Alps Alpine - Mechanical Pushbuttons with Memory
While various circuits that enable an on-off latch function are known, a simple electromechanical solution can be the best approach. Alps provides a variety of high-quality push buttons - in this tutorial, we will look at the SPEF210101 and SPEF220100 SKUs and provide information on how to select buttons intelligently.
Why Waste Time on Selecting Buttons Physically?
Marinetti's discovery of tactileism paved the way for this article - he postulated that human beings interact with their environment not only via the eyes but also via the sense of touch imparted via the hands.
Especially when it comes to test equipment, more valid words have rarely been spoken - an excellent example is the lukewarm response to a recent release of portable test equipment by a popular American vendor. The main complaint was not concerned with the relatively high price of the trinket. Instead, users complained bitterly about the use of a foil keyboard, which feels cheap. Due to that, anyone working on a product where a perception of high value is required is well advised to perform a physical casting of buttons before committing to a particular SKU.
In the case of Yours Truly, this takes the shape of a large distributor order with ten or more different SKUs, which are then tested by handling them collectively.
Adding bodies to the handling panel is a good idea - usually, a consensus for one particular unit will emerge. Continuing the casting run makes good sense for flexibility reasons: he who qualifies more than one SKU can switch in case of shortages. Early price comparison is of high importance. Electromechanicals are traditionally a field of business where the final price is widely elastic.
The same component can cost significantly more at one vendor. Due to this, performing a price comparison is vital and should be done early during the selection phase. If this is omitted for reasons of time pressure, many SKUs might be ignored because they are expensive at the distributor used for selection. As an example, look at the table below, which compares one SKU at various price breaks.
Table 1:
Keeping the Pushbutton and Tactile Switch Apart
In principle, a pushbutton and a tactile switch - by and large - look the same: distributors tend to mix them up sometimes, which is why Yours Truly's latest button sourcing run yielded a variety of non-suitable button SKUs. From a definitional (and functional) point of view, keeping the two types apart is relatively simple.
A tactile switch holds its state when pressed, reverting to its default state when physical pressure is removed.
A pushbutton, on the other hand, changes its state each time it is pushed. This makes them ideally suited as a power switch using a circuit topology similar to the one shown in the flux.ai sketch below.
Figure 2.
When determining the suitability of an unknown button, using an ohmmeter is highly recommended. The reason for this is that many pushbuttons, nowadays, are constant - as an example, look at the two figures that show an SPEF210101 and an SPEF220100 next to one another.
Figure 3.
Figure 4.
While the SPEF210101 changes its physical shape when switched on and off, the mechanical toggling action in the SPEF220100 does not affect the idle position of the plunger. This has benefits from a mechanical point of view. On the other hand, the depressed state of the pushbutton is a visual cue for users who want to quickly find out whether the product is turned on or off.
An additional factor of importance is the resistance of the switch: given that the power switch is usually directly in the current flow, its resistance can lead to temperature losses and (in the worst case) to early degradation. In the case of the button used here, the data sheet contains the resistance data in the figure.
Figure 5.
Of course, additional measurements can be performed with an Ohmmeter. If a Kelvin capability is present, use it to get better results. At https://youtu.be/LAmc1_PtYAs, a video illustrating the benefits of a Kelvin connection and an example probe design can be found. Another important criterion is the maximum breaking voltage of the part.
If it is too low, the current flow can either not be broken reliably or will be broken down with issues. Further information on switch-related topics, such as fretting corrosion, can be found in the book “Reference Guide Trilogy of Connectors English” - interestingly, Wurth does not provide PCB mountable pushbuttons such as the ones discussed in this article as of this writing. They do, however, offer larger versions of a similar functionality.
Selecting the Correct Component Footprint
Engineering experience shows that connectors and similar parts work best when selected from a through-hole SKU. Sadly, as of this writing, the Alps family is only available in surface mount variants. A careful look at the bottom of the components reveals the structure shown in the figure.
Figure 6.
The snout structures do permit a through-hole-like matching; furthermore, the way the pads are arranged ensures that shearing forces to the button are limited. This way, users can expect relatively high robustness even though the part is surface mount.
Visuals Using a Video
Should you prefer a video looking at the topic at hand, don't shy away from clicking the link https://youtu.be/nE87wy9RB8s. It compares a wide variety of additional buttons and also has a bit of additional information about how to use the OEMsecrets website more efficiently.
Conclusion
While the relationship between mechanical and electrical engineers is known to be somewhat rocky, selecting smart pushbuttons for electronic devices is not an impossible task, even for an electrical engineer. Following the steps outlined here should enable you to find affordable and high-quality buttons yielding great user satisfaction - may your designs always work flawlessly!