Looking for a circuit switch operated by velocity

I'm not at all electronically minded, but am looking for a small electronic circuit switch that is operated by velocity. Many years ago, I used to have a Casio watch that had some sort of switch for the light, where if you turned your wrist 90 degrees at a reasonable speed, the light would come on for 10-15 seconds.

I am now looking for some sort of component that would do the same job, and don't have a clue where to start.

Any help would be greatly appreciated.

Thanks.

Making something opereated by velocity is difficult because it needs to measure the relative velocity of surrounding objects or whatever you consider your reference frame. For example, a GPS can do this because it determines its position relative to the earth periodically and can therefore calculate velocity.

It is much easier to measure accelleration, because that can be done from inside a inertial reference frame. Note however that the force of gravity then can't be distinguished from accelleration due to change in velocity.

There small electronic chips that can measure accelleration and report it either via a voltage or digitally. This, with some firmware, can be used to detect specific gestures like you describe. Just like there are small electronic accellerometers, there are also small electronic gyrsoscopes. These report the angular velocity instead of angular position like mechanical gyroscopes do. These small gryros and accellorometers together are used in various motion and gesture detection schemes, like the wii toy, for example.

There are also very basic motion sensors. These look like a switch electrically that randomly opens and closes as a result of motion, and freezes state when stationary. They detect motion only, and are not useful for detecting specific gestures or determining velocity.

• Olin, thank you for your reply. I had the simpler version in mind - it would just need motion to trigger a timed switch that would keep a light on for a set period of time, but would need to be strong enough to only be triggered by a deliberate movement. Any idea on who might make or supply something like this? Smaller the better. – Chachi Dec 17 '11 at 16:52
• @Chachi: Various companies make rolling ball motion sensors. I evaluated one a few years ago, but can't seem to find the company name right now. I sortof remember the company was in Vermont or New Hampshire, although maybe that was just a local sales office. Search around for "motion sensor". – Olin Lathrop Dec 17 '11 at 17:02

In its simplest form, what you are looking for is basically a weighted spring inside a metal tube. As the device is moved, inertia causes the weight on the spring to lag behind the rest of the module. Enough change of velocity and the weighted end of the spring will contact the wall of the tube.

Sufficient "debouncing" will be required in your circuit, as the spring will oscillate back and forth afterwards maybe contacting the side of the tube many times in the course of one movement.

The amount of movement required to trigger it will depend on the stiffness of the spring, the size of the weight and the diameter of the tube.

At the other end of the scale you have Accelerometers. These are small chips which you often find in smart phones and such to detect movement. They are far more sensitive than the spring-weight-tube system (and smaller) and can measure precise amounts of movement and the direction they occurred in.

• No, velocity does not cause the weight to change position inside the tube. It seems you are confusing velocity and accelleration. – Olin Lathrop Dec 17 '11 at 15:44
• That should have said "change of velocity" - edited, thanks. It's not just acceleration, but deceleration - which I supposed could be expressed as negative acceleration ;) – Majenko Dec 17 '11 at 15:45
• Thank you Majenko. Yes, that sounds about right, but it sounds like quite a bulky solution... did you have a particular item in mind? The accelerometers sound like they would do the job, but would be overkill for the application I have in mind. – Chachi Dec 17 '11 at 16:54
• I used to have a bicycle alarm based around one of these items. In total the sensor measured about 1.5cm by 1cm and detected whenever anyone moved my bike. It didn't stop my wheels from being stolen though :( It is perfectly possible to scale the device down somewhat. The stiffer the spring the smaller the tube can be. If you're not careful though it turns into more of a vibration sensor. – Majenko Dec 17 '11 at 16:58