I've been asked to help with security at a farm. It's been troubled by late night theft and vandalism. Several of the fields and barns involved are several miles away from the farmhouse and don't have power. The current plan is to use an Xbee / arduino combination to detect when a farm gate or barn door gets opened, and then transmit that data back to the farm house.
My question is how to power the device. What I'd like to do is to harness the force used to open the gate to power the device, I was thinking of adapting this little wind-up LED torch.... http://www.maplin.co.uk/wind-up-micro-keyring-torch-353499 I figure I could remove the wind up handle and devise a mechanism whereby opening the gate turns the generator.
The plan is that instead of having an always-on device constantly monitoring a reed switch (or similar mechanism), the normal state of the device would be off, and as soon as the door is opened the tiny generator would produce enough power to switch the device on for just long enough to transmit a single message via an Xbee board.
Whatever solution I choose needs to be small and discrete, so wind or solar power are probably out. I could probably just use a battery, but where's the fun in that? Has anyone worked on similar problem or come up with a similar solution? Does my suggestion sound feasible? I know there are pizo devices that have been used to convert movement to power to charge phones, but I've not seen anything suitable for adaption for this situation.
OhMyCod:
The plan is that instead of having an always-on device constantly monitoring a reed switch (or similar mechanism), the normal state of the device would be off...
An AVR processor in the deepest sleep mode is essentially off. A pin-change interrupt (the reed switch changing states) will wake it. You can use the processor + MOSFETs to control the power of other devices.
Were I in your shoes I would first plan to use batteries to power the device for two reasons: KISS; time to delivery is shorter (miscreants caught sooner).
Were I in your shoes I would first plan to use batteries
To be honest, I probably will. But it won't be as much fun as a home made linear generator!
miscreants caught sooner
The miscreants only turn up every few months. One of the reasons for NOT using a battery is that (knowing my luck) the batteries would be flat the night they next strike!
My approach for such very low frequency events would be to have an extremely low power circuit monitor the (fence gate?) switch, then power up the rest of the circuit up to send a message when the switch is activated. If done properly, ordinary alkaline batteries could last for years.
OhMyCod:
The miscreants only turn up every few months. One of the reasons for NOT using a battery is that (knowing my luck) the batteries would be flat the night they next strike!
I have battery powered gadgets that are used several times a day and go 1 to 1½ years between battery changes. Once you get everything powered off / asleep the internal battery discharge is essentially the only thing left. That number is available from the battery datasheet. A bit of math to determine how often the batteries should be changed (plus a "safety factor") and the problem is gone.
But it won't be as much fun as a home made linear generator!
Agreed. But before pursuing that path you should calculate how much energy is available. Otherwise you could spend a significant amount of time building something that doesn't work.
It's a farm after all, so there should be plenty of mice, rats and other assorted rodents, put them to good use.
Build some rodent wheels (in cages) and connect small motors to them and you have rodent powered generators.
Build some rodent wheels (in cages) and connect small motors to them and you have rodent powered generators.
By far the most interesting idea, but how would you get wild rodents into the wheel and running? If you trap them first, then you have to go out every day to feed them, or build an automated, ultra low power feeder...
Design a device that will trap the rodents, feed them into the wheel until it dies. When the first rodent dies, dump the next one in. You might need to replenish bait though.
Once caught and in the cages, they will naturally run around in the wheel/s.
As for sustenance, there are plenty of feeders and waterers that work on good old gravity
and only need replenishing once a week or so.
jremington:
OK, got that. How do you get them out when they die?
Trap door on the bottom? Maybe some way to tilt or shake the cage into a hole? Or you could just clean them out every few weeks/months/years. Maybe when you'd replace the battery instead?
Can you connect your sensors so that they connect power with a relay to your alarm system. Your relay needs extra pair of contacts. The extra pair keeps the relay coil powered. A kind of latching or holding system. Then when processor has done its job, it turns the power off.
UnoDueTre:
I know the OP said that wind or solar is out of the question, but how about a small water generator if there is a small stream nearby?
My other thought would be a small solar panel that could be concealed and a super Cap. Charge it through the day and then power it over night. You can also add some extra security by including a 'check-in' pulse every day or few hours to ensure that the system is still running. This is assuming you'll have the extra power during the day to do the check. I still think either a super-complicated rodent powered device, or a battery, would be a better solution.
You also need to make sure leaving the door open doesn't drain the battery
