Six Position and Level Sensing Applications for Home Appliances (Part I)

Even with today's energy-efficient designs, modern appliances, especially refrigerators and freezers, are still major energy consumers in any home. In fact, according to the U.S. Energy Information Administration's Residential Energy Consumption Survey, refrigerators are the second-largest electricity user (13.7%), second only to air conditioners (14.1%).

As with most modern home appliances, the microprocessor-based electronics in today’s appliances provide greater control over a variety of functions, enabling designs to be more energy efficient. These modern electronics also make use of a variety of sensors to ensure reliable, safe, and more energy-efficient operation (Figure 1).

Figure 1: Common sensor applications in a typical refrigerator

Both reed switch sensors and Hall effect sensors are very reliable and will operate for millions of cycles with long term integrity when used with an electrical load at a microprocessor logic level. These devices are both operated by a magnetic field. Reed switches and sensors consume no power. Hall effect devices are semiconductors and consume a small amount of current in both the activated and inactive states.

Application 1: Door Position Sensing

Refrigerators equipped with microprocessor control units are compatible with contactless sensing technologies such as reed switches/sensors and Hall effect sensors. These technologies are very compatible with the low DC voltage and current requirements of microcontrollers.

The traditional mechanical and plunger switches that were previously used to directly switch 120 VAC incandescent lamps are no longer necessary. Modern refrigerators, freezers or wine cabinets using microcontrollers have switched to LED lights to illuminate the interior compartment when the door is opened, which has huge advantages in terms of energy saving.

Figure 2: Reed switch sensor

A reed switch sensor (Figure 2) or a Hall effect sensor can be used to detect the position of a door. For example, in a refrigerator, the reed switch sensor is mounted on the frame of the unit and a permanent magnet actuator is mounted on the door. Each reed switch or Hall effect sensor and magnet actuator are placed in such a way that when the door is closed, the reed switch or Hall effect sensor is activated and the light turns off. When the door is open, the magnet on the door is not within the activation range of the sensor, so the sensor is disabled and the light turns on. The controller uses inverse logic to control the light in relation to the sensor activation.

The sensor units are not visible as they are mounted behind a panel of a frame or door, greatly enhancing the aesthetics of the device. Unlike mechanical or plunger switches with visible levers or buttons, these contactless sensors operate without the need for physical contact.

Application 2: Water/Ice Dispenser Lever Position Sensing

Currently, the water and ice dispensers located on the front of the refrigerator door do not include sensors to detect their own open/closed position. Adding sensors will allow the microcontroller to support closed loop operation of the dispenser. There are several options available, including digital signals or rotary ratio metric outputs, or even linear ratio metric outputs.

Figure 3: Littelfuse 55140 Series Hall Effect Sensor

The digital version would be a simple reed switch or Hall effect sensor mounted on a fixed frame, while the magnet would be attached to a moving rod (Figure 3). When using this system, the water or ice is in one state of fill or flow rate. That is, it is either on or off.

However, using a ratiometric output Hall effect sensor provides continuous feedback relative to the precise moving rod/magnet position on the linear scale. With a microcontroller monitoring the precise position, the system can dispense water at a variety of flow rates. To improve long-term reliability, a non-contact sensing solution is preferred. A programmable rotary Hall effect sensor would be ideal for this application.

For example, the output voltage of the Hall Effect sensor can be set to 0.5 Vdc when the lever is in its normally open position, while the maximum swing of the lever movement can be set to 4.5 Vdc. Between these values, the flow of water can be controlled depending on the exact position of the lever arm.

Application 3: Drawer and compartment cover position sensing

Many potential sensing applications focus on individual compartments within refrigerators and freezers. A butter compartment door or drawers for produce, meat, and deli items could also be fitted with sensors to detect opening or closing.

All of these sensors can be reed switches or Hall effect elements with digital outputs. The magnets will be mounted in the moving tray or drawer, while the sensors will be fixed in a fixed position on the frame. If the tray or drawer is not closed properly, a light may be activated on the device to alert the user of the problem.

To be continued:

This Friday we will continue to discuss: ice bucket level sensing, temperature control dial position sensing and drain pan level sensor applications