Our project consists of an old wooden barrel, whose top lid is opened/closed with a simple linear actuator. I am a bit afraid that it might crush somebody's fingers while closing.

How could I detect trapped objects effectively?

Elevators often have light barriers, but the problem here is that the opening has a 360° border. Force sensitive resistors seem to be an option too. However, since the border is a circle, I guess applying them would not work too well (folding issues) and look bad on a vintage barrel.

I'm very sorry if I maybe created a duplicate here and I would like to encourage me to provide links to similar questions. I simply couldn't come up with useful search words for my problem.

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    Surely detecting objects that might become trapped would be a better idea? Commented Oct 27, 2017 at 7:09
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    Preventing would be the best option, but detecting it while it is happening is good enough too. E.g. force sensors cannot foresee it, but they enable us to stop and go backwards before things are getting crushed.
    – dube
    Commented Oct 27, 2017 at 7:22
  • Have a look a this table saw with finger detection emergency brake. youtube.com/watch?v=FbndZtkfcqs
    – user31481
    Commented Oct 27, 2017 at 7:53
  • Some work was done, IIRC, with leaky fibre optics on the top of electric car windows. Pressure makes the light leak out and the window stops moving.
    – Majenko
    Commented Oct 27, 2017 at 8:44
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    sounds like a good use case for those new cheap microwave radar motion sensors, partially shielded to cover just opening.
    – dandavis
    Commented Oct 27, 2017 at 11:39

1 Answer 1


The easiest way to prevent things becoming trapped is to take reasonable steps to prevent things getting near the "danger zone", so a barrier 2 metres around the barrel would be a good start. Probably it would be an impractical idea, but...

A physical barrier of some sort would be the best idea, because that is less likely to suffer a failure than an electronic system (well assuming it has been designed right). If you are using an electronic system then it needs to "fail-safe", when it suffers a failure it must adopt its safe state (barrel open).

You could surround the lid with IR lights and receivers in the lip. They would need to be spaced at a distance where an arm/finger couldn't get between the lid and lip without breaking a beam. This solution would only work over the last few inches, because of the angle of the beams. If all receivers had a signal the it would be safe to close. The problem is the receivers would need to be close together to prevent transgression.

Monitoring the current drawn by the actuator and the position of the actuator might be another. I am assuming that there is an increase in current draw when the lid actually closes, and a similar increase in draw when something is trapped. If the lid isn't fully closed when the current increases then something is trapped.

The final way I can think of is probably the most effective. As the barrel closes don't close it the last few inches, allow it to 'fall' as the actuator descends. This means if there is an object in there the actuator isn't pulling the lid down and the object will hold the lid against gravity. This would be done by having a channel in the mounting block for the actuator 'pin'/ connection point to slide up and down.

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OK Its not an electrical solution.

  • I really like the last idea. I have to check what type of actuator I have, because I was under the impression that it holds the position when unpowered which would make "sliding down" impossible.
    – dube
    Commented Oct 27, 2017 at 8:20
  • I don't think I phrased that very well. When the actuator first pull the lid it will be at the bottom of the 'channel'. Once gravity takes over the lid will 'jerk' downwards as the channel moves and the actuator pin slides to the top of the channel (you might want to damp that with a spring to make it look slicker). Then the actuator's downward movement will slow the effect of gravity and it will effectively be controlling the fall. If you stick you arm in the lid will stop on your arm, and your arm will prevent gravity closing the lid. [Continued....] Commented Oct 27, 2017 at 10:01
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    Thinking about it, if little Joey does stick his head in your barrel you might want to have a cut off if the actuator starts drawing to much current as well as a backup. I suspect the change in current draw will not be enough to prevent saw fingers, but it will just give him a sore head to remind him not to be so stupid in future. Commented Oct 27, 2017 at 10:08
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    That wasn't what I meant, but I think your idea is a much better idea. It hides the ugliness much better. Commented Oct 27, 2017 at 12:13
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    Ah now that I think about it, that would actually enable me to watch for the lower end of the actuator raising, e.g. with a physical switch or a plug which the actuator would tear off! And I could implement it inside the barrel, well hidden!
    – dube
    Commented Oct 27, 2017 at 12:15

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