you mean that the transmitters wont be able to send through the boat? wireless networks work very well in the boat, and that works on 2,4 ghz does it not? why would these be so much worse? there are not metal walls inside the boat, and there are plenty of windows aswell as a main hatch where i plan to put the base unit.

 

Im sorry for the confusion. I actually wrote to replys, one before the one that has been posted. for some reason dod the first reply dissapear. It went something along these lines:

I found this 2,4ghz transceiver which is both cheaper and i think will work better for my purposes. I should be able to get less noise aswell as save energy on the module by having them sleep until they are activated by the base. Does any of you know of a microcontroller except for the AVRs that can be programmed using a 15 dollar programmer?

as far as i can tell, i wont be needing anythin but the uC, the transceiver and a battery for the modules?

I'll try to paint a picture of the boat in paint to give you guys as clear an idea as possible of the setting. We are on a very limited budget and i dont want to get into a project that wont work. As you say, false alarms will destroy the whole point of the alarm.

The windows of the boat are plexi, and the main hatch will almost allways be open. This alarm is not meant for stormy days, but rather the calm days when your on low alert.

As i see it, from a very uneducated point of view, even 90% lost packages would be okay if you accept a response time of 30 seceonds. it cold be implemented so that when 15 seconds passed since the base spoke to module A, it will start pinging it. If it has had no response for 15 sec, it sounds the alarm. Or am i way off track here?

Could one long antenna along the whole length of the boat do the job?

Thanks for all your help guys!

 

Ok, so you think i'm better of sticking with plain transmitting units? Will the tranceiver be using that kind of power even when its not receiving any data?

What would happen if i used the original 400ish Mhz receiver with an antenna that went along the length of the boat? Or the 2,4Ghz tranceiver for that matter, but only using the sendfunction for the modules.

Battery time will be an important factor, as having to remove the modules every couple of days will be very inpractical. As long as i get over three weeks i will be fine, because i doubt we'll do any longer runs than that. While ashore the modules can be left by themselves recharging.

Thanks