With increasing concerns over the toxicity of mercury and a growing number of restrictions that target the substance, product designers across the world are seeking alternatives to the classic, mercury tilt switch, and many suppliers have stepped up to the plate to provide new options.

The traditional mercury tilt switch has a number of advantages and disadvantages. On the plus side is the virtue of its simplicity. Mercury (Hg) is a heavy, silvery metal that is liquid at room temperature, which gave rise to its popular name, quicksilver.

In a basic, normally open switch application, a sealed capsule containing mercury is tilted off its horizontal axis. Gravity causes the mercury to roll to one end of the capsule, where it covers two electrical contacts, completing an electrical circuit. In an opposite, normally closed implementation, the angular disturbance moves the mercury off a set of contacts to open the circuit.

In addition to simplicity, the ability of mercury to wet over the entire surface area of the contacts gives the arrangement a current-carrying capacity that permits a mercury switch to directly handle line voltage without the need for relays or other control devices.

Fig. 1. Conceptual top view of a simple ball tilt switch. Four pins, isolated from case, form a cage around a ball resting in a hole (left). When the ball rolls out of its resting hole, it makes contact with pins, closing a circuit between pins and case (right)

Fig. 2. Cutaway and external illustrations of a typical Aerodyne TO-5 tilt switch where the tilt angle is determined by the conical shape of the case. (Fourth pin not shown.)

One drawback of such configurations is the lack of accuracy in detecting smaller angular movements, which is why mercury tilt switches historically were used to detect a very large change in angle, typically a change in orientation from a vertical to horizontal axis, or vice versa. In the past, applications included steam irons, lids on clothes washers and chest freezers, space heaters, vending machines, and so on.

But the biggest downside to mercury is its toxicity. Growing awareness of how mercury in the product waste stream threatens human health inspired a migration away from mercury switches in many applications years ago. New developments, such as the impending Restriction on Hazardous Substances regulation in the European Union, have accelerated the mercury-free trend, and have further inspired vendors to seek more inventive alternatives.

Rolling Over

Unlike a more sophisticated sensor providing angular measurements, a basic tilt switch typically yields an either/or type output (on/off, high/low). They are commonly used to indicate a significant change in axis orientation: detecting when a lid has been opened or an appliance has been tipped over. But they can also be used to detect smaller angular changes off axis, as in the old style of home thermostats where mercury switches responded to the slight tug of a bimetal thermostatic coil.

When looking for something other than mercury that responds to gravity, the idea of a ball leaps out as the most obvious solution. In its simplest implementation, a change in angle causes a metal ball to roll off of its resting spot. In doing so, the action either makes or breaks a connection. A ball arrangement is not the only way to create a mercury-free tilt switch, but it is one of the easiest.

The most significant constraint with this approach is that the ball makes contact at only a small point on its surface, limiting its current-carrying capacity. As a result, most ball-type devices can’t handle line voltage directly and are, therefore, used to either trip a relay or provide a signal input to a controller.

On the plus side, a ball-type device can be designed to provide more precise detection of angular change, and the concept provides the flexibility to target switch designs for specific applications.

One company that has a lot of experience with these types of tilt switches is Aerodyne Controls, Ronkonkoma, N.Y. The company designs and manufactures all kinds of motion switches — tilt, acceleration, impact, rotation, spin, disturbance, along with specific variations of each — for all kinds of markets. With literally thousands of designs on file, the firm has developed switches for exotic high-tech applications in the military and aerospace areas, as well as for more mundane products, such as chest freezers and alarm clocks.

One of its more popular basic designs consists of a sealed-glass header where four contact pins form a cage around a ball resting in a conical hole. (See Fig. 1.) When the ball rolls out of the hole, it hits a one of the pins to complete a circuit. In a similar, alternative style, the ball rests at the bottom of a conical case whose shape determines the actuation angle. (See Fig. 2.) A simple disturbance switch, this device is frequently found in gas meters, where it prevents consumers from cheating the gas company. If a consumer attempts to reinstall a gas meter upside down to prevent if from recording gas consumption, the tilt switch detects the change of orientation and activates a latching solenoid that shuts down the meter.

At the other end of the spectrum, one of Aerodyne’s more complex custom designs was for an emergency radio/marker buoy that would be deployed by a submersed submarine in distress to broadcast its location. A tilt switch is necessary to prevent the buoy from transmitting when the antenna is in the water, which would cause circuit-damaging feedback.

To ensure that the buoy only transmits when it is less than 45 degrees off vertical, the switch design combines several features. At the center of it all is ball resting in a conical hole. When the ball rolls out of the hole, it transfers its weight to the switch case, and the shift in weight permits a spring to push open a set of contacts. When the ball rolls back to the hole, its velocity and mass overcome the force of the spring, quickly re-closing the contacts.

The application demanded a tight tolerance: little more than one degree between open and closed states. This required complex calculations of ball mass and velocity working against spring force to achieve the desired accuracy.

Another feature that Aerodyne can add to a ball-style tilt switch is dielectric damping fluid, which would be desired in situations where vibration might cause “jitter” between the ball and contacts, creating a flickering connection. The silicone fluid comes in different viscosities, depending on the degree of damping required. As noted, ball-type tilt switches are more typically used to switch small signal voltages. However, they can be designed to handle line voltage by simply making the ball and contacts bigger. Aerodyne did exactly that for a chest freezer, where the orientation of the door lid operated an interior light. The penalty for this approach, of course, is size. This particular switch has a diameter of about an inch and a slightly longer length.

NKK DS Series tilt switch uses a nickel-plated ball to reflect IR light beam from LED to phototransistor. When the switch is tilted beyond 30 degrees in any direction, the ball rolls off its perch and ceases to reflect the beam.

Eye On The Ball

NKK DS Series switch is available with right-angle or straight PC terminals. In both designs, the ball still operates against gravity in horizontal position.

Using a ball as an electrical contact to make or break a circuit is not the only way to utilize a ball’s sensitivity to angular change. Another approach is to employ sensors to monitor a ball’s position.

For example, NKK Switches, Scottsdale, Ariz., has developed a tilt switch using a photo-interruption technique. Its DS Series switch contains an IR LED, phototransistor, and a steel ball plated with bright nickel to give it a mirror finish. These elements are configured such that, when the ball is in its resting, level position, its mirror surface reflects the IR beam from the LED to the phototransistor. In the neutral, horizontal position, the phototransistor is on, and output voltage is low side at 1.0 VDC.

The ball will roll off of its resting place when the switch is tilted beyond 30 degrees in any direction and will cease to reflect the beam. This turns the phototransistor off, and output voltage goes to high side at 4.0 VDC. The return angle required to reset the ball is a minimum of 10 degrees.

The compact component measures about 9 mm in all three dimensions. One important design consideration is that the component must be constantly powered to operate the LED and phototransistor. The device draws 50 milliamps at normal operating voltage of 5 VDC.

Omron is targeting small portable electronics with its D6B Micro Tilt switch. The device employs a small, magnetic ball traveling in a V-shaped groove. The ball’s position is determined by two Hall-effect sensors, one on each side of the V.

Another interesting way to follow a ball is to make it magnetic and detect its position with Hall-effect sensors, the method used in the D6B Micro Tilt switch, made by Omron Electronic Components, Schaumburg, Ill. The device uses a small, magnetic ball that travels up and down an integral, V-shaped groove. The position and relative movement of the ball is tracked by two Hall-effect sensors, one each side of the V-groove, permitting the component to provide two switched outputs, one left, one right.

When the device is in the neutral position, both outputs remain high. After it is tilted somewhere between an angle of 45 to 75 degrees, either left or right, the relevant output switches to a low signal of 0.5 VDC. Typical return (reset) angles for the device are 50 to 20 degrees, where the output returns to high.

While the tilt switch can be used in common applications such as space heaters and arcade games, its miniature form factor was designed with portable electronics in mind, products like PDAs, cell phones and video cameras. About the size of a pencil eraser, the surface-mount unit measures only 5.5 mm x 5.5 mm x 3.75 mm. Operating voltage is 2.7 VDC to 3.3 VDC.

This switch also requires constant power, but current consumption is extremely low — only 10 microamps (typical) and 20 microamps (maximum) — which Omron hopes will make it even more appealing to the power-sensitive, battery-operated, portable electronics segment.

One important consideration for utilizing the device is to ensure that its location is not influenced by any external magnetic fields (motors, solenoids, etc.) that would interfere with its operation.

Conflicting Forces

A different means of using magnetism is to employ it as an opposing force to gravity. This approach can be found in a switch design developed by Magnasphere, Brookfield, Wis., which exploits the principle of spherical magnetism. The actuator was initially developed to provide an alternative to reed switches in security systems. In its simple version, a spherical rare earth magnet is held in place at the top of a case by its magnetic attraction to the component’s ferromagnetic cap. When an external magnet below the case exerts a pull on the magnetic sphere, the ball snaps down to the bottom and bridges a connection between a center electrode and the case.

Magnasphere has subsequently taken this concept and adapted it into two different tilt switch configurations. In the single-axis implementation, a tube contains a ferrous metal ball that rests atop a Magnasphere switch. Magnetic attraction holds the internal magnetic sphere to the top of the case. When the tube is tilted far enough off the vertical axis, the outer ferrous ball rolls off the top of the switch case, and the magnetic sphere falls back down, either making or breaking a connection, depending on the design.

The omnidirectional implementation works in similar fashion, except that the ferrous metal ball rests in a half-sphere cup.

According to Rick Kirschman, president, the Magnasphere tilt switch offers a several advantages over some of the other mercury-free designs. One is that the holding power of magnetic attraction makes the switch more resistant to vibration-induced contact chatter than some ball-type tilt switches. And that same holding power leads to a crisp make or break of connection that emulates a snap-action switch. In addition, the Magnasphere switch does not need to constantly draw power, like a sensor-based switch.

One of the drawbacks of this design is that is does not respond to angular changes with the same degree of precision as other methods, but Kirschman says that most of the tilt switch applications the company sees are simply detecting the difference between vertical or horizontal status and don’t typically require a high degree of precision.

Specifications for stock parts are not available, since most Magnasphere deals primarily with custom applications.

In this normally closed version of a Magnasphere tilt switch, a magnetic ball (the smaller one) bridges a connection between an electrode and the switch case. It is held in position by its magnetic attraction to a ferrous metal ball (larger ball). When gravity moves the ferrous metal ball off its spot, the magnetic ball falls, breaking the connection. Illustration shows both single-axis (top) and omni-directional (bottom) implementations.