Micro Switch PB Series

Contents

• Overview
• Output circuits
  • Untimed pulse contact buffer
• Keyboard types
• Catalogue listing numbers
• Types
• Documentation
• Further reading

Overview

PB Series is a general classification of switches from Micro Switch that take the form of pushbuttons built around existing circuits and switches. There is no specific shape, size or style of PB switch. Some include a control circuit (such as a debounce circuit) and others contain only a microswitch. There seems to be a tendency for them to contain SM Series microswitches, but details on these switches are scarce.

PB was not the only series to enclose a microswitch. KS Series—with models 1KS1-T and 1KS3-T at least—also took the form of an actuator assembly enclosing a microswitch. Here, the microswitch is riveted in and cannot be removed for inspection. These two 1KS models appear to be Micro Switch mount, using the same general style of mount adapter as KB (7A1) switches.

Pictured at right are 1PB871 one-shot keyboard switches (using 311SM29 microswitches) and a 1PB2001 assembly containing a Series 2ED untimed pulse contact buffer.

Output circuits

Untimed pulse contact buffer

The untimed pulse contact buffer circuit provides simple de-bouncing of the pushbutton to which it attaches. The circuit is a flip-flop, connected to the normally open and normally closed terminals of a switch. As the pushbutton is operated, the normally-open terminal becomes live, and this activates the flip-flop. The flip-flop then remains active as the contacts bounce and settle, as the state of the switch contacts has no further effect. When the switch is released, the normally-open output is shut off, and the normally-closed connection conducts again, deactivating the flip-flop. This approach is straightforward, but will only function on a switch with both normally open and normally closed terminals.

The complete 2ED untimed pulse contact buffer circuit as published in the Bulletin ED brochure from 1966 is as follows:

Keyboard types

It appears that the range 1PB870–1PB879 was used for switches suitable for keyboards. These are tall clicky switches that contain an SM Series microswitch, which has both a normally-closed (NC) and a normally-open (NO) terminal. The PB switch is a tall plastic housing that contains the plunger and microswitch. The plunger rides on a pair of coil springs, and drives the microswitch’s actuator arm by way of a horizontally-sliding block worked by an arm extending down from the plunger that runs in a slot in the sliding block. This arrangement provides a longer travel distance and overtravel.

At least two types of PB keyboard switch exist: continuous and one-shot. Continuous switches hold the microswitch in its operated position so long as the plunger is held. One-shot switches have a triangular cam track within the slide block that allows the slide block to spring back once the switch has passed the operating point. The design is very similar to that of alternate action switches, but the simpler cam track does not hold the plunger down. One-shot operation was used in Hall effect switches (the sink pulse sensor types) to achieve N-key rollover; here, the reason for one-shot switching is not known, but it could be for the same purpose. The one-shot type is covered by US patent 3375340 filed in February 1967. The patent does not provide the rationale for the design; it only indicates that the switch is designed such that it actuates when it is pressed but not when it is released (that is, the trigger mechanism only operates when the plunger is moved downwards past the actuation point). Examination of the UNIVAC keyboards in which these switches were used show that all the switches are wired separately, which fits with the use of pulse-type switches.

With no means to conduct accurate examinations, the switch characteristics are impossible to gauge reliably. Total travel of the 1PB871 one-shot type (the only type available for inspection) is around 5 mm. Actuation of the internal 311SM29 microswitch occurs at somewhere around 2.0 to 2.5 mm into the switch travel. The microswitch has hysteresis, with a release point around 0.3 to 0.4 mm higher. The one-shot mechanism that releases the microswitch does not take effect until at least 4 mm travel, making the design sub-optimal: the switch has two unrelated tactile events around 1.5 to 2 mm apart, and switch can be held in its actuated state in between these positions. There is a faint click and tactile event as the microswitch actuates, and a separate strong tactile event and loud click as the plunger actuator arm passes off the slide block and it is fired back to the start position. The combination of two separate click sounds and two separate tactile points gives it a strange feel and sound. The patent does not seem to make any mention of tactile or audible feedback, and it is not clear whether these characteristics are intentional or coincidental.

The keycap mount is very similar design to that of SW and SN switches, but the mount dimensions are smaller and 2SW keycaps fall off.

1PB877 and 1PB878 were used in the following keyboards:

• UNIVAC 1701 keypunch keyboard
• UNIVAC 1710 keyboard

Both of those types use a 311SM29 microswitch. 1PB877 is one-shot and is used for most keys (including space bar); 1PB878 is continuous and is used for keys that the operator will hold down.

In those keyboards, the switches only date back to 1971. Older switches—dating back to the start of 1968—are found in a UNIVAC keyboard marked “4118511-01”. This example uses 1PB871 for most keys, and 1PB872 for at least one of the unmarked modifier keys, suggesting that 1PB872 is continuous (1PB871 is proven to be one-shot). Of the dates shown, most are 6840 and 6850, but one of the 1PB871 switches is marked 6804, nearly a year older than the others. The date of introduction of the series is not known, but it is likely to be around 1967 based on the filing date of the patent. There is (or was) eBay listing 263690218445 for 1PB877 switches with date codes 7804 and 8216, giving us an early indication of how long these remained in production.

The UNIVAC keyboards that use these switches take advantage of the double-throw switches by chaining the common and normally-closed terminals. That is, the normally-closed terminal of each switch is used to supply power to the common terminal of the next switch (or vice versa):

In this arrangement, while a particular switch is held in its actuated state, all switches down the chain become inoperable as their source of power is cut off:

Chaining the terminals in this way simplifies the PCB routing, although the exact reason for choosing it is not certain. However, this does produce a keyboard that is incapable of arbitrary rollover.

Transistor Electronics Corporation offered a nearly identical switch in 1967. It is not clear at this stage which manufacturer arrived at the design first, and there does not appear to be a patent for TEC’s version.

Catalogue listing numbers

PB Series catalogue listing numbers are typically prefixed with the number of poles. The prefix may also encode other characteristics. Examples include:

Types

The following table gives a small number of examples. The complete PB Series range is outside of the scope of this site.

Documentation

• Installation Sheet, Series 2ED untimed pulse contact buffer (PK 8702 0)

Further reading

• Micro Switch Bulletin ED (1966) — covers ED Series electronic pulse circuits, and Series 2, Series 6 and PB switches built around them
• Micro Switch Catalogue 30 Issue 8 (1988)
• PB Series Pushbutton Switches (catalogue extract, 1998)