12
2.4.12 2.4.2.2 Rotating Instruments With No Pulse Output
This category of meters includes consumer service meters (Revenue Meters). If the
instrument has no pulse output then an optical sensor is utilized to sense disk rotation. This sensor
generates a pulse which is used in place of the pulse output of the SC-10. Otherwise the test is the
same as that described above.
The optical sensor consists of a light source and a light detector. In one type, the light source
illuminates a portion of the meter wheel and the light is reflected from its surface back to the light
detector. The surface of the wheel at the index point is non reflective interrupting the light as the
index point passes by the light detector. Thus, once per revolution (or in some instances more often)
the light detector senses the short interruption of the light source.
Other sensors operate with meter wheels which allow transmission of light through a small
hole in the wheel at the index point. This type senses a short pulse of applied illumination.
The light source can be an incandescent lamp or light emitting diode (LED). A modulated
LED source can reduce the interference from high levels of ambient light. The pickup can be a
simple photo-resistor. If the source is modulated there must be a demodulator after the pickup to
benefit from this technique. The output may be a TTL level signal. In the 811A a level of < +0.8V DC
(low state) to +2.0V DC (high state) must be applied.
Optical sensors are commercially available or may be ordered through the factory.
Some sensors operate from the AC line voltage while others require only excitation for the
light source. The Model 811A has provisions to adapt to a wide variety of sources through
connections on J48, settings of S-3 on the inside rear panel and selectable resistance values on the
rear panel assembly to match impedance's. Refer to Figure 2-4.
If the sensor has a incandescent 5 to 6.3V lamp and a photo-resistive pickup, the 5V
regulated supply on pin C of J-48 may be used for lamp excitation. The photo-resistive element may
be either a high or low impedance device. If it is a high impedance device it will be matched best by
resistors R33 or R34, and if low impedance R30 or R31. Select one of the resistors by moving the
mini-jumper connector (on rear panel p.c. board) to the appropriate resistor. With excitation on the
lamp and the lamp installed to provide maximum and minimum illumination of the photo element, the
voltage swing on the emitter of transistor Q13 is measured. The optimum matching will cause the
greatest swing with a low value of < +0.8V and a high value of +2.0V DC. If the photo pickup
provides a TTL level output the mini-jumper should be connected to R4 which is the highest value
resistor. If the pickup provides an open collector or switch closure at the index point, such as the SC-
10 and many of the electronic transducers, then the 4.7K resistor, R32 to +5V DC, should be
connected by the mini-jumper.
Switch S-3 inverts the internal TTL signal so that in all cases the occurrence of the index
point of the meter's wheel or pulse input creates an 'ON' condition of the EVENTS/GATE MODE
indicator on the front panel. Proper operation will occur in any instance but it is much better to see
the pulsing 'on' of an indicator than the reverse.
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