Using Arduino & servo to create snapping motion

Hi all,

Firstly, I’d better start by saying I’m a complete newbie to all things Arduino so taking my 1st steps into a larger world!

… add to that, I’ve never really done anything much with electronics either, so I’m trying to learn lots fast.

I’m happy to learn and figure stuff out, but please just bear with me whilst I do as I’ll inevitably ask a few silly/newbie questions along the way.

Right, now that’s out the way :wink:

I’m working on a project for my daughter where I need to make something that has a “snapping” motion (like a dinosaur gnashing)

I’ve tried adapting a few kids toys but they havent been powerful enough for what I need so I’ve decided to have a go at doing it with Arduino.

My thinking (and I could be wrong so feel free to correct me!), is that I’m going to need a minimum of:

a) an Arduino board
b) a battery pack
c) a switch (it needs to be tiny and easy to push as there will be another part of a seperate system pressing this to trigger the snapping, so I’m thinking 1 of these should do the trick:
d) a high speed servo (After a bit of research I’m thinking this should do the trick as I need something capable of moving approx 1kg at 3cm at high speed:

Optional upgrade:
e) potentially, I’d like to be able to add a soundboard at a later date so we can add a noise to co-incide with the snapping motion. To keep things simple I thought I’d try to focus on ‘a-d’ first, but thought I’d better flag ‘e’ now in case it impacts on the questions below.

Now for the questions!
1)Type of board:
Initially I was looking at a Arduino Uno, but space is limited so I’m wondering if a Micro or Nano would be a better option.

I’m anticipating that all should cope with what I want to achieve, but I’m unsure if there’s anything I may not be aware of as to why 1 would be a better option over the others?

(Also, can Micro & Nano be used as standalone starter boards? My reason for asking is some places seem to imply you need to hook them up to a larger UNO board (or equivalent) in order to transfer the data, but I’ve seen they have a USB on so unsure why this would be necessary if you can simply hook plugin to the board itself. Perhaps the threads/articles I’ve read elsewhere as just out of date now?)

  1. Breadboards:
    Do I need to use a breadboard, or can I simply connect the switch, batteryholder, and servo directly into the inputs/outputs on the board to save space?

  2. Power source:
    Again, this is my lack of electrical knowledge coming through :blush:
    I was thinking of using a 9v battery. However, if I understand it right, the Arduino board needs 5v and the servo needs between 4.6v-6v, so would I be better providing each with it’s own power supply?

Also, I’ve seen threads about avoiding using 9v batteries and instead use a bank of AA’s, so given the above, what do you think would be the best solution?

Thanks all!

I need something capable of moving approx 1kg at 3cm at high speed

That is very big, note that servo you linked to was only rated at 25g so you need one that is 40 times more powerful at least. Such a thing is going to be big and quite dangerous if it is going to snap with that force.

can Micro & Nano be used as standalone starter boards


Do I need to use a breadboard,

You need to NOT use a bread board at all in any way. They are very unreliable especially in something that moves like this. Wire things up directly or use strip board.

I was thinking of using a 9v battery.

One of those small rectangular things? Forget it they have very little capacity and will soon be flat.

The Arduino can take between 7V and 12V on the barrel jack, it contains it's own voltage regulator. Servos are best driven with their own supply but you can regulate it down from a common voltage, but I don't think you design will end up using a servo.

Thanks Grumpy_Mike

I think there may be a bit of confusion re:

That is very big, note that servo you linked to was only rated at 25g so you need one that is 40 times more powerful

As I understand it, that's the weight of the unit. Not what it's capable of lifting?

High Speed Servo 4.5kg/ 25g/ 0.07sec

.... so I thought that meant the servo weighs 25g, is capable of lifting 4.5kg at 1cm (so 1.125 at 4cm) and can rotate to 60degrees in 0.07s at 1cm mark. Is that not right?

I don't need a lot of force so perhaps I've got my understanding or calculations wrong. (It certainly shouldn't be a dangerous amount of force I'm trying to create so perhaps I've calculated too much.)

I wonder if you could get a better snap, if you used the servo to disengage a lever in a pawl and have a spring do the snapping; then on the way back the servo would pull the lever back to re-engage?

First off, that's a goteck servo. I just purchased another model of theirs, and hobbyking failed to list the maximum movement, which was 55 degrees, but with modifications (built in) to my transmitter, I upped that to about 90 degrees. I needed about 150 degrees, so I'm going to have to rig a different block and tackle system to make up for the lack of movement. The goteck manufacturer page does list the total movement if thats an issue for you. I also could not find a programmer for their digital servo's, so if you need to change some of the internal servo settings, I believe you're SOL (Unless somebody knows of one, if so, please link).

Secondly, the movement speed is listed. It's .07 sec to move 60 degrees with no load. You will have a large load, so that number will increase, significantly.

Thirdly, it is rated at 4.2 kg / cm. The farther away your load is from the servo, the higher the torque on the servo will be. I forget the actual math, but I'm assuming your load will not be right against the servo, and may quickly exceed that capacity. There are similar sized servos that are around 15kg/cm, and they get bigger. One of those may be more appropriate for your application.

And finally, think of the inertia in a 1kg mass moving fast enough to make a snapping noise. That's a lot of ummph behind it, and kid's fingers are fragile. 1kg is roughly 1 liter of water in weight, and since it's going to snap, it's going to be hard shelled. I'd be afraid to get my fingers under a hard water bottle dropping to the floor, let alone this.

Maybe if you posted more specifics, we might understand the problem better. I may have misunderstood the snapping part, I'm assuming your making a sound, but if it's gnashing teeth or just quickly closing a jaw, then I'm even more scared for her fingers.

Marmotjr: it is rated at 4.2 kg / cm. are around 15kg/cm

Bzzzzt..... not kg/cm

I think a servo is simply the wrong mechanism to make anything snap. You will burn out the motor or strip the gears trying it to drive a servo beyond where it is free to move.

Maybe if you posted more specifics, we might understand the problem better

Fair point. Right, so the idea is to make my daughter a remote control "Monster Book of Monsters" from the Harry Potter films. In case you haven't seen it, here's a quick video from the film:

As the base for the body, I'm using a decopatch book very similar to this: |500x271

In terms of weight, I'd estimated and rounded up as I thought it was safer to have the capacity to lift more than necessary, but in light of the above I've tried to breakdown the weight of each item to give a more accurate figure:

It's the flap of the book which I need to make "snap" so that in itself isn't very heavy.

However, I need to factor in: a) the fur fabric (approx 125g) b) underlayer of thin leather (125g) c) 4 x eyes (unknown as haven't made them yet) d) tentacles (rubber)... known as I haven't made them yet either... but I'm guessing around 125g) e) teeth (only the top teeth will be attached to the top flap so I'm guessing around 125g but it's going to be clay so potentially might be slightly heavier. f) any cables, clips, glue, etc for attaching all the elements (unknown weight at the mo)

That said, it looks like that's instantly halved the amount of weight it's likely to need to lift in case that helps?

Thanks, makes more sense now.

I would recommend that you use the techniques that you use when making automaton, I have found that this book has a lot of good tips and information in a very easy to follow format.

P.S. I bought mine last year from Tullie House the museum in Carlisle for £10.00 not the silly price on that link. There was an exhibition about automata. I was there about a month ago and they still had a copy in the gift shop.

I did initially look at using something like a cam, but I wanted to make it a remotely operated snapping system (I've got a remote control setup from a donor toy that takes care of the movement of the book in terms of left/right/forwards/backwards, etc), and can remotely trigger the snapping via the same mechanism, but as it's not powerful enough to use the existing setup I'm trying to create a seperate system which is triggered by the 1st.

I think the problem of using a cam (if I understand it right) is that I won't necessarily have control to ensure a "snap" is fully completed?... unless perhaps I figure out the size of cam I'd need to perform X "snaps" completely. Possible I guess.

The way you do it with a cam is to drive it by a motor and have a photo reflector switch turn on an off the motor for just one revolution. You can get the snap by biasing the cam follower with an elastic band.

Anything that "snaps" probably needs to be very light weight so it can move quickly, and importantly, won't hurt a child if the child somehow gets into the snapping action. Maybe a continuous rotation servo with the servo horns flipping the end of a craft stick and rubber band setup.