Hack into a stopwatch to make a phototimer

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 illuminated, photocurrent flows through R₁, raising its voltage to more than 0V, which drives the right side of C₁ above the 1.5V rail. The base of Q₁ is reverse-biased and has no effect. However, Q₂’s emitter is now forward-biased because R₄ holds the base near 1.5V. As Q₂ turns on, the charge in C₁ dissipates across R₂, raising its voltage and creating a positive pulse. You convey this pulse to the stopwatch through R₅, which is necessary in the case of extreme illumination of the photodiode. It limits the current into the stopwatch circuitry so that a large pulse cannot latch or overpower the internal stopwatch circuitry. The photocurrent creates a difference between 1.5V and the voltage of R₁; this difference causes C₁, under illumination, to enter a final voltage.


Your selection of the value of C₁ depends on the exposure time to be measured and on the photodiode used. The response rate of this circuit is approximately 500 msec. This example uses an Everlight PD333-3C/HO/L2 photodiode with a large spectral bandwidth, but any other photodiode or even a photoresistor will also work.