JPH0411275Y2 - - Google Patents

Info

Publication number
JPH0411275Y2
JPH0411275Y2 JP1985122829U JP12282985U JPH0411275Y2 JP H0411275 Y2 JPH0411275 Y2 JP H0411275Y2 JP 1985122829 U JP1985122829 U JP 1985122829U JP 12282985 U JP12282985 U JP 12282985U JP H0411275 Y2 JPH0411275 Y2 JP H0411275Y2
Authority
JP
Japan
Prior art keywords
leaf spring
spring member
tape cassette
cassette holder
tape
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
JP1985122829U
Other languages
Japanese (ja)
Other versions
JPS6235488U (en
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed filed Critical
Priority to JP1985122829U priority Critical patent/JPH0411275Y2/ja
Publication of JPS6235488U publication Critical patent/JPS6235488U/ja
Application granted granted Critical
Publication of JPH0411275Y2 publication Critical patent/JPH0411275Y2/ja
Expired legal-status Critical Current

Links

Landscapes

  • Packaging Of Annular Or Rod-Shaped Articles, Wearing Apparel, Cassettes, Or The Like (AREA)

Description

【考案の詳細な説明】 産業上の利用分野 本考案はテープレコーダのテープカセツトホル
ダに関する。
[Detailed Description of the Invention] Industrial Application Field The present invention relates to a tape cassette holder for a tape recorder.

従来の技術 テープレコーダのテープカセツトホルダには、
テープカセツトホルダが回動させてホルダ内に収
容してあるテープカセツトを所定位置に装着した
ときに、テープカセツトの上面を押してテープカ
セツトを上記所定位置にばね付勢する板ばね部材
が取り付けられている。従来は、例えば実開昭47
−1128号公報、実開昭47−14122号公報に示され
ているように、上記の板ばね部材はねじによりテ
ープカセツトホルダにねじ止めされている。
Conventional technology The tape cassette holder of a tape recorder has a
When the tape cassette holder is rotated and the tape cassette housed in the holder is mounted in a predetermined position, a leaf spring member is attached that presses the top surface of the tape cassette to spring bias the tape cassette to the predetermined position. There is. Previously, for example,
As shown in Japanese Unexamined Patent Application No. 1128-1128 and Japanese Utility Model Application No. 47-14122, the above-mentioned leaf spring member is screwed to a tape cassette holder with a screw.

考案が解決しようとする問題点 このため、板ばね部材をテープカセツトホルダ
へ取り付けるには、板ばね部材を片方の手の指先
で所定位置に押さえて、他方の手でねじ止め作業
をすることになり、作業が面倒で自動組立化も困
難であるという問題点があつた。
Problems that the invention aims to solve: For this reason, in order to attach the leaf spring member to the tape cassette holder, it is necessary to hold the leaf spring member in place with the fingertips of one hand and tighten the screws with the other hand. However, there were problems in that the work was troublesome and automatic assembly was difficult.

問題点を解決するための手段 本考案は、カセツトホルダに、挿入された板ば
ね部材を上下左右方向の変位を制限した状態で保
持する保持溝を設け、且つ板ばね部材の挿入時に
板ばね部材の係止孔を係合する係止突起を設けて
なる構成としたものである。
Means for Solving the Problems The present invention provides a cassette holder with a holding groove that holds the inserted leaf spring member in a state in which displacement in the vertical and horizontal directions is limited, and also provides a retaining groove for holding the inserted leaf spring member in a state in which displacement in the vertical and horizontal directions is limited. The structure includes a locking protrusion that engages the locking hole.

保持溝と板ばね部材とを、板ばね部材の両端の
折り返し形成されている板ばね部材が弾性変形さ
れる構成とする。
The holding groove and the leaf spring member are configured such that the leaf spring member, which is formed by folding back both ends of the leaf spring member, is elastically deformed.

係止突起は挿入される板ばね部材に対向する側
に傾斜面を有する。
The locking protrusion has an inclined surface on the side facing the inserted leaf spring member.

作 用 保持溝及び傾斜面を有する係止突起は、板ばね
部材を挿入するだけで板ばね部材の固定を可能と
する。
Function: The retaining groove and the locking protrusion having the inclined surface enable the plate spring member to be fixed simply by inserting the plate spring member.

板ばね部が弾性変形することにより、この弾性
力が、板ばね部材を保持溝内に弾性的に押し付け
る。
When the leaf spring portion is elastically deformed, this elastic force elastically presses the leaf spring member into the holding groove.

実施例 次に本考案になるテープレコーダのテープカセ
ツトホルダの一実施例について説明する。
Embodiment Next, an embodiment of a tape cassette holder for a tape recorder according to the present invention will be described.

第1図中、1は樹脂成形品であるテープカセツ
トホルダ本体であり、挿入されたテープカセツト
を保持するテープカセツト保持部2を有し、左側
の腕部3の先端のピン部4をテープレコーダの軸
受部(図示せず)に支承され、右側の腕部5の先
端をロツド6により支承されてばね7により回動
付勢されており、矢印A1,A2方向に回動しうる。
In Fig. 1, 1 is a tape cassette holder main body which is a resin molded product, and has a tape cassette holding part 2 for holding an inserted tape cassette. The tip of the right arm 5 is supported by a rod 6 and is biased to rotate by a spring 7, so that it can rotate in the directions of arrows A 1 and A 2 .

テープカセツトホルダ主体1の左右両側には板
ばね部材8を保持溝9(右側の保持溝は図示せ
ず)が形成してある。保持溝9は板ばね部材8の
形状に対応した構造であり、第2図A,Bに併し
た示すように、溝部9aと突条部9bとを有す
る。保持溝9の奥部には更に第3図に併せて示す
ように、係止突部9cとリブ状のストツパ9dが
形成してある。係止突部9cは、保持溝9の入口
部9eに対向する側に傾斜面9c1を有する。
Grooves 9 for holding the leaf spring member 8 are formed on both left and right sides of the tape cassette holder main body 1 (the right-hand holding groove is not shown). The holding groove 9 has a structure corresponding to the shape of the leaf spring member 8, and has a groove portion 9a and a protruding portion 9b, as shown in FIGS. 2A and 2B. As shown in FIG. 3, a locking protrusion 9c and a rib-shaped stopper 9d are further formed in the inner part of the holding groove 9. The locking protrusion 9c has an inclined surface 9c 1 on the side facing the entrance portion 9e of the holding groove 9.

板ばね部材8は、第4図A乃至Dに併せて示す
ように、細長のベース部8aと、ベース部8aの
夫々の端よりベース部8aに重なるように折り曲
げられて形成された第1、第2の板ばね部材8
b,8cとよりなる。第1、第2の板ばね部材8
b,8cは共に逆V字形状を有し、矢印B,C方
向に弾性的に撓みうる。第1、第2の板ばね部材
8b,8cのベース部8aとの付け根部には、
夫々テープカセツトを押圧するばね力を弱めて所
定の値とするための切欠8d,8eが形成してあ
る。この切欠8d,8eを形成することにより板
ばね部材8の幅寸法wは大となつている。このこ
とは後述する係止孔を形成し易くし、且つテープ
カセツトを押圧する部分を広くしており、好まし
いことである。またベース部8aのうち第1の板
ばね部8b側の端部の近傍には、小判形状の係止
孔8fが穿設してある。
As shown in FIGS. 4A to 4D, the leaf spring member 8 includes an elongated base portion 8a, and a first portion formed by bending each end of the base portion 8a so as to overlap the base portion 8a. Second leaf spring member 8
It consists of b, 8c. First and second leaf spring members 8
Both b and 8c have an inverted V shape and can be elastically bent in the directions of arrows B and C. At the base of the first and second leaf spring members 8b and 8c with the base part 8a,
Notches 8d and 8e are formed to respectively weaken the spring force pressing the tape cassette to a predetermined value. By forming the notches 8d and 8e, the width w of the leaf spring member 8 is increased. This is preferable because it makes it easier to form a locking hole, which will be described later, and also widens the area on which the tape cassette is pressed. Further, an oval-shaped locking hole 8f is bored near the end of the base portion 8a on the first leaf spring portion 8b side.

この板ばね部材8は、第1図、第2図B、第5
図A乃至Cに示すように、保持溝9内に挿入する
ことにより取り付けられる。まず板ばね部材8は
第1図及び第2図Bに示すように、第1の板ばね
部材8cを先頭にして保持溝9の入口9eより保
持溝9内に矢印D方向に挿入される。第5図Aに
示すように、ベース部8aは溝部9aに嵌合して
溝部9aの奥方に進入し、第1、第2の板ばね部
材8b,8cは突条部9bの下面に当接して弾性
変形し、突条部9bにより案内されつつ移動す
る。板ばね部材8を第5図Aに示す状態より更に
挿入すると、ベース部8aの挿入方向先端が係止
突部9cの傾斜面9c1に案内されて、第5図Bに
示すように第1の板ばね部8bを撓ませつつ、傾
斜面9c1に乗り上げ、係止突部9cを乗り越え、
ストツパ9dに衝突し、板ばね部材8はそれ以上
の挿入を制限される。板ばね部材8がストツパ9
dに係止される最終位置まで挿入されると、係止
孔8fが係止突部9cに対向し、特に第1の板ば
ね部材8bによるばね力によりベース部8aが下
がり、第5図Cに示すように係止孔8fが係止突
部9cに係合し、板ばね部材8が取り付けられ
る。
This plate spring member 8 is shown in FIGS. 1, 2B, and 5.
It is attached by being inserted into the retaining groove 9, as shown in Figures A to C. First, the leaf spring member 8 is inserted into the holding groove 9 from the entrance 9e of the holding groove 9 in the direction of arrow D, with the first leaf spring member 8c at the beginning, as shown in FIGS. 1 and 2B. As shown in FIG. 5A, the base portion 8a fits into the groove portion 9a and enters the depths of the groove portion 9a, and the first and second leaf spring members 8b and 8c abut against the lower surface of the protrusion portion 9b. It is elastically deformed and moves while being guided by the protrusion portion 9b. When the leaf spring member 8 is further inserted from the state shown in FIG. 5A, the tip of the base portion 8a in the insertion direction is guided by the inclined surface 9c1 of the locking protrusion 9c, and as shown in FIG. While bending the leaf spring portion 8b, climbs onto the inclined surface 9c1 , climbs over the locking protrusion 9c,
The leaf spring member 8 collides with the stopper 9d, and further insertion of the leaf spring member 8 is restricted. The leaf spring member 8 is the stopper 9
d, the locking hole 8f faces the locking protrusion 9c, and the base portion 8a is lowered by the spring force of the first plate spring member 8b, as shown in FIG. As shown in the figure, the locking hole 8f engages with the locking protrusion 9c, and the leaf spring member 8 is attached.

なお、テープカセツトホルダ本体1の寸法のバ
ラツキを考慮して、上記傾斜面9c1は、基準面で
ある底面9a1より寸法a低い位置まで延在するよ
うに定めてある。このため、各テープカセツトホ
ルダ本体1毎に寸法誤差がある場合にも、傾斜面
9c1の下端は依然として底面9c1より低い位置に
あり、底面9a1より上方に位置することはないた
め、挿入された板ばね部材8のベース部8aの先
端には確実に傾斜面9c1に当接する。即ち、全て
のテープカセツトホルダ本体1に対して、板ばね
部材8は確実に取り付けられる。
In addition, in consideration of variations in the dimensions of the tape cassette holder main body 1, the inclined surface 9c1 is determined to extend to a position a dimension a lower than the bottom surface 9a1 , which is a reference surface. Therefore, even if there is a dimensional error in each tape cassette holder body 1, the lower end of the inclined surface 9c 1 will still be at a lower position than the bottom surface 9c 1 and will not be located above the bottom surface 9a 1 . The distal end of the base portion 8a of the plate spring member 8 that has been rotated is surely brought into contact with the inclined surface 9c1 . That is, the leaf spring member 8 is securely attached to all tape cassette holder bodies 1.

また、板ばね部材8は、第1に、ベース部8a
が溝部9a内に嵌合していることにより、矢印
E1,E2で示す左右方向の変位を制限され、第2
に第1、第2の板ばね部8b,8cが突条部9b
に係止されていることにより、矢印F1,F2で示
す上下方向の変位を制限され、第3に係止孔8f
が係止突部9cに係合していることにより、矢印
Gで示す挿入方向とは逆の離脱方向の変位を制限
され、第4に、ベース部8aの挿入方向先端がス
トツパ9dに当接していることにより、矢印Dで
示す挿入方向への変位を制限されて、テープカセ
ツトホルダ本体1に取り付けられている。
Further, the leaf spring member 8 firstly has a base portion 8a.
is fitted into the groove 9a, so that the arrow
The left and right displacements shown by E 1 and E 2 are limited, and the second
The first and second plate spring portions 8b and 8c are connected to the protrusion portion 9b.
By being locked in the locking hole 8f, displacement in the vertical direction shown by arrows F 1 and F 2 is restricted.
is engaged with the locking protrusion 9c, thereby restricting displacement in the withdrawal direction opposite to the insertion direction shown by arrow G, and fourthly, the tip of the base portion 8a in the insertion direction abuts against the stopper 9d. As a result, displacement in the insertion direction shown by arrow D is restricted, and the tape cassette holder body 1 is attached to the tape cassette holder main body 1.

また第1、第2の板ばね部8b,8cのばね力
により、ベース部8aは矢印F2方向に付勢され、
溝部9aの底面9a1に押し付けられている。この
ため、ベース部8aは底面9a1よりの浮き上がり
を制限され、然して係止孔8fが係止突部9cよ
り外れる危険は解消されている。
Furthermore, the base portion 8a is biased in the direction of arrow F2 by the spring force of the first and second leaf spring portions 8b and 8c.
It is pressed against the bottom surface 9a1 of the groove portion 9a. Therefore, the base portion 8a is restricted from rising above the bottom surface 9a1 , and the danger of the locking hole 8f coming off from the locking protrusion 9c is eliminated.

テープカセツトホルダ本体1を矢印A1方向に
カセツト装着位置まで回動させると、第6図に示
すように、第1、第2の板ばね部材8b,8cは
更に撓み、テープカセツト10は上面を第1、第
2の板ばね部材8b,8cにより、矢印H方向に
板ばね部材8b,8cが予め弾性変形していない
場合に比べて強い力で押圧され、下面を位置決め
ピン11,12に係止されてテープレコーダの所
定位置に装着される。
When the tape cassette holder main body 1 is rotated in the direction of arrow A1 to the cassette mounting position, the first and second leaf spring members 8b and 8c are further bent, and the tape cassette 10 has its upper surface bent. The first and second leaf spring members 8b and 8c press the leaf spring members 8b and 8c in the direction of the arrow H with a stronger force than when the leaf spring members 8b and 8c have not been elastically deformed in advance, and their lower surfaces are engaged with the positioning pins 11 and 12. The tape recorder is then stopped and mounted in a predetermined position on the tape recorder.

板ばね部材8は別の保持溝に対しても上記と同
様に、単に挿入することにより取り付けられる。
The leaf spring member 8 can also be attached to another holding groove by simply inserting it in the same manner as described above.

考案の効果 上述の如く、本考案になるテープレコーダのテ
ープカセツトホルダによれば、板ばね部材をテー
プカセツトホルダ本体の保持溝内に単に挿入する
だけで板ばね部材がテープカセツトホルダ本体に
取り付けられる構成であるため、ねじ止めに比べ
て、板ばね部材を非常に簡単に取り付けることが
出来、然して組立作業の作業性の向上を図ること
が出来、更には自動組立を可能になる。
Effects of the Invention As described above, according to the tape cassette holder for a tape recorder according to the present invention, the leaf spring member can be attached to the tape cassette holder body simply by inserting the leaf spring member into the holding groove of the tape cassette holder body. Because of this structure, the leaf spring member can be attached much more easily than with screws, and the workability of the assembly work can be improved, and furthermore, automatic assembly is possible.

また、取り付け状態で板ばね部が弾性変形せし
められているため、この板ばね部の弾性力によつ
て、ベース部が溝部内に押し付けられており、係
止孔が係止突部から外れにくい状態となり、板ば
ね部材を脱落しにくい状態で取り付けることが出
来る。
In addition, since the leaf spring part is elastically deformed when installed, the base part is pressed into the groove by the elastic force of the leaf spring part, making it difficult for the locking hole to come off from the locking protrusion. This allows the leaf spring member to be attached in a state where it is difficult to fall off.

また、板ばね部が弾性変形せしめられているた
め、カセツト装着時に、カセツトに十分大きな押
圧力を付与することが出来、カセツトを装着位置
に強固に固定することが出来るという特長を有す
る。
Further, since the leaf spring portion is elastically deformed, a sufficiently large pressing force can be applied to the cassette when the cassette is installed, and the cassette can be firmly fixed in the installed position.

【図面の簡単な説明】[Brief explanation of the drawing]

第1図は本考案になるテープレコーダのテープ
カセツトホルダの一実施例の分解斜視図、第2図
A,Bは夫々第1図中保持溝の構造を取り出して
示す図、第3図は保持溝の奥部の係止突部及びス
トツパを拡大して示す斜視図、第4図A乃至Eは
夫々板ばね部材の平面図、正面図、底面図、右側
面図、左側面図、第5図A,B,Cは夫々板ばね
部材を単に挿入することにより板ばね部材が取り
付けられる過程を順を追つて示す図、第6図はテ
ープカセツトが装着された状態を示す図である。 1……テープカセツトホルダ本体、2……テー
プカセツト保持部、8……板ばね部材、8a……
ベース部、8b……第1の板ばね部、8c……第
2の板ばね部、8d,8e……切欠、8f……係
止孔、9……保持溝、9a……溝部、9a1……底
面、9b……突条部、9c……係止突部、9c1
…傾斜面、9d……ストツパ、9e……入口部、
10……テープカセツト、11,12……位置決
めピン。
Fig. 1 is an exploded perspective view of one embodiment of the tape cassette holder of the tape recorder according to the present invention, Fig. 2 A and B are views showing the structure of the holding groove in Fig. 1, and Fig. 3 is a holding groove. A perspective view showing an enlarged locking protrusion and a stopper at the deep part of the groove, and FIGS. Figures A, B, and C each show a step-by-step process of attaching a leaf spring member by simply inserting the leaf spring member, and Figure 6 shows a state in which a tape cassette is attached. DESCRIPTION OF SYMBOLS 1... Tape cassette holder main body, 2... Tape cassette holding part, 8... Leaf spring member, 8a...
Base part, 8b...first leaf spring part, 8c...second leaf spring part, 8d, 8e...notch, 8f...locking hole, 9...holding groove, 9a...groove part, 9a 1 ... Bottom surface, 9b ... Protrusion section, 9c ... Locking protrusion, 9c 1 ...
...Slope, 9d...Stopper, 9e...Entrance,
10... Tape cassette, 11, 12... Positioning pin.

Claims (1)

【実用新案登録請求の範囲】 テープカセツトホルダ本体内に収容されたテー
プカセツトを該テープカセツトホルダ本体に設け
られた板ばね部材によりばね付勢してテープレコ
ーダの所定位置に装着するテープレコーダのテー
プカセツトホルダにおいて、 上記板ばね部材を、その端部近傍に係止孔を有
するベース部と、該ベース部の端より該ベース部
に重なるように折り曲げられて形成してあり弾性
変形して上記テープカセツトをばね付勢する板ば
ね部とよりなる形状とし、 上記テープカセツトホルダ本体を、上記板ばね
部材の上記ベース部が嵌合する溝部と上記板ばね
部を上記ベース部に近づく方向に弾性変形させた
状態で案内する突条部とよりなり、該板ばね部材
が挿入され、挿入された該板ばね部材を上下左右
方向の変位を制限した状態で保持する保持溝を設
け、且つ挿入される板ばね部材に対向する側を傾
斜面とされた係止突部を上記溝部内に突出させて
設けてなる構成とし、 上記板ばね部材が、上記係止孔を有する側の端
より上記保持溝内に挿入されて、上記ベース部が
上記傾斜面に案内されて上記係止突部に乗り上
げ、上記係止孔が上記係止突部に係合した状態と
され、且つ上記板ばね部が上記突条部により上記
ベース部に近づく方向に弾性変形され、該板ばね
部のばね力によつて上記ベース部が上記溝部内に
弾性的に押し付けられた状態で上記テープカセツ
トホルダ本体に取り付けられてなる構成のテープ
レコーダのテープカセツトホルダ。
[Claims for Utility Model Registration] A tape recorder tape in which a tape cassette housed in a tape cassette holder body is biased by a leaf spring member provided on the tape cassette holder body to be mounted at a predetermined position on a tape recorder. In the cassette holder, the above-mentioned plate spring member is formed by forming a base part having a locking hole near the end thereof, and being bent from the end of the base part so as to overlap with the base part, so that the above-mentioned tape is elastically deformed. The tape cassette holder main body has a shape consisting of a leaf spring portion that biases the cassette, and the tape cassette holder main body is elastically deformed in a direction where the base portion of the leaf spring member fits into a groove portion and the leaf spring portion approaches the base portion. The plate spring member is inserted into the plate spring member, and a retaining groove is provided to hold the inserted plate spring member in a state in which displacement in the vertical and horizontal directions is limited. A locking protrusion whose side opposite to the leaf spring member is an inclined surface is provided to protrude into the groove, and the leaf spring member is arranged in the retaining groove from the end on the side having the locking hole. the base portion is guided by the inclined surface and rides on the locking protrusion, the locking hole is engaged with the locking protrusion, and the leaf spring portion is inserted into the locking protrusion. The tape cassette holder is attached to the tape cassette holder main body in a state in which the base part is elastically deformed in a direction approaching the base part by the protrusion part and is elastically pressed into the groove part by the spring force of the leaf spring part. A tape cassette holder for a tape recorder with the following configuration.
JP1985122829U 1985-08-10 1985-08-10 Expired JPH0411275Y2 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
JP1985122829U JPH0411275Y2 (en) 1985-08-10 1985-08-10

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
JP1985122829U JPH0411275Y2 (en) 1985-08-10 1985-08-10

Publications (2)

Publication Number Publication Date
JPS6235488U JPS6235488U (en) 1987-03-02
JPH0411275Y2 true JPH0411275Y2 (en) 1992-03-19

Family

ID=31013385

Family Applications (1)

Application Number Title Priority Date Filing Date
JP1985122829U Expired JPH0411275Y2 (en) 1985-08-10 1985-08-10

Country Status (1)

Country Link
JP (1) JPH0411275Y2 (en)

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5830902B2 (en) * 1976-10-30 1983-07-02 広栄化学工業株式会社 Stabilization method for chlorine-containing resins

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5447421U (en) * 1977-09-09 1979-04-02
JPS5830902U (en) * 1981-08-20 1983-02-28 松下電器産業株式会社 cassette lid device
JPS59153647U (en) * 1983-03-30 1984-10-15 パイオニア株式会社 Cassette storage case

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5830902B2 (en) * 1976-10-30 1983-07-02 広栄化学工業株式会社 Stabilization method for chlorine-containing resins

Also Published As

Publication number Publication date
JPS6235488U (en) 1987-03-02

Similar Documents

Publication Publication Date Title
JPH0218527Y2 (en)
CA1201496A (en) Electrical connectors with quasi-terminal pins
KR100296460B1 (en) IC carrier
JPH0411275Y2 (en)
JP2501496B2 (en) Mounting device to rail
JPH0466077U (en)
JPH0541468Y2 (en)
JPH0431615Y2 (en)
JPS6243125Y2 (en)
JP2583879Y2 (en) Floating connector housing
JP2767483B2 (en) connector
JPH0218390Y2 (en)
JPH0713072Y2 (en) Cassette holder device
JPH0322571Y2 (en)
JPS6028811Y2 (en) parts mounting device
JPS642298Y2 (en)
JPH0125459Y2 (en)
JPH0129830Y2 (en)
JPS6342731Y2 (en)
JPH0726650Y2 (en) Magnet case attachment
JPS62215Y2 (en)
JPH056590Y2 (en)
JPH0627027Y2 (en) Head carriage drive
JPH0345399Y2 (en)
JP2533524Y2 (en) Semi-fixed variable resistor