Modulated Light Barrier

Description: The light barrier described here can be used to guard an entrance. It can signal when someone is walking through a corridor or check if a car has been parked far enough in a garage to close the door. The circuit consists of a transmitter that sends modulated infrared light and a receiver that recognizes this signal. This circuit is designed to be largely insensitive to daylight or fluorescent light, making it suitable for outdoor use. The transmitter generates a burst of 36 kHz approximately 1000 times per second for a duration of 540 ms. IC1, configured with capacitors C1 and resistors R1 and R2, operates at a frequency of about 1000 Hz. The output of IC1 drives IC2 to oscillate at around 1000 times per second for 540 ms. IC2 is set to a frequency of 36 kHz using components C2, P1, R4, and R5. The output from IC2 drives the infrared LED D1 through transistor T1. Capacitor C3 and resistor R3 mitigate potential interference on the power supply rail caused by the high current through D1. The receiver, which is a simple design, utilizes IC3, which performs most of the necessary functions. When IC3 detects an IR signal at 36 kHz, its output switches to 0. The transmitter alternates between sending the IR signal for 540 ms and being quiet for 470 ms. When the signal reaches IC3, capacitor C4 discharges through diode D2. The non-inverting input of IC4a is set to 2.5 V using resistors R10 and R11, causing the output of IC4a to be 1. During the quiet periods, C4 partially charges through R8 but does not exceed 2.5 V. The output of IC4a toggles to 0 only when the light barrier is interrupted. IC4a has an open-collector output, which discharges C5, causing the output of IC4b to become 1. The signal is stretched to approximately one second with R9 and C5. Increasing R9 to 100 k will extend this duration to about 10 seconds. Resistors R12 and R13 help prevent output chatter around the trigger point, although this circuit is not likely to experience such issues. The output from IC4b provides a clean logic signal for further processing. Calibration of IC2's frequency to 36 kHz can be efficiently performed using an oscilloscope. If an oscilloscope is unavailable, point the IR LED D1 at the receiver IC3 and adjust P1 to minimize the voltage on the inverting input of IC4a. Care should be taken to ensure that IC3 does not receive an excessively high signal during calibration by positioning the IR LED at a distance or not directly aiming it at the receiver. If calibration proves difficult, setting P1 to the center position usually suffices. The circuit is designed to minimize issues with ambient light. However, if direct sunlight affects IC3, it may be necessary to enclose it in a small tube directed at the IR LED to prevent sunlight interference. If the IR LED and receiver are too close, reflected light from walls may be detected even when an object is present between them. In such cases, using short opaque tubes for both the transmitter and receiver can help. The wires connecting the IR LED can extend several meters without issue, but the receiver IC should not be placed too far from the circuit.

The light barrier circuit operates by using a transmitter and a receiver to create an invisible detection zone. The transmitter generates a modulated infrared signal that is detected by the receiver. The frequency modulation at 36 kHz allows the system to differentiate between the transmitted signal and any ambient light sources, thus enhancing reliability in various lighting conditions. The use of integrated circuits (ICs) simplifies the design and reduces the need for discrete components, which can improve the overall robustness of the circuit.

The transmitter's operation involves generating a precise timing sequence that alternates between active and inactive states. This modulation is crucial for ensuring that the receiver can accurately detect the IR signal. The receiver circuit is designed to respond specifically to the 36 kHz frequency, allowing it to filter out noise from other light sources. The output from the receiver can be used to trigger other devices, such as alarms or automated doors, making the circuit versatile for different applications.

The design considerations include the physical arrangement of the transmitter and receiver to minimize false triggers from reflected light and interference from ambient light. Using opaque tubes for both components helps to focus the IR signal and reduce the likelihood of unintended activation. Additionally, the calibration process ensures that the system operates effectively within its intended range, allowing for adjustments based on specific environmental conditions.

Overall, this light barrier circuit is an effective solution for monitoring entrances or spaces where presence detection is required, providing a reliable and efficient means of signaling based on infrared light modulation.The light barrier described here can be used to guard an entrance. You can use it to signal of someone is walking through the corridor, or to check if the car has been parked far enough in the garage to be able to close the door. The circuit consists of a transmitter, which sends modulated infrared light and a receiver, which recognises this.

The circuit used here is almost insensitive to daylight or fluorescent light and therefore can be used outside. The transmitter (Figure 1) generates about 1000 times per second, for a period of 540 ms, a burst of 36 kHz. IC1 has been set with C1, R1 and R2 to a frequency of about 1000 Hz. The output of IC1 ensures that IC2 will oscillate about 1000 times per second for a period of about 540 ms.

IC2 is set to a frequency of 36 kHz with C2, P1, R4 and R5. The output of IC2 drives the IR LED D1 via transistor T1. C3 and R3 prevent the reasonably high current through D1 from generating too much interference on the power supply rail. The receiver (Figure 2) is quite a simple design, because IC3 already does a lot of the work for us. When the IC sees` an IR-signal with a frequency of 36 kHz, the output of IC3 will become 0`. The transmitter circuit alternates between sending an IR-signal of 36 kHz for 540 ms and is quiet for 470 ms.

When this signal arrives at IC3, C4 will discharge via D2. Because the non-inverting input of IC4a is set to 2. 5 V, with the aid of R10 and R11, the output of IC4a will be a 1`. In the intervening quiet periods of 470 ms, C4 will partially charge via R8, but this is not of sufficient duration to exceed the voltage of 2. 5 V. Only when the light barrier is interrupted will C4 charge far enough that the output of IC4a will toggle and become a 0`.

Because IC4a has an open-collector output, C5 will be immediately discharged and the output of IC4b will become a 1`. With R9 and C5 this signal is stretched to about one second. If you increase the value of R9 to 100 k, then this will become about 10 seconds. R12 and R13 are included to prevent chatter of the output around the trigger point, although there is not really a risk of that happening in this circuit.

Together with R14, the output of IC4b delivers a clean logic signal that we can use for further processing. The quickest way of calibrating the frequency of IC2 to 36 kHz, using P1, is with the aid of an oscilloscope.

If you do not have one of those, then point the IR-LED D1 at the receiver IC3 and turn P1 so that the voltage on the inverting input of IC4a is as low as possible. Make sure that IC3 during the calibration does not receive too high a signal by placing the IR-LED a considerable distance away or by not pointing directly at the receiver.

If this procedure is not that successful then just set P1 to the center position, this works just fine usually. You should not have a problem with ambient light with this circuit. If you do have a problem because, for example, there is direct sunlight on IC3, then you will need to place it inside a small tube and point it at the IR LED.

In this way no direct sunlight can reach the receiver. If the IR LED and the receiver are placed too close together it is possible that the receiver will sense light reflected off the walls, even when someone is standing between the transmitter and receiver. In this case the solution is also a short piece of tube for both the transmit LED as well as the receiver (Figure 3).

Make sure that the tubes are opaque (paint black or use water pipe, for example). The wires to the IR LED can be several meters long without any problems. Do not place the receiver IC too far from the circuit.