Circuit layout arrangement for key switch signal recognition

1. Field of the Invention

The present invention relates to a key switch signal recognition circuit, and more particularly to a simple circuit layout of key switch signal recognition circuit.

2. Description of the Prior Art

Key switch circuits are intensively used in office automation equipment, industrial instrument, electric and electronic consumers products, and electric home appliances to serve as input interface means between the user and the machine. According to the conventional key switch circuit designs, one key switch button is matched with one circuit loop. The key switch buttons can be switches, micro switches, and membrane switches.

FIG. 1 shows a prior art switch button type key switch signal recognition circuit including a number of key switches. According to this design, a number of key switch contact areas may be arranged on a substrate 10 such as a printed circuit board, and a number of key switch buttons 11 are suspended above the key switch contact area. Each key switch contact area is generally formed of two corresponding copper foil wires 12 and 13 . One copper foil wire 12 is connected to a common point, such as a grounding terminal or a power source terminal. The other copper foil wire 13 is connected to a first connector 14 . The pins of the first connector 14 are respectively connected through a cable 15 to a second connector 17 mounted on a circuit board 16 . The second connector 17 of the circuit board 16 is further connected to a key switch signal reading interface 18 or a key switch signal processing circuit. According to this conventional key switch circuit design, a number of independent circuit loops are needed to transmit click signal from the key switch buttons, the cable 15 must have a number of wires, and the connectors 14 and 17 each must have a number of pins and one grounding wire. Further, when the numbers of the key switch button are changed, the model of the connector, the number of wires of the cable, and the number of pins of the key switch signal reading interface must be relatively changed.

In some key switch circuit designs, an encoder is used to achieve the object of using a limited number of wires to recognize a number of key switch buttons. However, the use of the encoder greatly increases the cost of the key switch circuit. Therefore, these conventional key switch circuit designs do not satisfy current market requirements. For example, in the design of a key switch circuit for a scanner, the factors of ease of use, highly expandability, less number of component parts, and lost cost must be taken into account.

Therefore, it is desirable to provide an improved key switch signal recognition circuit to mitigate and/or obviate the aforementioned problems.

Accordingly, the primary object of the present invention is to provide a key switch signal recognition circuit, which uses the architecture of a circuit layout to achieve accurate signal recognition of a number of key switch buttons, so that the number of key switch contact wires, the number of wires of the cable, and the number of pins of the related connector can be minimized.

It is another object of the present invention to provide a key switch signal recognition circuit, which uses a limited number of wires to achieve signal recognition of a number of key switch buttons. No signal encoder is required.

The present invention will be apparent to those skilled in the art by reading the following description of preferred embodiments thereof, with reference to the accompanying drawings, in which:

FIG. 1 illustrates a circuit diagram of a key switch signal recognition circuit according to the prior art;

FIG. 2 is an exploded view of a key switch signal recognition circuit according to the present invention;

FIG. 3 shows a circuit layout in accordance with a first embodiment of the present invention;

FIG. 4 is a cross sectional view of the key switch button taken along line 1 — 1 of FIG. 2;

FIG. 5 is a circuit layout of a second embodiment of the present invention;

FIG. 6 is a circuit layout of a third embodiment of the present invention, showing the key switch signal output lines respectively connected to an input of an AND gate; and

FIG. 7 is a circuit layout of a fourth embodiment of the present invention showing each of the key switch signal output lines is further connected with a debouncing circuit.

FIG. 2 is an exploded view of the key switch signal recognition circuit of the present invention. FIG. 3 shows the circuit layout in accordance with the first embodiment of the present invention. As illustrated, the key switch signal recognition circuit comprises for example seven key switch buttons 21 ˜ 27 , and three keys witch key switch signal output lines B 1 ˜B 3 .

Each key switch button is provided with an electrically conductive element. FIG. 4 is a cross sectional view of the key switch button, taken along line 1 — 1 of FIG. 2, showing that an electrically conductive element 210 is mounted at the bottom side of the key switch button 21 . The electrically conductive element 210 is preferably made of electrically conductive rubber.

During installation, the key switch buttons 21 ˜ 27 are mounted in respective through holes 21 a˜ 27 a arranged on a frame 2 corresponding to respective contact areas 31 ˜ 37 arranged on a substrate 3 below the frame 2 . A connector 4 is mounted on the substrate 3 , having a set of pins respectively connected to the key switch signal output lines B 1 ˜B 3 .

When clicking either of the key switch buttons 21 ˜ 27 , the corresponding contact area of the substrate 3 is triggered to output a signal to the corresponding pin of the connector 4 through the key switch signal output line B 1 ˜B 3 .

According to the present design, each contact area of the substrate comprises a first wire set and a second wire set. The first wire set comprises at least one wire. Each wire of the first wire set has one end connected to a common line. The other end of each wire of the first wire set is an open end. The second wire set comprises at least one wire respectively disposed in parallel to and electrically insulated from the at least one wire of the first wire set. The other end of each wire of the second wire set is respectively connected to an assigned bit line, which is selected subject to a bit encoding mode. The bit-encoding mode can be known BCD code or any suitable encoding format.

Therefore, when depressing one key switch button to touch the corresponding contact area, the respective electrically conductive element electrically connects the first wire set and second wire set of the touched contact area, causing the respective key switch signal output line to send a key switch signal corresponding to the depressed key switch button.

As illustrated in FIG. 3, the wires of the first wire set (left-sided wires) of the first contact area 31 are connected to a common line B 0 . The second wire set (the right-sided wires) of the first contact area 31 includes three wires respectively connected to key switch signal output lines B 1 , B 2 and B 3 . The right side of the first wire set of the first contact area 31 is an open side. The left side of the second wire set of the first contact area 31 is also an open side. The wires of the first wire set of the first contact area 31 are disposed in parallel to and electrically insulated from the wires of the second wire set of the first contact area 31 .

The wires of the first wire set (left-sided wires) of the second contact area 32 are connected to the aforesaid common line B 0 . The second wire set (the right-sided wires) of the second contact area 32 includes two wires respectively connected to the first, and second and third key switch signal output lines B 1 and B 2 . The wires of the first wire set of the second contact area 32 are disposed in parallel to and electrically insulated from the wires of the second wire set of the second contact area 32 .

The wires of the first wire set (left-sided wires) of the third contact area 33 are connected to the aforesaid common line B 0 . The second wire set (the right-sided wires) of the third contact area 33 includes two wires respectively connected to the first and third key switch signal output lines B 1 and B 3 . The wires of the first wire set of the third contact area 33 are disposed in parallel to and electrically insulated from the wires of the second wire set of the third contact area 33 .

The wires of the first wire set (left-sided wires) of the fourth contact area 34 are connected to the aforesaid common line B 0 . The second wire set (the right-sided wires) of the fourth contact areas 34 includes one wire connected to the first key switch signal output line B 1 . The wires of the first wire set of the fourth contact area 34 are disposed in parallel to and electrically insulated from the wires of the second wire set of the fourth contact area 34 .

The wires of the first wire set (left-sided wires) of the fifth contact area 35 are connected to the aforesaid common line B 0 . The second wire set (the right-sided wires) of the fifth contact area 35 includes two wires respectively connected to the second and third key switch signal output lines B 2 and B 3 . The wires of the first wire set of the fifth contact area 35 are disposed in parallel to and electrically insulated from the wires of the second wire set of the fifth contact area 35 .

The wires of the first wire set (left-sided wires) of the sixth contact area 36 are connected to the aforesaid common line B 0 . The second wire set (the right-sided wires) of the sixth contact area 36 includes one wire connected to the second key switch signal output line B 2 . The wires of the first wire set of the sixth contact area 36 are disposed in parallel to and electrically insulated from the wires of the second wire set of the sixth contact area 36 .

The wires of the first wire set (left-sided wires) of the seventh contact area 37 are connected to the aforesaid common line B 0 . The second wire set (the right-sided wires) of the seventh contact area 37 includes one wire connected to the third key switch signal output lines B 3 . The wires of the first wire set of the seventh contact area 37 are disposed in parallel to and electrically insulated from the wires of the second wire set of the seventh contact area 37 .

Subject to the aforesaid circuit layout, a bit definition table illustrating the relation between the key switch buttons and the bit codes for the key switch signal recognition is obtained as shown in TABLE 1:

It is noted that the present invention requires only three bit lines (key switch signal output lines B 1 , B 2 , B 3 ) for recognizing signals from the seven key switch buttons 21 ˜ 27 without the use of any encoding circuit or encoder.

In actual practice, the key switch signal output lines B 1 , B 2 and B 3 are connected to the respective pins of the connector 4 , and the connector 4 is connected to a cable 5 . According to the preferred embodiment of the present invention, the cable 5 comprises three signal lines and one grounding line. A second connector 6 is used to connect the cable 5 to a circuit board or signal-reading interface 7 .

FIG. 5 shows a second embodiment of the present invention. According to this embodiment, the wires of the first wire set (left-sided wires) of the first contact area 31 are connected to the common line B 0 , the three wires of the second wire set (right-sided wires) of the first contact area 31 each has one end respectively connected to the first, second and third key switch signal output lines B 1 , B 2 and B 3 and an opposite end terminating in two parallel end portions B 11 and B 12 disposed in parallel to and electrically insulated from the wires of the first set of the first contact area 31 .

FIG. 6 shows a third embodiment of the present invention. According to this embodiment, the first, second and third key switch signal output lines B 1 , B 2 and B 3 are respectively connected to the signal input end of a respective AND gate 51 . The AND gate 51 is induced to output a triggering signal Sense to the key switch signal reading circuit (not shown) when the corresponding key switch signal output line B 1 , B 2 or B 3 is logically high.

FIG. 7 shows a fourth embodiment of the present invention. According to this embodiment, the first, second and third key switch signal output lines B 1 , B 2 and B 3 are respectively connected to respective debouncing circuits 91 , 92 and 93 capable of preventing the respective key switch signal output line to output an unusable signal at the initial stage when the respective key switch button is depressed.

Although the present invention has been described with reference to the preferred embodiments, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims.