Making light barriers insensitive to sunlight

The light barrier is in itself a relatively simple construction. Most of us are probably familiar with it from our physics lessons or from electronics kits, where the apparatus consists of a light source, usually in the form of a light bulb or LED, and a receiver element, mostly a light-dependent resistor (LDR), a photodiode or a phototransistor.

Such an array may just about function in "lab conditions"; it is, however, miles away from being suitable for practical use. This is primarily due to the lack of consideration paid to external interference factors. The majority of these are other light sources " or quite simply ambient light or even direct rays from the sun.

In practice we thus resort to a number of measures in an attempt to reduce these optical disturbance variables " with varying degrees of success. We employ an entire range of "tricks" to this end, from the mechanical to the optical. Placing a tube in front of the receiver element, for example, greatly reduces the angle of irradiation of ambient light. In some applications, however, there just isn't room for a "mechanical" solution such as this.

Another possibility is to use an optical filter to reduce the range of the receivable light spectrum. Here, photodiodes with IR filters are a viable option, especially when infrared LEDs are used. However, even here the long-wave component of sunlight, for example, will continue to act as a disturbance variable. In addition light barriers with a "visible" beam are easier to adjust. The figure on the home page gives a diagram of such a setup " and also illustrates the difficulty of setting a suitable trip limit for the evaluation electronics.

If there is no object in the way of the light beam, the received optical power (1) is the sum of the transmitted light plus the ambient light and lies above the switching threshold (2). If the beam is interrupted, the level of ambient light (3) must remain below the trip limit. If this is not the case, any object passing through the light beam cannot be recognized.

It is easy to understand that with high levels of ambient light, such as direct, unfiltered rays from the sun (4), a correspondingly high trip limit and thus an enormous power of transmission would be necessary, both usually impossible to implement and/or detrimental to the lifetime of the light source. It is also evident that safe operation of a light barrier is not possible in all conditions. The difficulty here is to differentiate between the DC transmission signal on the receiver side and the disturbance variables.