Hack into a stopwatch to make a phototimer

Ralf Kelz, Seefeld, Germany; Edited by Paul Rako and Fran Granville - November 3, 2011

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.

Hack into a stopwatch to make a phototimer figures 1 and 2

When the photodiode is not illuminated, capacitor C₁ charges to 1.5V (Figure 3). The charge initially comes through the base-emitter junction of Q₁ with a time constant that R₁×C₁ sets. Once C₁ charges to 1.5V minus the base-to-emitter voltage, R₃ tops off the charge on C₁ until it reaches 1.5V. Because R₃ and R₁ are in series during this time, this topping off occurs with a slower time constant that (R₁+R₃)×C₁ sets.

Hack into a stopwatch to make a phototimer figure 3

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.

When the photodiode is not illuminated, no photocurrent goes through R₁, so C₁ can charge back up as its left side goes to ground and its right side goes first to a base-emitter drop below 1.5V and subsequently all the way to 1.5V. Because the initial charge conducts through the base-emitter junction of Q₁, that transistor again turns on, delivering a pulse across R₂ and halting the stopwatch.

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.