Infrared beam barrier

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We have built an infrared beam barrier by using a pair of an infrared transmitter and a photoelectric receiver. The system is able to detect the presence of an object when an infrared light beam is blocked from getting from the transmitter to the receiver. 

The infrared beam barrier can be used on burglar alarms, for automatic door opening, for building an automatic electronic doorbell or on any other automation where a detector of presence of an object is required.   

The maximum range of the invisible beam is approximately 5 to 8m and the transmitter and receiver pair operates on 12V. When the infrared light beam is blocked from any object, the detector activates a relay for 5 to 60 seconds.

The infrared transmitter circuit

The infrared transmitter consists of an oscillator operating at a frequency of approximately 1KHz. The oscillator generates a digital pulse sequence with a 10% duty cycle. This means that during one cycle, the signal at the oscillator output is at logic - high for the 1/10 of the cycle and for the remaining 90% of the cycle is at logic - low. The oscillator is a classic circuit of an astable multivibrator at 1KHz, which is based on the popular 555 integrated circuit. 

The infrared transmitter circuit
Figure 1. The circuit of the transmitter of the infrared beam barrier

The generated pulse sequence is applied to the base of the transistor T1. T1 operates as an electronic switch and turns-on and off two infrared LEDs connected in series (D3 and D4). The LEDs flashes during the on-state of the oscillator and this way they produce short light bursts. The R3 resistor is used to limit the electric current of the infrared LEDs within their maximum ratings.

The generated pulse sequence at oscillator's output
The generated pulse sequence at oscillator's output

R3 value has been calculated for a supply voltage of 12V. The transmitter can also operate at different voltages, typically from 6 to 15V. However, if you choose to use a different than 12V supply voltage, you should change the R3 value proportionally. R3 should withstand about 1/2W.

Assembly guide for the infrared transmitter
Figure 2. How to assemble the infrared transmitter

In our prototype, we have used Vishay's TSAL6200 infrared LEDs, emitting infrared radiation at 940nm. These LEDs withstand a current of 80mA and when they are in their on-state, they have a voltage drop of about 2V. Actually the circuit works with any type of infrared LEDs. You may use different infrared LEDs or/and different wavelength (for some types you may have to use a different value for R3 in order to adjust the current within its maximum ratings), however, the detector used in the receiver should operate at the same wavelength with the IR LEDs in the transmitter.

The infrared receiver circuit

At the receiver, we use an infrared phototransistor that operates at the same wavelength (at 940nm) with the transmitter LEDs. In the prototype we used the INFD3940 phototransistor and we have also successfully tested an OP550A. In practice, you may use any phototransistor you prefer, that operates at the same wavelength as the transmitter LEDs. Of course, the quality of the phototransistor also affects the sensitivity of the receiver.

The electronic circuit of the infrared receiver
Figure 3. The electronic circuit of the infrared receiver

On every received light pulse, the photo-transistor produces a positive electric pulse at the upper end of R3. The IC2-A op-amp amplifies the received pulses and drives the IC2-B Schmitt trigger.

IC2-A acts as a non-inverting amplifier and has a voltage gain equal to 1 + R10 / R7 = 34. Due to C8, the amplifier does not work at DC and at low frequencies, in order not for the receiver being affected from natural infrared sources. The response of the amplifier at much higher frequencies than 1 KHz is also limited by the performance characteristics of the op -amp (due to the gain-bandwidth product).

IC2-B functions as a hysteresis comparator (Schmitt trigger) and also inverts the signal. The lower and higher threshold values of the Schmitt trigger are set from R12, R13, R11 and R9 to about 0.798 and 0.802V, respectively. 

In the absence of a received signal, the output of the Schmitt comparator is in its high-logic level (at about 11V). During each received pulse of infrared light, the output of the comparator goes to a logic-low level (at about 1V). This means that during the 1ms cycle of the received signal, the output of the IC2-B goes at logic-low for 0.1ms and then returns to a logic-high level for 0.9ms.

The output of the Schmitt trigger
The received pulse sequence at the output of the Schmitt trigger

The R14-C11 network acts as an integrator. In the absence of received signal, C11 is fully charged to the supply voltage through R14. However, when the receiver receives the light beam from the transmitter, the negative pulses produced at the output of IC2-B, discharges C11 to logic zero within a short period of time.

IC3 contains internally a pair of quad-input NAND gates. By connecting the three of the four inputs of each gate to the supply voltage, we use these gates as NOT gates. The green LED lights up when the receiver receives the light beam from the transmitter. During any interruption of the light beam, the green LED turns off and a negative pulse is applied to the pin 2 of IC4 via C12.

Assembly guide for the infrared receiver
Εικόνα 4. How to assemble the infrared receiver

The IC4 (555) functions as a one-shot circuit. It is triggered from a negative pulse at pin 2 and produces a single pulse at its output (pin 3). The duration of the one shot pulse depends on the time constant (R16 + R17) × C13. Once triggered the circuit will remain in this state until the set time is elapsed, even if it is triggered again during this interval.

This means that once the infrared light beam is interrupted, the output of the IC4 switches to logic 1, the red LED lights up (D8) and the relay switches on. The relay remains energized for 5 to 60 sec (depending on the R17 trimmer setting) and then turns off again. The green LED lights up again once the light beam link between the transmitter and the receiver is restored.

How to build the Infrared beam barrier

To make things easy, we have designed two printed circuit boards. One printed circuit board is for the transmitter and the other one is for the receiver. You may download the printed circuit board’s artwork from the attachments – section below.

All components should be soldered to these printed circuit boards according to the assembly guides of Figures 3 and 4, for the transmitter and the receiver, respectively.

All the resistors we use in the circuit are of 1/4W type and of 5% tolerance. We use low loltage electrolytic capacitors (16V) and the remaining capacitors are of low voltage (50V) polyester or ceramic type. 

The transmitter does not need any initial adjustment. Once you assemble the boards and power them up, the green LED of the receiver should light up when you place the transmitter directly opposite to the receiver sensor. 

The receiver needs only one initial adjustment. You need to adjust the R17 potentiometer to set the time that the relay will remain on after each interruption of the infrared beam. This time interval can be set from about 5 to 60 seconds. If different setting limits are required, then you may change the value of R16 and /or R17.

You may use any type of infrared LEDs and phototransistor you prefer on the transmitter and the receiver. However, you should keep in mind that the infrared LEDs and the phototransistor should operate at the same wavelength (eg at 940, at 925, or at 835 nm or at any other wavelength). For some infrared LEDs or for some phototransistors that have very different characteristics than those we used in the prototype, you may need to change the values of R3 and / or R5 resistors. R3 should be selected on the basis that the maximum current specified by the manufacturer is not exceeded in the LEDs. R5 should be selected on the basis that the phototransistor should operate at an operating current within the limits recommended by its manufacturer.


The 2 printed circuit boards of the infrared beam barrier in a pdf file


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