Need ideas for a very low power latch release mechanism.

I want to build an programmable animal feeder to dispense small amounts of food.

Rather than deal with some complex auger or motorized system, and worrying about it jamming, or dispensing too much, etc. I decided the most simple and efficient way to do this could be to just build a few small compartments, that basically have a trap door at the bottom (the device will be hung from above).

That has the benefit of perfect proportions, nearly impossible to jam even with almost any shaped food, simplicity, small size, and very low power usage because gravity is doing most of the work.

However I'm having a tough time figuring out what sort of release mechanism to use to let the trap doors drop open. I want to make this whole device battery powered so the less power usage the better. It would be nice if they could be triggered with less than 40ma so that they could be triggered directly from an arduino pin with no relays involved. Just need to pull a pin, or open a latch, or something like that. Any ideas?

I would still use a transistor or MOSFET a to drive the release to protect the Arduino from excess current or spikes.

A pin attached to a solenoid would be simplest method. A quick pulse would not use too much power.

Weedpharma

I know this is not exactly what you need but I found This video about a parachute release mechanism. I'm pretty sure you could use some of the ideas for your project. Those tiny servos will run from the 5v of an arduino with no problem.

Saw a unique idea once involving a "trip" solenoid.

The armature was held closed using one of the rare earth magnets that are everywhere these days.

The magnet was located in the centre core of the solenoid close to the armature.

When the release was required, the coil was energised which put a reverse field on the armture and it trips.

Unit was inside the workings of an earth leakage combination circuit breaker.

Very small unit so not able to open feeder trap doors but the idea is there, not at 40mA though.

Have a look at escarpment (tick tock) mechanisms. Also small relays, although the travel is small the force is high at the hinge end. Look at how gun triggers work.

Its possible to make an escarpment mechanism to do what you want using a rubber band cardboard and scissors and a few pushpins. I tend to use laser cut acrylic these days though. Finding the right solenoid is the normal problem but electric clock mechanisms are cheap and use very little power, the minute hand is geared and will easily do what you need

Edit Another tip. A cd has a 15 mm centre hole which is a common size of dowel. A small notch in the rim makes an effective release mechanism. Driling a hole at a suitable part of the radius allows mounting a rubber band. As the disc turns a fair amount of travel can be had.

do you have to fill these ? or is there an automaitc fill ? if you have to fill, then you could latch them.

I think a weighted trap door that would remain closed by it's own weight, and then, the latch. fill the chamber and the weight sits on the door. once the latch is released, the weight inside would force the trap door open. and then once empty, the weight of the door would swing it back up. possibly to latch itself.

with some clever engineering, you might be able to design a latch that also resets on gravity, an eccentric latch comes to mind. something that carrries a weight that only needs a slight push to activate, but re-sets when it swings down.

how do you know which animal is at the feeder. switching on is only needed when animal is there, so a simple leverswitch is good, then for your output a coil and a transistor is best energize only 0.1 seconds is enough to open. use a servo to turn the puller below the needed hopper.

Thanks for the input. It seems right now its a debate between a modified servo, or solenoid.

The idea of a servo came up a few times, and I thought of it myself, however typically the way a servo works I believe it always draws quite a bit of current just while operating, and holding its position. I could get around this by taking a micro servo, removing all the control circuitry and also removing the end stops so it just runs 360, then simply wiring up a positive and negative wire directly to the motor. That way it will draw no power at rest, and will run simply off a bit of positive voltage applied. This could be an option, however it is a bit of work having to do all this modification to each servo. It would be nice if there was something that did what I wanted out of the box.

So the solenoid option may also be good, except that I think they may draw much more current than desired, based on the way they operate. I looked up micro solenoids on ebay, and the first one I found that had actual specs wanted 170ma at 6v or 80ma at 3 volts, and they seemed a bit pricier with more limited selection. It might be a good idea if I do use a relay, but definetly doesn't seem likely I can power it off a arduino pin.

Guess I'm leaning towards modified servo right now, I think I could run a couple modified micro servos directly off the pins with no issues.

On the cam grinder I built a few years back, I used a low power solenoid (all I had available at the time) and a trip mechanism for the end-of-grind much the same as the old style rat trap.
Works very well, sensitive, able to hold large force, small trip force, manual reset.

jetpack_gorilla: Thanks for the input. It seems right now its a debate between a modified servo, or solenoid.

Guess I'm leaning towards modified servo right now, I think I could run a couple modified micro servos directly off the pins with no issues.

This was why I suggested clocks. They are basically miniature stepper motors with a gearbox.

instead of all the work on a servo, just buy a gear motor.

simple h-bridge.

He wants to work from an arduino output pin.

That motor requires far too much current.

Clock motor can be driven directly.

http://www.cibomahto.com/2008/03/controlling-a-clock-with-an-arduino/

I can see the desire to use a tiny motor and then gear the heck out of it. uses less power, just takes more time. for this project, it should not be an issue.

the OP said he might go with a servo, and then cut out the running circuitry to element the stand-by current drain. not sure why not just taking power off it would not do the same thing.

then add a relay, (another coil and more current) to turn it on and off.

I think the release mechanism is the key. how much power will be needed. a clever design, like an over-center lock or even a gun lock could work. low power, controls a high power device.

dave-in-nj:
I think the release mechanism is the key. how much power will be needed.
a clever design, like an over-center lock or even a gun lock could work. low power, controls a high power device.

Yes that’s the tricky bit it needs precision to work properly.

Speed, cat flap solenoid broken , above method opens cat flap in under 0.5 sec.
Driven at about 100 hz.

Used a 555 though not an arduino.

Edit
This makes a fair force with a very sensitive sear (gun) type release.
cries out for a low power solenoid but they are ver hard to find.
Clock motor can be had for £1

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