This device (see schematic) will convert the current from a silicon photo diode into a time pulse between audible clicks. The same number of clicks counted will indicate the same exposure.  The current from a silicon photodiode is linear with the intensity of the incident light.  This device integrates that current and produces a click when a given amount of light has been measured.  The time between clicks will shorten as the light intensity increases and the time between clicks will lengthen as the light intensity decreases.  Even if the intensity varies the device will integrate to calculate the amount of accumulated light.  The time between clicks will always indicate the same amount of exposure.

Instructions for use:

SET UP:

Place silicon photodiode in a fixed position to look at output of cold lamp and so as not to block or shadow negative.

Wire to + and - inputs of Monitor.

Turn on lamp.

Adjust RATE resistor for 1 second pulses at optimum light.

     Note:  GAIN and ZERO are adjusted to compensate for the
                      components used.
               ZERO is adjusted to give zero output for zero input.
               GAIN is adjusted to keep maximum input below
                       but close to 5 mV.

Do not readjust unless the photodiode is replaced or moved.

OPERATION:

Negative is in carrier and film is in easel ready for exposure.

Place dark card so as to cover output from lens.

Turn on lamp.

On third click remove card.

Count number of clicks for exposure.

When exposed place the card so as to cover output from lens.

Turn off lamp.

 

Schematic and part list follow.

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