You can examine the schematic diagram in the figure-1. The famous 555 chip is used to produce a 38KHz square wave. Frequency of the wave can be adjusted by turning the R5 potentiometer. The receiver side uses a TSOP1738 (HS0038) IR receiver module. It typically reacts to the 38KHz IR pulse and activates its output.
Figure-1, Schematic diagram of the IR obstacle detector
According to the TSOP1738 datasheet: “The TSOP17XX– series are miniaturized receivers for infrared remote control systems. PIN diode and preamplifier are assembled on lead frame, the epoxy package is designed as IR filter. The demodulated output signal can directly be decoded by a microprocessor. TSOP17XX is the standard IR remote control receiver series, supporting all major transmission codes.”
So these features, especially its embedded IR filter, make it suitable for our application. The figure-2 shows the designed PCB for the circuit. The PCB board mostly designed by SMD components which only take a small 3.2cm*1.8cm board size.
Figure-2, PCB board design of the IR obstacle detection circuit
As you can see on the PCB silkscreen, you must install an isolation barrier between the IR diode and the receiver. The two components are installed next to each other, so you have to make sure that the receiver only reacts to the “front-reflected” IR light.
If you could not find something to put between the sensors, just cover the 5mm IR diode with a black heat-shrink and allow the light to exit just from the front.
The figure-3 demonstrate a 3D view of the assembled board. You can inspect the installed components.
Figure-3, A 3D view of the PCB board
The LED D3 shows the correct power connection and the LED D1 lights up when an obstacle is located in the sensor detection range.
The R1 potentiometer determines the transmission power which of course affects the detection range. The potentiometer R5 adjust the frequency. If you have an oscilloscope, you should be able to see a signal like the figure-4 at the U1, pin-3 (output). Adjust the R5 to read 38KHz. Actually, the R5 allows you to install a wide range of similar IR receivers that might operate with different frequencies.
Figure-4, The 38KHz signal at the 555’s pin-3
A 3-pins male pin-header connects the board to an external circuit. You must connect the +5V supply to the + terminal and the ground to the – terminal. The “S” output acts as an active low trigger. It means when the sensor detects an object, the “S” output will be connected to the ground (as the LED D3 lights up). The SMD package of the SMD components is 0805.
You can download the PDF schematic, Gerber and NC-Drill files from the reference.