An accurate and inexpensive measurement of incident illumination was thus produced, and incidentally, a name for the instrument. Over the past years these instruments have been constructed in a number of school districts and summer institutes, and periodically we receive inquiries concerning development of a less expensive solid-state version. In this article we describe an inexpensive design which is extremely easy to build and use. It is constructed on a small perforated circuit board, and is powered by a 9-volt transistor radio battery.
All components are readily available from common electronic hobbist outlets.
Since the resistance of readily available cadmium sulfide cells is low, the frequency of oscillation is high at normal room light levels. This is far to rapid to observe visually, and divider circuit IC-2 is called into action to reduce the flash rate to an observable value. Hz, an easily observable value. One can visually count pulse rate up to about 3 per second. The performance of this circuit is illustrated in Figure 2.