In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Inexpensive Digital Monitoring of Signals from a Spectronic-20 Spectrophotometer John R. Amend* Department of Chemistry and Biochemistry, Montana State University–Bozeman, Bozeman, MT 59717;
[email protected] Matthew E. Morgan Department of Chemistry, U.S. Air Force Academy, Colorado Springs, CO Alex Whitla Department of Chemistry, Mount Allison University, Sackville, NB, Canada
Obsolete analog-readout vacuum-tube and solid-state Spectronic-20 family spectrophotometers can be found in most college and university chemistry laboratories. While optically these are good single-beam instruments, their electronics package does not permit attachment of recorders or digital displays. In fact, in vacuum tube instruments the two meter terminals float at more than 100 V above ground. These instruments may be commercially converted to digital or recorder readout, but commercial electronics modification packages cost approximately $400. Ed Vitz (1) described an inexpensive method that involves replacing the phototube by placing a photoconductor in the Spectronic-20 filter holder. This article will describe a simple modification that costs about $10.00 for a two-conductor earphone plug, a closed-circuit jack, and a simple IC amplifier and will permit readout of the Spectronic-20 signal with a digital voltmeter or a laboratory interface and a computer. The extremely linear Spectronic-20 phototube is retained in this modification. When the external plug is removed, the Spectronic-20 reverts to its normal operating mode. Spectronic-20 instruments of the vacuum-tube and transistor generation use gas or vacuum phototubes. Light striking the photocathode of these tubes causes electrons to be ejected, which are then collected by a +90-V charge placed on the collecting electrode of the phototube. The instrument’s electronics measure the electron flow in this circuit, presenting it as a percent transmission signal on a 0–100 analog meter (Fig. 1). Our modification retains the Spectronic-20’s optical components and phototube power supply. A 1⁄4-inch closedcircuit earphone jack is mounted next to the phototube by drilling a 3⁄8-inch hole in the base of the Spectronic-20, or a similar hole in the back of the Spectronic-20 case. The ground lug of the earphone jack is connected with a soldered wire to the ground lug on the phototube socket (Fig. 2). The photocathode lead (pin 8 of the phototube socket) is disconnected and run through the closed-circuit connections of the jack, connecting the photocathode to the tip of the earphone plug and to the external amplifier input when the plug is inserted. When the plug is removed, the closed-circuit contacts simply connect the original photocathode circuit to the
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Spectronic-20 electronics package, and the instrument runs normally. The 90-V collecting voltage lead (pin 4 of the phototube socket) remains connected to its power supply in the Spectronic-20’s electronics.
Figure 1. The Spectronic-20 family spectrophotometers use a vacuum or gas phototube to measure light. The phototube signal may be connected to an external circuit for monitoring while still using the Spectronic-20’s high voltage supply to power the phototube.
Figure 2. A 1⁄4-inch two-conductor earphone plug and closed-circuit jack permit connection of the photocathode signal to an external amplifier. When the plug is removed, the Spectronic-20 reverts to normal operation. Two operational amplifiers present a 1.00-V signal to the digital readout at 100% transmission. 100% T is set with the Spectronic-20’s light beam control (right-hand knob). Phototube connections are shown from the wiring side of the socket.
Journal of Chemical Education • Vol. 77 No. 2 February 2000 • JChemEd.chem.wisc.edu
In the Laboratory
amplifier IC-1 will be positive and its output negative. IC-2 is a gain of one inverting amplifier to produce a positive signal to the display. This amplifier is not necessary if one is willing to have the digital readout display negative100 at 100% transmission. (The DVM can be set to its 0–2-V range for transmission measurements; 1.00 volts corresponds to 100% T.) The amplifier may be built on a solderless breadboard; a layout is illustrated in Figure 3. Two 9-V batteries provide power for the amplifier circuits. One can check dark current by removing the sample tube from the Spectronic-20, thus causing the light path to be blocked. Dark current is so small as not to require compensation. To set 100% transmission, simply insert a blank sample and adjust the right hand Spectronic-20 control, which moves a wedge across the light beam between the grating and the sample. Materials Figure 3. The amplifier circuit may be constructed on a small prototyping board. It is powered by two 9-V batteries. Amplifier gain can be doubled by substituting a 20K resistor for R3.
Two wires run from the earphone jack to a small integrated circuit amplifier (Fig. 3), which can be constructed on a prototype board. Note that there is no high voltage on the wires to the amplifier. The amplifier consists of two CA3140 or equivalent operational amplifiers. IC-1 is a current follower amplifier. With a 1-MΩ feedback resistor, each microampere of phototube current produces 1 V at the amplifier output. Because this is an inverting amplifier circuit and the electron flow is away from the amplifier input toward the phototube, the input of
Closed circuit, 1/4 inch, two-conductor earphone jack, Radio Shack 274-252 or similar. ⁄4-inch, two-conductor earphone plug, Radio Shack 2741536 or similar.
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Integrated circuits (2) CA3140, OPO7, or similar operational amplifiers. Resistors: (1) 1 MΩ, 1⁄4 W, (2) 10 k Ω, 1⁄4 W. Two 9-V transistor radio batteries and battery clips Solderless breadboard to construct amplifier circuit, Jameco 20600 or similar.
Literature Cited 1. Vitz, E. J. Chem. Educ. 1994, 71, 879–885.
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