I'm looking at making a very basic, automatic cat feeder for the odd occasion I'm not around at 2pm when she gets fed. I figured I'd keep things simple by having a door that is spring loaded, but held closed by some kind of mechanism.

I'm so blissfully ignorant with regards to electronics, but figured that perhaps a solenoid valve (valve, switch?) would work well, keeping the door shut so the cat cannot open the door itself.

When 2pm comes around, a timer (not figured that bit out yet) would hit the solenoid thus releasing the door to open under spring tension.

Is there an easier approach than this, any other suggestions as to how to keep the door closed?

Many thanks, Ben


3 Answers 3


Solenoids are simple but fairly inefficient ways of holding things for a long duration of time - in fact, a lot intended for momentary use will overheat and suffer damage if they are left on for minutes/hours, though obviously there are those rated for that.

A solenoid that releases a latch could be preferable to one which actually holds the door shut, since the latch release only has to occur briefly. If your timing mechanism can't produce a brief pulse, you may be able to wire a switch into the door such that once the door opens the power to the solenoid is cut; still, think about what happens if the door jams and fails to open. Conversely, if you are holding the door shut with the solenoid you will probably find out soon (while you are around to see the smoke) if it can survive that duty cycle, and if it fails, the door opens early rather than not at all.

Motor based solutions are more complicated, but by taking a longer duration of time to accomplish the opening they can use mechanical advantage to do it with lower amounts of power. Still, you have the cutoff problem. An RC Servo with a 555 pulse generator setup so that when the power comes on it produces a pulse width to go to the open position could be an option, but in order to manually close it you'd need a way to bypass the when-to-open timer to power it, and also a switch to change the pulse width to command the motor to the closed position.

For something that can perform more than one feeding you might look at either a rotary disk where a new compartment opens every 45 degrees and either step motor or more likely a gear motor and simple one channel encoder (probably and photo interrupter). Or you could do something with an auger feeding food out of a hopper. For any multi-day solution, you could then add a webcam or even just a still camera that takes a picture 30 seconds after opening and emails it to your phone, so that you can see the pet happily munching away on food that was delivered... or find out you need to call a friend to go check on the situation.


What you want is a door buzzer catch.

That will open for as long as you apply power, so the door can swing open on the spring. You can then just close the door with a click and it's ready for the next time.

As for the timer, yes, a simple 24 hour socket timer and wall-wart would do the trick.

Of course, you could get a lot more complex with say an Arduino, an RTC shield, an Ethernet shield, darlington array, etc, and have it so you can trigger the opening of the door through the internet...

  • \$\begingroup\$ Careful. A lot of cheap timers (at least the mechanical ones) can't be set for more than a minimum duration of "on" time. Conversely, a lot of solenoids aren't rated to be constantly on and can overheat and suffer damage \$\endgroup\$ Oct 6, 2011 at 16:13

Nothing wrong with a spring-opened door, unlocked by a solenoid, which is activated by a timer.

small one: http://www.goldmine-elec-products.com/prodinfo.asp?number=G18567 larger one: http://www.goldmine-elec-products.com/prodinfo.asp?number=G16036

For the timer you could use one of those cheap timers for switching lights on on a 24-hours pattern. Don't forget to use an appropriate transformer to get the right voltage for the solenoid.

  • \$\begingroup\$ I've got one of those 24 hour timers, can you confirm that a solenoid will close (releasing door) when power is flowing and open (moving back to lock position) when power is taken away? I suspect that would probably depend on how I wire the thing. \$\endgroup\$ Oct 6, 2011 at 11:25
  • \$\begingroup\$ Actually, the second link seems to be spring loaded, so when power is applied it opens, and then springs back in position after. Correct? \$\endgroup\$ Oct 6, 2011 at 11:26
  • \$\begingroup\$ Another question, is there a solenoid that retracts when power is applied? If so what terminology would you describe this as? \$\endgroup\$ Oct 6, 2011 at 12:03
  • \$\begingroup\$ @BenEverard, not worried about grabbing the cat with it? \$\endgroup\$
    – Kortuk
    Oct 6, 2011 at 14:57
  • \$\begingroup\$ There are probably all kinds of solenoids, these two are just two I rembered seeing in a shop I sometimes buy from. But why do you want the latch to move back? I had the impr5ession that all you wanted was to unlock the door at a certain moment. \$\endgroup\$ Oct 6, 2011 at 15:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.