Ultrasonic Range Finder Using 8051

A simple ultrasonic range finder using 8051 microcontrollers is presented in this article. This ultrasonic rangefinder can measure distances up to 2.5 meters at an accuracy of 1 centimeter. AT89s51 microcontroller and the ultrasonic transducer module HC-SR04 form the basis of this circuit. The ultrasonic module sends a signal to the object, then picks up its echo and outputs a waveform whose time period is proportional to the distance. The microcontroller accepts this signal, performs necessary processing, and displays the corresponding distance on the 3 digits seven-segment display. This circuit finds a lot of application in projects like automotive parking sensors, obstacle warning systems, terrain monitoring robots, industrial distance measurements, etc.

1. 8051 microcontroller

2. HC-SR04 ultrasonic module

3. 3 digits 7segment display

4. 2N2222 NPN BJT

5. 100ohm resistor

6. 8.2kohm resistor

6. 11.0592MHz crystal

7. 33pF capacitor

8. 10uF capacitor

HC-SR04 is an ultrasonic-ranging module designed for embedded system projects like this. It has a resolution of 0.3cm and the ranging distance is from 2cm to 500cm. It operates from a 5V DC supply and the standby current is less than 2mA. The module transmits an ultrasonic signal, picks up its echo, measures the time elapsed between the two events, and outputs a waveform whose high time is modulated by the measured time which is proportional to the distance. The photograph of an HC-SR04 module is shown in 1st photo.

The supporting circuits fabricated on the module make it almost stand alone and what the programmer needs to do is to send a trigger signal to it for initiating transmission and receive the echo signal from it for distance calculation. The HR-SR04 has four pins namely Vcc, Trigger, Echo, GND and they are explained in detail below.

1) VCC: 5V DC supply voltage is connected to this pin.

2) Trigger: The trigger signal for starting the transmission is given to this pin. The trigger signal must be a pulse with 10uS high time. When the module receives a valid trigger signal it issues 8 pulses of 40KHz ultrasonic sound from the transmitter. The echo of this sound is picked by the receiver.

3)Echo: At this pin, the module outputs a waveform with high time proportional to the distance.

4) GND: Ground is connected to this pin.

From the timing diagram, you can see that the 40KHz pulse train is transmitted just after the 10uS triggering pulse and the echo output is obtained after some more time. The next triggering pulse can be given only after the echo is faded away and this time period is called the cycle period. The cycle period for HC-SR04 must not be below 50mS. According to the datasheet, the distance can be calculated from the echo pulse width using the following equations.
Distance in cm = echo pulse width in uS/58

The ultrasonic module is interfaced to the microcontroller through P3.0 and P3.1 pins. Port0 used for transmitting the 8-bit display data to the display and port pins P1.0, P1.1, P1.2 are used for transmitting display drive signals for the corresponding display units D1, D2, D3. Push-button switch S1, capacitor C3 and resistor R9 forms a de-bouncing reset circuitry. Capacitors C1, C2, and crystal X1 are associated with the clock circuit.

The first part of the program sets the initial conditions. Port 0 and Port 1 are set as output ports for sending digit drive patterns and digit drive signals respectively. Port pin 3.0 is set as an output pin for sending the trigger signal to the ultrasonic module for starting transmission and port pin 3.1 is set as an input pin for receiving the echo. TMOD register of the microcontroller is so loaded that Timer 1 operates in mode2 8 bit auto-reload mode. Timer 0 of the microcontroller is not used here. In the next part of the program (loop MAIN), the TL1 and TH1 registers of Timer1 are loaded with the initial values. TL1 is loaded with the initial value to start counting from and TH1 is loaded with the reload value. This is how timer 1 in mode 2 works: When the TR1 bit of the TCON register is set the TL1 starts counting from the initial value loaded into it and keeps counting until rollover (ie; 255D). When a rollover occurs, the TF1 flag is set and TL1 is automatically loaded with the reload value stored in TH1 and the sequence is repeated until TR1 is made low by the program. The TF1 goes high at the first rollover and if you want it as an indicator for each rollover, you have to clear it using the program after each rollover. In the next part of the MAIN loop, P3.0 is set high for 10uS and then cleared to make a 10uS triggering pulse. The ultrasonic module issues a 40Khz pulse waveform after receiving this trigger and the program waits until a valid echo is received at P3.1. The pulse width of the echo signal is proportional to the distance to the obstacle and so the next job of the program is to measure the pulse width. Whenever there is a valid echo pulse at P3.1, the Timer1 starts and it counts from the initial value to 255 ie: 255-207= 48 counts. Then the counter restarts and accumulator increments by one for every restart. This sequence is repeated until the echo signal at P3.1 vanishes (ie; P3.1 goes low). Now the content in A will be equal to the number of Timer1 reloads which is in fact proportional to the distance. From the datasheet, it is clear that 58uS echo pulse width indicates 1cM distance. When the processor is clocked by a 12MHz crystal, 58 counts of Timer1 indicate 1cM. That means 1 reload is equal to 1cM. But here we are letting the Timer1 count only 48 times before reload and this is done in order to compensate for the time lags caused by the branching instructions used for checking the status of P3.0 and P3.1 pins. If this trick is not done, the individual time lags caused by the branching instructions will be cumulatively added to the observed pulse width and the range finder will show a reading higher than the original distance. Some trial and error were required for getting the correct Timer1 to reload value and with the 207D (ie; 48 counts) used here the error was found to be less than half a centimeter which is quite fine in this context. The next part of the program does necessary mathematics on the current content in A and displays it as 3 digit readout on the display.

the hex file for the microcontroller is provided below or u can generate it using the Keil software step are given in pdf

Ultrasonic library file to download click here

(copy library files to

C:\program files (x86)\labcenter electronics\proteus professional\LIBRARY)

the ultrasonic hex file for simulation to download click here

Proteus simulation files click here