cheap light meter

7-14 elektor july/august 1980 When developing photographs a light meter can be a useful gadget. Of course, the more luxurious the better, but as the diagram here shows, a simple circuit can also be very effective. The circuit is con- structed around a photodiode. In a camera the diode generates enough voltage to control a meter directly. Often, however, there is not enough light under the enlarger. That is why a transistor has been added. Unfortu- nately, this also puts an end to the original design"s simplicity, for now a battery will also be necessary. One solution is to connect the light meter to an ordinary multimeter switched to the resistance range. This is indicated inside the dotted line. Remember that the positive test lead of most meters is the negative pole for the resistance range! BPW 20 BPW 21 18 cheap light meter Operation is as follows. The test diode is connected between the collector and base of the transistor. The more light that falls on the diode, the more the...