Hack into a stopwatch to make a phototimer

摘要

The exposure tester in this Design Idea measures the on time of a light source, whether an LED, an incandescent lamp, a halogen lamp, or another source. It can be made with an ordinary stopwatch and a few simple components (figures 1 and 2). An electronic stopwatch needs two pulses to operate; one starts the internal counter, and another one stops it. A light source provides only one pulse, corresponding to the time the light is illuminated. This circuit generates a short trigger pulse whenever the luminous intensity changes. When the photodiode is not illuminated, capacitor C_1 charges to 1.5V (Figure 3). The charge initially comes through the base-emitter junction of Q_1 with a time constant that R_1×C_1 sets. Once C_1 charges to 1.5V minus the base-to-emitter voltage, R_3 tops off the charge on C_1 until it reaches 1.5V. Because R_3 and R_1 are in series during this time, this topping off occurs with a slower time constant that (R_1+R_3)×C_1 sets.