These user-facing components at the heart of most remote controls have many technical subtleties.
I haven’t yet upgraded to using a single, universal remote-control unit to manage the diverse electronic products in my “media center.” That fancy designation is very misleading, as the room includes ancient electronic devices, including a 26-inch CRT TV (circa 1995), which is still in use paired with an over- the-air (OTA) digital-converter box, a 15-year-old VCR, which recently stopped working but also has a DVD player that is still used occasionally, and some other oddball items.
As a result, as I suspect is the case with many of you, I also have a collection of infrared (IR) remote controls. Some of them were shipped with whatever device they control, but a few are $15 “universal” replacements for which you have to one-time key in the vendor code for the one it is replacing, shown in Figure 1.
Note that there is potential terminology confusion here: a “universal” remote control can be a single, sophisticated unit that can be user-programmed to support multiple, unrelated units; however, it also refers to a simple replacement unit that can be configured to replace a basic, single-unit remote control supplied by the OEM. Same word, but with very different implications.
These remote controls are now standard peripherals for almost every consumer appliance, device, or product: air conditioner, ceiling fan, thermostat, video screen, controller boxes, and more. As infrared devices, they do not cause or are affected by RF interference (RFI), they are line of sight, they are safe, and they (and their complementary receivers) are low cost compared to an RF wireless device using Wi-Fi or another unlicensed RF band, and they have no regulatory restrictions. They are immune to spoofing or eavesdropping, unless the interloper is in very close proximity.
In most of these units, the keypad is a single-piece rubber-like mat which is made from a sophisticated silicon elastomer, as seen in Figure 2. This technology is used in many devices, ranging from low-end remote controls to higher-end, partially sealed devices such as industrial-grade keypads suitable for harsh and even wet conditions. They are even used in some full-size QWERTY keyboards as they are spill and dirt-resistant.
An elastomeric keypad uses compression-molding properties to form webbing around an electrical switch. Silicone rubber is soft and flexible, so it compresses under force. When the user pushes down on a button on an elastomeric keypad, the silicone rubber flexes to make contact; when the user lets go of the button, it reverts to its original shape. This behavior is the basis on which elastomeric keypads operate.
The bottom of each key, which touches the underlying PC board switch-contacts tracks, is coated with a conductive material such as graphite (although there are several other options). When the key bump is pressed by the user, its underside conductive pad contacts two traces on the underlying circuit board and effects a contact closure.
There are many production virtues to this keypad. First is cost, both the low bill on materials (BOM) cost as well as the overall assembly cost. The rubber pad has all the keys built into it as a single item, and so no discrete, individual keys are needed. Assembly is simplified because a single part – the rubber pad – is simply placed on top of the remote control’s circuit board.
Unlike a flat membrane-type keypad, the rubber-bump keypad has a tactile, tangible feel when the key buttons are pressed; that’s an important consideration for many users. As seen in the photos of various remote controls, there is almost unlimited flexibility in layout, keypad button size (and they do not have to be the same), color, and more.
In short, what’s not to like? You have a somewhat sealed keypad assembly, tactile operation, almost any layout arrangement you want, low BOM cost, and simple assembly — all positive attributes.
The underlying technology
The keypad is made from silicon rubber and is “activated” by user compression. The elastomer can be translucent, which allows backlight, or almost any opaque color
Despite the conceptual simplicity of these keypads, there are many critical aspects to the material properties and dimensions. One representative example, shown in Figure 3, shows the many important dimensions of a typical keypad switch. Of course, these dimensions are related; if one is changed, such as for a larger key, other dimensions will likely have to be changed as well to ensure proper overall switch “feel” and action.
The relationship between actuation force and stroke (travel) is also carefully analyzed, as seen in Figure 4. Longer strokes of 1.5 to 2.5 mm offer better tactile feel as they have 50 to 120 grams of actuation force, but also have less life when compared to shorter strokes of under 0.5 to 0.75 mm with higher actuation force of 150 to 200 grams; one good compromise is 1.2 mm and 135 grams.
There are many other fabrication options. The key legends can be printed on the surface of the associated plastic housing of the keyboard assembly (lower cost), or on the key tops themselves. Printing on the key tops can be done using screen printing, longer-lasting laser etching, or it can be molded in place. For surface protection and longevity, the keypad can have a matte (non-shiny) finish, polyurethane coating, or even an epoxy coating for increased ruggedness.
There are even choices in contact-pad materials, which can include graphite pads, black carbon pills, electrically conductive printing ink, or metal domes with gold, copper, or nickel for maximum longevity, as shown in Figure 5.
Potential for problems — and a solution
After a few years, some (but not all) of these low-end, single-device, OEM-supplied remote controls often start acting erratically and even stop responding to key depressions. While the problem may be their ultra-cheap batteries — easy enough to check and replace — in many cases, it is the degradation of the material in the key buttons themselves. That problem doesn’t lend itself to an easy solution.
Here’s the situation: A high-quality elastomeric keypad has a long life. Ruggedized ones are resistant to moisture, temperature extremes, physical abuse, and more, and can provide reliable service for many years and use cycles.
However, many lower-quality ones are also in use. These are usually the ones that come packed along with a product that typically retails for $100 and below; sometimes, they are also sent with higher-end systems.
These quality concerns are also relevant to the replacement remote controls available from online stores and retail for $15 to $25. This includes both the “universal” units that can be programmed to control any one of many end-products, as well as product-specific identical replacements. Clearly, their BOM and manufacturing costs are very low, as is the quality in many cases. The problem is that you can’t necessarily distinguish a better one from a lesser one just by looking at them.
The problem is that over time and accelerated by use, the elastomeric material deteriorates and oozes out a thick liquid plasticizer as part of a “natural” depolymerization process. This substance flows between the key button and the associated PC board traces and prevents electrical contact. The keys become intermittent when pressed and eventually don’t work at all; the problem is more severe with the most-used key contacts.
What can you do? There are several options once you open up the remote control case; a task can range from trivial to hard, depending on the way the top and bottom halves are joined. Then you take a cotton swab soaked in rubbing alcohol, clean the contact areas on the bottom of the button and on the circuit board, and reassemble the unit.
For more definitive results, you can cut tiny pieces of aluminum foil and glue them to the button contact areas to compensate for the problems caused by oozing and loss of low-resistance material at the button surface. This process requires nimble fingers and dexterity and is certainly not for the average person. (For a good discussion of the problem and various solutions, see “How to Repair a TV Remote Control or Alarm Keypad With Kitchen Foil”).
Then you go test it, and that’s the problem. Determining if the fix worked requires pointing the reassembled remote control at the unit it manages and trying it out. Sounds simple enough, but it’s a hassle. If it doesn’t work, you have to repeat the disassembly and cleaning process and try again.
The entire open-clean-reassemble-test cycle may mean going from workbench to TV area several times, and it is both frustrating and time-consuming. Some remote controls have a single small LED that flashes when any button is depressed and makes contacts This very small feature makes it much, much easier to assess if you have properly cleaned the button and PC board so they are making consistent and reliable contact. As an alternative, some smartphones can be programmed to recognize the presence or absence of the IR pulse from the remote control, but I haven’t investigated this further.
Conclusion
The modern infrared-based remote control is a clear story of the evolution of a solution to meet a perceived need, starting with hard-wired controls, progressing to an alternative ultrasonic approach, and then taking a radical shift to optical technologies. It is made possible by the development of IR LEDs as emitters and phototransistors as complementary receivers, along with ICs for signal coding and decoding. The result is a low-cost, generally reliable, highly effective unit which is now ubiquitous and is taken for granted. It also demonstrates how technology advances filter down in price as they increase in functionality, while being simple to use without the need for setup, initialization, or other complications.
References
Silicone rubber keypad, Wikipedia
Pros and Cons of Elastomeric Keypads, Nelson Miller Group
Rubber Keypad Designer’s Guide, Diamond Electronics Ltd (via Internet Archive Wayback Machine)
Silicone Rubber Keypads, Diamond Electronics Ltd.
Silicon Keyboard Design Guide, Xiamen Better Silicon Rubber Co., Ltd
Custom Design & Manufacturing of Silicone Rubber Keypads, Silicone Dynamics
Rubber Keypad Comparison: Conductive & Non-Conductive Construction Differences, Epec LLC
How to Repair a TV Remote Control or Alarm Keypad With Kitchen Foil, Hub Pages
Related EE World content
The winding path to the infrared remote control: part 1
The winding path to the infrared remote control: part 2
Insight: The internal working of a remote control
Is Li-Fi “To Be” or “Not To Be”? Part 1: Context
Is Li-Fi “To Be” or “Not To Be”? Part 2 – Attributes
Is Li-Fi “To Be” or “Not To Be”? Part 3 – Status
