Direction Sensitive Light Barrier

Two light barriers positioned closely together can determine the direction of passage. For instance, placing them at the entrance of a toilet allows for automatic control of lights: turning on when entering and off when exiting. The circuit shares similarities with other modulated light barrier designs. There are two installation methods: a completely duplicated setup in opposing directions to avoid interference, and a configuration with one infrared (IR) transmitter and two receivers. The choice of installation depends on the specific application. In a doorway, a single transmitter is adequate if the receivers are spaced approximately 5 cm apart. For wider openings, using two separate IR transmitters is recommended. This circuit operates effectively over several meters, even under direct sunlight. The same type of IR transmitter used in the modulated light barrier is employed here. For the dual transmitter setup, it is sufficient to duplicate components R6, T1, D1, C3, and R7 from the modulated light barrier circuit. The output OUT (pin 3) of IC2 can easily drive two of these IR drivers. The receivers differ slightly from those in the modulated light barrier, but the circuit remains consistent for both installation types. The TSOP1736 is utilized, which is responsive to IR light modulated at a frequency of 36 kHz. Components D2, R8, and C4 ensure that the pulses received from IC3 at the output of IC5a yield a logic high (1) when the beam is unobstructed. Conversely, when the beam is interrupted, this output changes to a logic low (0) within approximately 1 ms. Similarly, IC5b outputs a logic low when IC4 ceases to receive IR light. The 4013 CMOS IC, containing two D flip-flops, is used here with only one being operational. The moment light barrier 2 (IC4) is unblocked, it triggers a clock signal that transfers the state of light barrier 1 (IC3) to output Q1. This signal actuates the relay via T2, controlling the room light. Consequently, the circuit turns the light on or off the instant light barrier 1 is interrupted.

The circuit operates based on the principles of infrared light transmission and reception. The IR transmitter emits modulated light that is detected by the receivers. When a person crosses the light barriers, the interruption of the IR beam results in a change of state in the output signals of the integrated circuits. The use of two receivers allows for detection of the direction of movement, which is essential for the intended application of controlling lights in a restroom.

For the installation, the IR transmitters should be positioned at an optimal height and angle to ensure reliable operation. The TSOP1736 receivers should be shielded from direct sunlight to prevent false triggering, although the circuit is designed to mitigate such interference. The relay used to control the light should be rated for the load it will be switching, ensuring safe and reliable operation. The design of the circuit can be adjusted based on the specific requirements of the installation environment, such as the distance between the barriers and the type of lighting being controlled.

Overall, this light barrier circuit effectively integrates sensing and control technology to automate lighting, enhancing convenience and energy efficiency in spaces such as restrooms. The modularity of the circuit allows for customization and adaptation to various applications beyond lighting control.With two light barriers closely positioned one after the other it is possible to establish in which direction they have been crossed. If, for example, you place it at the entrance of the toilet then you can use it to control the lights: on when entering and off when leaving the room.

The circuit for this has many similarities with the modulated light barrier appearing else-where in this Summer Circuits issue. There are two ways to position the light barriers, namely a completely duplicated installation in opposing directions (this to prevent mutual interference) and a version with one IR transmitter and two receivers. Both types of installation are shown here, which one is most suitable depends on the actual application.

When used in a doorway, one transmitter is sufficient if the receivers are placed about 5 cm apart. With a wider passage, an installation with two separate IR-transmitters is a better solution. This circuit has a range of several meters, even if the sun shines directly on the receiver! We use the exact same IR-transmitter(s) as for the modulated light barrier. For the installation with two separate IR-transmitters it is sufficient to duplicate R6, T1, D1, C3 and R7 from the circuit of the modulated light barrier. Output OUT (pin 3) of IC2 can drive two of these IR-drivers without any difficulty. The receivers are slightly different than those of the modulated light barrier and the circuit is the same for both types of installation.

We again use the TSOP1736, which is sensitive to IR-light that is modulated at a frequency of 36 kHz. D2, R8 and C4 ensure that the received pulses from IC3 at the output of IC5a result in a 1` when the beam is not interrupted.

When the beam is interrupted this output will become a 0` within about 1 ms. In the same way IC5b generates a 0` when IC4 stops receiving IR-light. The 4013 CMOS-IC used here contains two D-flipflops, of which we use only one. The instant that light barrier 2 (IC4) is unblocked again, is used to clock the state of light barrier 1 (IC3) through to output Q1. This signal drives the relay via T2, which operates the light in the room. The circuit therefore turns the light on or off the moment that light barrier 1 is uninterrupted. 🔗 External reference