Phototransistor-IrED Pairs

Phototransistor-IrED pairs are very commonly used, yet they are often under utilized.

First, most application use a continuous current to energize the IrED, although the sampling of the phototransistor is fairly infrequent. Although the phototransistor-IrED pair does have a lag time due to physical characteristics, the lag time is often in low tens of microseconds. This means the IrED only needs to be switched on about 50 $\mu \mathrm{s}$ before the phototransistor is sampled.

Let us consider a situation in which a sensor is read 100 times per second. If the IrED is energized for only 50 $\mu \mathrm{s}$ for each sample, the power consumption is reduced to $\frac{50\mu \mathrm{s}}{10\mathrm{ms} = 0.5\%}$ of that of the continuously energized design.

Another trick commonly done to phototransistor-IrED pairs is to connect IrEDs in series. Because each IrED only has a forward voltage drop of 1.7V (at 20mA), two can be connected in series when powered by a source of 5V. Obviously, the current limiting resistor value must be adjusted. Given a voltage source of 12V, up to 6 IrEDs can be connected in series.

Be careful not to connect too many IrEDs in series. Although some current will flow at a forward voltage of less than 1.7V, the amount is too small to emit any infrared.

The last, but most important, trick is to use a high pulse current to energize the IrEDs. Because the pulsing technique reduces the duty cycle of the on-time, we can now use a larger current and stay within the average power dissipation parameters. This turns a dimly lit light into a strobe. It helps to improve the signal-to-noise ratio (most noise comes from background lighting), and reduce the value of the pull-up resistor (to improve recovery time). The one drawback of this approach is the loss of repeatability. The pulsing circuit introduces uncertainty because it is difficult to guarantee a constant current during a pulse.