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Omron B2H-F7W

Contents

Overview

B2H-F7W is a clicky medium-profile Hall effect keyboard switch from the B2C/B2H/B2R family from Omron. It has the following characteristics:

Per the rest of the family, it offers:

The shell is just under 19 mm square.

The force curve is comparatively shallow, and only reaches around 45 cN at its highest. A combination of low force gradient and precise tactile drop (of 10 cN) gives it a smooth, clean feel and distinct tactile sensation.

The click system has a high level of hysteresis. The tactile event occurs at around 2.7–2.9 mm, while the release click occurs around 1.2–1.3 mm. Being Hall effect, the click sound and feel do not physically correspond with actuation, and the keyboard is required to define these points.

The Matsushita/Panasonic DN837 four-terminal redundant-output Hall IC was introduced in 1976. It was superseded in 1979 by DN6837, an improved model with only three terminals. DN837 has since been discontinued, with DN6837 being listed as a replacement in the Hall IC Series Application Notes from Panasonic published in 2002. The terminals are (from left to right) VCC, Output O1, Output O2 and Ground (the Hall IC faces the front of the switch). The Hall element is 1 mm² and is located in the centre of the plastic SIL package.

Design

The switch uses exceptionally long terminals: those of the Hall IC are 8–9 mm, and those of the diode are 11 mm in length. The securing leg is around 6.2 mm long.

Just as with RAFI RS 76 C, the Hall sensor uses a tiny magnet, far smaller than that of the late 1960s Micro Switch SW Series. The magnet is 2.5 mm long and 2.2 mm in cross section. A thin wash of white substance marks the face that points towards the Hall sensor.

View full-size image Side, showing the model number and what appears to be the lot number
View full-size image Base, showing the six terminals and dummy post; the Hall IC terminals are very thin and often bent

The base of the switch has nine holes, eight of them numbered. Positions 1–4 (centre rear) are for the Hall sensor; adjacent to this is a large square hole without a number. Positions 5 (left) and 7 (right) appear to be for the LED terminals in illuminated models. Next to position 7 is position 8, and these two together are used for the diode in B2H-F7W. Position 6 is a square hole that contains a T-shaped metal piece that seems to form part of the damping base for the movable magnet. Since the diode is not securely held and the Hall IC legs are flimsy, this sturdy metal leg is the only solid fixture and it may be used as an anchor for securing the switch to the PCB.

The switch is also highly unusual in not providing stand-offs to hold it away from the PCB. There are two shafts that hold the LED spring legs, but as these are at the rear of the switch, they cannot function as stand-offs. It is not clear whether the switch is supposed to perch precariously on these shafts, or whether they are intended to sit in their own holes in the PCB. B2R has similar shafts for the LED spring legs, but it also has stand-offs.

Operation

B2H-F7W contains a click mechanism invented by Ashikaga Norihiko and Seiyama Shinkichi and described in Japanese patent S53136680 filed in May 1977.

A pair of identical magnets provide the tactile and audible feedback. One magnet is inserted into the plunger, and does not move. The other magnet is inserted into the shell, and is free to move by a small amount. The magnet in the plunger moves past the magnet in the shell as the switch is operated. The magnets are placed such that (according to the patent) their north poles face each other. The magnets repel each other, and this holds the movable magnet in its Down position, preventing it from rattling around within the switch.

As the plunger is depressed, the magnets get closer together. Although you would expect a sharp rise in tactile force as a result, this does not occur.

As the magnets cross over each other, the plunger magnet’s north pole passes over the movable magnet’s north pole and approaches its south pole. At this point, the magnets start to attract instead of repel:

The attraction between the magnets pulls the plunger downward, providing a drop of over 10 cN in force. The movable magnet is pulled upwards towards the plunger magnet, and is driven into the end of its track, causing a sharp impact sound:

As the movable magnet has slid upwards, the plunger will have to be released some distance before the two north poles come into contact, giving physical hysteresis. As this switch is Hall effect, this hysteresis has no effect on the actual electrical actuation, which would have to be implemented in the keyboard controller.

Disassembly

The switches obtained from Yahoo! Auctions in Japan have stiff, fragile plastic. As such, attempting to open a switch is liable to at least weaken one of the four retaining prongs, if not completely snap it off. Casual disassembly is not advised.