Positioning the Optical Sensor

The optical transducer can be correctly positioned using only a voltmeter, measuring the output voltage of the circuit shown above.

Test the fork by measuring the output voltage both for unobstructed light and when the light is blocked by an opaque object. This will give the working range. An example is shown in the table.

Note that stray light in the room will significantly affect the result, MORE INFO TO BE SUPPLIED SOON.

Table 1:

Fork gap	Unblocked [V]	Blocked [V]
9.5 mm	5.63	3.15
6 mm	5.3	3.0
3 mm	5.3	2.9

Select the resting point as the voltage midpoint between the two extremes. Adjust the position of the fork so that the midpoint voltage is reached. Start from the unobstructed position and move the fork slowly.

For example, with a violin G string of .75 mm diameter, we might have: Fork gap 9.5 mm, unblocked 4.8 V, blocked 3.4 V, giving a midpoint voltage of 4.1 V.

The position must be accurate to a few tenths of a millimeter. When acquiring the string vibrations, the oscilloscope and/or sound card should be AC-coupled. Sound cards are almost always AC-coupled.

The optical method requires 10 times higher accuracy for positioning than does the magnetic method. The magnetic velocity signal can be time-integrated to give displacement, which is easier to understand.