A friend of mine is biologist and needs to capture regularly ducks to control their health.
He has made a jail which door can be released easily (guillotine door), but this jail is going to be placed on a platform in the middle of a small lake. He has asked for my help to build a device to remotely close the door.
Our first idea was a Hobby remote controller and receiver to activate a servo to close the door.
But there are some limitations which worries us:
- The platform is in the middle of the lake (very hard to access), so the batteries should last at least 24h.
- Would it be better to use some kind of clock to disconnect it during the night?
- The servo cosumes energy, even when it is not moving, and the door can be closed only once. After that it is necessary to reopen it manually. Would it be better to use a motor and a relay? What kind of receiver could we use?
- The humidity of the lake could be a problem?
- High temperatures during the day could be a problem?
When it comes to humidity I would dip everything in Plasti Dip http://www.plastidip.com/ to make it waterproof. And if size or weight is of no concern, then I’d use a regular motorcycle battery. That would probably give you months of operation.
So, just closing the trap door huh? Opening is done by hand.
A very simple latch should be able to that. Gravity will make the door fall down. The latch only has to hold it up until a motor retracts the latch. Could be as simple as a pin through a hole in the door, then pull the pin.
Humidity and heat are a packaging issue.
A receiver would hardly need more than .5 Ampere continuously. The motor requires no more than a few Ampere when it operates for several seconds. So your battery would need a capacity exceeding 12 Ah. This is my most pessimistic assessment. The battery should also be able to deliver a short current of few A.
I would not worry about power consumption during the nightly hours. That receiver is propably way more economic than my estimated .5 A.
I would use a transistor, not a relay, to switch the motor.
Ordinary RC receivers give off servo signals, rather than on/off digital signals. There lies a nice electronics challenge.
An RC servo signal is basically a PWM signal with a period of 20 ms and a duty cycle varying from 5% to 10% (1ms on to 2ms on). The trick is to detect a signal change. For example a change from 5% to 10%.Perhaps the right choice of Capacitor/Resistor combo could be charged by the PWM. Chose the combination so that a 5% signal would sink through the Resistor, but a 10% signal would charge the Capacitor above the base-emitter voltage of the transistor.
Although, you might be able to just use the absence/presence change in the signal. Just switch your RF transmitter off/on.
So question for the RC gurus in this thread: What kind of signal does a receiver give to a servo when it is not receiving any RF signal from the transmitter?
You know, with setint and flags, you can wake up a picaxe from sleep with a background serial data receive. I.e. just a set of $5 Rx/Tx units to 2 picaxes --Rx and the transmitter. The reciever is basically in sleep mode all the time saving a ton of power. Yes the the RF unit has to stay on but hey, 1 outta 2 ain’t bad. You send a little wake-up bit or two and bam, the unit wakes up and sets the trap.
Also, in my mind a solenoid from a car’s electric locks seems to be the best choice. Sure both the relay and the coil take a few amps but really, they will be on once --for what, 1 second? That power consumption does not even figure in.
If you’ve found a good high capacity battery, but it can’t supply the required current to quickly activate the solenoid, you can add a tank capacitor to the solenoid power lines. This will allow the solenoid driver to briefly supply more grunt than you’d get out of the battery alone.
Here’s a thought about your environmental concerns. Choose one or the other:
1) make it all water tight and keep the whole unit under the platform, nice and cool; the other) make it all heat proof and keep the unit high and dry on the roof of the trap.
