A Very Economical Strip Chart Pen Recorder A recorder can he constructed very economiedly by undergraduate (or even high school) students. The pen drive is provided by the dizplaeement of a speaker core. The movement of the eore is magnified by a.lever. A roll of adding machine paper may he used instead of normel recorder graph paper. I n the example shown the paper drive was provided by a small fan motor with the speed reduced through a train of homemade pulleys. The speaker may be of virtually any size and broken speakers (which abound in radio repair shops) are normally as good as new ones. We have used speakers with and without the paper cone. Removal of the cone gives the advantage of greater care travel hut neeesitates a horizontal mountine so that the weieht of the Den and lever will return the Den to
can give a stronger pen movement through asingle pushrod connecting the two cores. The 8.0-in. speaker shown has a eore movement of 0.7 em which is amplified to the 7.2 ern width of the paper roll by a 10.3: 1 lever. The lever has severs1 pen positions which allow other scales to be employed. A small plastic disc glued to the speaker core provides the lever motion through a pushrod. The pushrod is connected to the shortside of the lever close to the rotating lever support. The lever support is made from a. '/,-in. rod reduced slightly at the extremes to provide positive alignment and rotation of the supports on either side of the speaker. The pen holding lever is bolted t o a. flattened section of this rotating lever support. The lever support is seen crossing the speaker face vertically. A felt marking pen is attached near the end of the lever. Although the pen motion is really an arc instead of s. straight line, this represents very little problem if the length of the lever is much longer than the width of the paper. To prevent pen skipping and large blots, which tend to start tears in the paper, the paper is raised slightly from the wooden bed by means of a '/sin. brass rod. The pen writes on the psper where i t is slightly elevated. Uniform movement of the paper is provided hy friction with a. thin rubber tube fitted tightly over the '/sin. axis of the final pulley in the drive chain. A short piece of acetate film under the paper a t the drive location minimizes friction with the recorder bed. A 1.5-V flashlight battery gives full scale deflection. For smaller signals (from 10 to 500 my), a linear amplifier may be used. One can he built with two transistors (or purchased as an integrated circuit). A common 2-5 W audio amplifier will also suffice. The simplest method to make quantitative measurements on the recorded graph is to include a known signal. under a glass d. a t e to which is afixed, on the underside, The granh on the unlined adding machine DaDer . may"he laced . a pie& df translucent graph This recorder has variable ranee h v either Den nosition or bv the use of a. voltaee amnlifier. Chart meed is ad-
T o whominquiries should be addressed.
JALIBCO, M~XICO GUADALAJARA,
142
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Journal o f Chemical Education
