Leonard Kruczynski, H. D. Gesser, D. W. McBride, and J. 0. Westmore University of Manitoba
I
Winnipeg, Canada
Construction of an Inexpensive "Spot Follower" Recorder (DC amplifier)
W e wish to describe a fully automatic spot follower (dc amplifier) which can be built quickly from readily available parts.' The spot follower consists of a photoelectric device mounted on the pen of a conventional recorder (Heathkit model E m - 2 0 ServoRecorder in this case) and coupled to the input of the recorder in such a way that the pen-photoelectric assembly will follow the light beam of a galvanometer impinging on it. The photoelectric device consists, schematically, of a Wheatstone bridge (Fig. I), in
Figure 1. R,J Matched pair of CdS RI ~hotoresistivecells VR '1.000-ohm potent-
VS
iometer variable-voltage dc
R
source 10-154 output t i recorder
three wires, which need not be exceedingly fine or flexible, since the servomotor can easily overcome the resistance encountered. The photocells are glued with epoxy adhesive on a thin aluminum sheet which in turn is slipped into the pen retainer spring assembly of the recorder. Output from the bridge is then fed into the recorder, which, however, requires some modification. The Heath model is recommended, since the modiica tions can be made by simply removing a few spring clips. These modifications, shown in Figure 3, consist of: ( I ) defeating the servopotentiometer circuit (hence cutting out the reference cell), (2) shorting the conventional input, and (3) feeding the bridge output through the parallel R-C network directly to the chopper (vibrator).
which two matched CdS photoresistive devices (type NSL-3P Cadmium Sulphide Photoconductive cells manufactured by National Semi-conductors Ltd. of Figure 3. The original circuit is shown in heavy type, the modifisotions os dotted liner.
7Pen Figure 2.
Diagram of "spot follower" recorder.
Montreal, Canada) play an integral part. The two photocells are mounted on the recorder pen (Fig. 2) and attached to the remainder of the bridge by 1 Commercial units rwaililable include the "Photodyne" lightspot follower by Sefram, Paris, France; and the "Mierogrs;ph2' dc Microvolt Recorder by Xipp and Zonen, Delft, Holland.
These changes were found necessary, otherwise a positive feedback loop will result in oscillation. The output of the bridge must be fed to the recorder with the correct polarity; reversed polarity will make the assembly shun the spot and become a "shadow follower." (The polarity can be reversed by reversing the output of the bridge.) The light beam originated from a Leeds Northrup mode1 2430-d (5 X lo-& ua/mm) galvanometer. The frosted glass scale was rembved; ;lowing the beam to impinge directly on the photocell assembly. The unit can be operated in subdued light; the ratio of beam intensity to background intensity determines the voltage needed to run the bridge with a significant output (hence the variable dc supply for the bridge, Figure 1). Response time is limited to that of the galvanometer itself. By placing the photocell 30 cm from the front of the galvanometer the sensitivity was found to be 40 pV/cm. This sensitivity could be appreciably upgraded by a judicious choice of galvanometer shunt and galvanometer-to-photocell distance. Volume 43, Number 7, July 1966
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A constant-impedance input attenuator was used, allowing the galvanometer to be critically damped at all attenuator settings. This device hns been used with success not only in
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DTA2 but also in recording any motion via a light beam or shadow, for example, in dilatometry, pressure changes etc. W E N D L I ~ TW. ,
w., J. CBEM.EDUC.,37,94 (1960).
