The familiar reed switch comes with a unique set of properties. These include ON resistance of the order of milliohms, OFF resistance of the order of tera-ohms, total immunity to ESD or electrostatic discharge, hot switching capability of about one watt, and almost zero power operation. However, as all electronic components are shrinking to surface mount sizes of 0402, 0201 and even to 01005 (0.4 x 0.2 mm), the large size of the reed switch is anachronistic. Since 70 years of its invention, the conventional reed switch has been steadily shrinking. What began with a 50 mm long glass tube in 1938 has come down to about 5 mm today.
However, even after a sort of following Moore’s law of ever-shrinking transistor size on integrated circuits, reed switches have now reached a brick wall. The fundamental limitations of physics and manufacturing are preventing the conventional reed switches from going below the 5 mm size. Now, a new technology promises to break this barrier of 5 mm size, while retaining all the desirable properties of the reed switch. Manufacturers are using HARM, or High-Aspect Ratio Microfabrication MEMS technology. For instance, reed switches such as the RedRock RS-A-2515 piezoelectric flexure actuators from Coto Technology is based on this technology.
Alternatives to reed switches also exist. For instance, there are Hall Effect switches, AMR or Anisotropic MagnetoResistive switches, and GMR or Giant MagnetoResistive switches in the market. However, all the above are active switches, requiring a power supply to operate them. This is a disadvantage related to these active switches, as they add to circuit complexity and take up PCB real estate. Active switches require three electrical connections instead of two – one for supply power, one for the return ground and the third for the sensor signal.
Active switches have further disadvantages in that they require external circuit elements such as bypass capacitors or pull-up resistors. This increases the cost and effective size of these multi-component switching systems. There is additional complexity as these can only switch milliamp-level currents, and extra buffer circuitry is necessary for switching higher currents. Active switches are also susceptible to damage from ESD. In contrast, reed switches made from the HARM MEMS technology has none of the above disadvantages.
Switches made from the HARM MEMS technology offer very small size, high-current hot-switching capability, and zero-power operation. This performance makes the technology suitable for a wide range of applications including automotive and medical. For instance, HARM MEMS technology allows making endoscopes the size of a pill that patients simply swallow, nearly invisible and tunable hearing aids, convenient insulin delivery systems, and some exciting new automotive switching applications.
Although motor vehicles are large systems with enough battery power, conventional affordable reed switches have been widely used for a variety of functions such as ABS systems, gear lever position sensing, and door lock control. Smaller reed switches are also necessary in vehicles for sensing various fluids, for instance, brake fluids. Usually, a single reed switch, triggered by a float magnet in the fluid reservoir indicates a binary position – there is either enough fluid, or there is none.