The PIC Ultrasonic distance meter works by transmitting a short pulse of sound at a frequency inaudible to the ear (ultrasonic sound or ultrasound). Afterwards the microcontroller listens for an echo with the help of a few transistors and a comparator.
The time from transmission to echo reception lets you calculate the distance from the object.
|Range||~5cm - 300cm (approx.)|
|Internal oscillator frequency||4MHz|
The project uses 5 standard NPN transistors. Three are used as amplifiers in self bias mode to amplify the received signal from the sensor, and two are used as a darlington pair to transmit with higher current output to the transmitter.
A comparator is used to detect the threshold echo detection level - so there are no special components other than the microcontroller and the ultrasonic TX/RX modules which are standard 40kHz types.
Note: As you approach 300cm it is more difficult to receive a reflected signal so the practical range is probably nearer 200cm. The SR04 unit will perform better since it uses higher gain opamps.
This ultrasonic distance meter circuit is experimental and educational since you can buy ready made modules such as the HC-SR04 which are undoubtedly convenient - you supply a pulse trigger and they provide a distance (pulse width coded). These are quick and very easy to use but do not demonstrate what is going on.
This design uses external components, comparators and transistors to achieve the same result.
The 16F88 also has a built in comparator and reference level generator which would save components but this design can be used with any microcontroller that has a capture module. It is probably possible to use it with a 16F84 using some careful coding for time measurement.
Note: To use opamps you should use 3 with each set to a gain of 10 to give an overall x1000 gain (if using 324 types). If you choose a more capable opamp such as a modern rail-to-rail MCP301 or MPC302 then you can get away with less (due to the GBP being higher).
Note that the internal oscillator of the PIC micro is used and this saves two pins - that can be used as normal I/O pins.
|Compiler||Mikroelectronika MikroC compiler Free!|
|Target||16F88 (retargetable to other PICs that have a CCP module).|
|Software notes||Drives multiplexed display, controls CCP, interrupt use.|
|Hardware notes||High gain transistor amp.|
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You can recompile the project files if you want examine code operation (using the built in simulator) or change the source code. Note the hex file is contained in the download so you do not have the recompile the source code.
The time from transmission of the pulse to reception of the echo is the time taken for the sound energy to travel through the air to the object and back again.
Since the speed of sound is constant through air measuring the echo reflection time lets you calculate the distance to the object using the DST equation :
Distance = (s * t)/2 (in metres)
You need to divide by 2 as the distance is the round trip distance i.e. from transmitter to object and back again.
|s [m/s]||the speed of sound in air|
|t [s]||the round trip echo time.|
Some delay times:
|Round trip echo time||Distance|
|t = 588us||10cm|
|t = 5.8ms||1m|
Note: The speed of sound in air is more or less constant at 330m/s (@ 0ºC) - it varies mainly with temperature (~340m/s @ 20ºC). In this project I am using a value of 340m/s i.e. it is assumed that the project is used indoors. You can change it to whatever you like by modifying the code.
You can get ultrasonic transducers optimized for 25kHz, 32kHz, 40kHz or wide bandwidth transducers. This project uses a 40kHz transducer but it will still work with the others if you make simple changes to the software. The receiver and generator circuits will work as they are.
Note: If you use a different transducer you must change the software to generate the correct frequency for the transducer as they only work at their specific operating frequency.
The 40kz signal is easily generated by the microcontroller but detection requires a sensitive amplifier. I have used a three transistor amplifier for the receiver.
This is followed by a peak detector and comparator which sets the sensitivity threshold so that false reflections (weaker signals) are ignored.
This project makes use of the CCP module (in its capture mode) to accurately measure the signal reception time at the CCP port pin. When a signal triggers the CCP module the value of timer 1 is stored in a CCP register (or captured).
If you store the value of timer 1 and then enable the CCP after transmitting an ultrasound pulse the CCP will trigger when the comparator activates i.e. as soon as an ultrasonic echo is received.
Subtracting the stored value from the CCP register value gives the time delay in machine cycles. Since the project uses a 4MHz main clock then the time delay will be measured in micro-seconds.
The minimum distance of this scheme is about 5cm. Looking at the output of the first receiver amplifier shows a that it should be more accurate at lower distances - it is inaccurate by about 2cm which is still quite good. Probably the addition of amplifiers for the longer range stops accurate short range operation.
The maximum distance is limited by the sensitivity, gain and noise performance of the receive amplifier and also the transmit power and duration of transmission.
For this circuit the maximum distance is about 3m.
The previous design incorrectly used RA5 as output and it is the MCLRn pin that can only be used as an input. So RA5 drive to the seven segment was removed and the DP pin (Decimal Point) is left unconnected. RA6 and RA7 were moved up one 7 segment drive position.
You can use any PIC microcontroller that has an internal CCP module and enough memory to hold the program and program the PIC in circuit through the ICSP connector.
The circuit uses a three transistor amplifier and a two transistor output driver and it can be constructed out of standard component). The comparator is a well known M311 type (you could even use an opamp as a comparator, with suitable circuit changes, as this system is not particularly high speed).
First of all why use them?
I wanted to see what you could do using only transistors and it seems that you can do quite well i.e. the system works to the same distance as other designs using op-amps.
Of course when using op-amps you can achieve lower power operation and use less components.
The first two transistor amplifiers use standard biasing to set the output at the collector in the middle of the supply. If you look at the dc conditions the two (100k) input bias resistors across 5V to 0V set the input bias point at 2.5V. When the Vbe voltage is dropped across the transistor's emitter junction the voltage at the emitter is Vbias - 0.6 (approx 2V). So the emitter current =2/2k2 ~ 1mA. Ic=Ie. So the dc bias point is 5V-IcRc 2.7k*1mA ~ 2.5V.
The AC gain of these transistors is RC over RE (but at AC the capacitor has impedance of 40Ohms at 40kHz) so the effective Re is the intrinsic transistor emitter resistance (re~25ohms) plus the impedance of the capacitor (re is temperature dependent). So the gain is 2k7/65 ~ 40. If used at different temperatures you will get some gain variation.
The last transistor uses fixed biasing to set the bias point. For a more stable amplifier (less affected by Beta variation) use the same amplifier as the other two. I have just used it to see how it works as it can be seen quite often in other circuits and it seems to work well. It will however be dependent on the exact transistor used (its Beta value) and it will also be dependent on temperature - which will both affect its bias point and gain.
The comparator is setup as a standard circuit will a small amount of hysteresis (to stop oscillation if the input changes slightly) - the 1M ohm feeds back to set the hysteresis level.
Using an oscilloscope monitor the signals RB3 and RB0. Use RB3 as the trigger as this is the signal that regularly generates the ultrasound. RB0 is the detected echo.
Set the output of the comparator (RB0) low by turning preset VR2 fully in one direction. Point the transducers at an object at about 1 metre away and turn the preset until a signal appears (at about 6ms after RB3).
Set the output of the comparator (RB0) low by turning preset VR2 fully in one direction. Point the transducers at an object at about 1 metre away and turn the preset until a the display generates '100' (approx).
Move the board back and forwards to check that it displays a larger and then smaller number. Check the longer distance e.g. point at the ceiling and then a closer object e.g. a wall 20-50cm away. Adjust the preset as necessary.
You could remove the comparator and use the internal analogue comparator but this would require more software to set the comparator level. It would require 'Up' and 'Down' buttons to control the level settings or an algorithm could change the levels to find the optimum threshold.
With a temperature sensor you could change the value used for the speed of sound (currently fixed at 340m/s for 20ºC operation i.e. indoors!). This would make the PIC Ultrasonic distance meter more accurate in different environments.
Piezo elements also generate more output when a higher voltage is applied so using a voltage doubling circuit would increase the range of the unit. Also using a negative voltage generator and doubler would increase the overall voltage across the transducer output.
Compiler project files
C Source files.
For a tutorial on compiling these project files click here.
This contains all the code except the bit manipulation routines found in bit.h.
It enters a continuous a continuous loop calling ulta_gen - the routine that generates the ultrasound at 40kHz.
The ultra_gen routine is set up using the simulator to set the timing of the output signal for a period of 25us (40kHz). This is then repeated every 40ms. The required refresh rate of the seven segment display is 20ms so the display update routine (seg_display_int) is called twice over the 40ms period. (I should really say that the display update routine takes 20ms and calling this twice creates the total 40ms delay).
The display relies on persistence of vision to make it appear that the display is not flickering - a refresh rate of 50Hz or more does the job ( 1/50Hz = 20ms).
In theory the maximum distance that you could measure is 40ms*340m = (13.6) 6.8m (half the round trip time delay ) but in practice this is limited by the signal conditioning circuits. If they were changed you could get more range.
If a capture occurs indicated by gCapInt then the DST calculation is performed and the value of variable val is updated. val is the value displayed by the seven segment display routine 'seg_display_int' so val is continuously refreshed to the seven segment display.
The interrupt routine is only enabled when required and when the capture occurs (if it does) only the first capture is stored - so that later reflections are ignored (by resetting gCapOn).
The first reflection should be the strongest and therefore the closest object. When captured the variables t_capL,t_capH and t_capO are set to the value of the capture register which will be the value of timer 1 when the capture module triggered.
At the moment I have not used t_capO (and should do so) as it accounts for the roll over when timer 1 overflows. All that happens is that occasionally (when an overflow occurs) the wrong value will be generated - for hand held use it is not noticeable at all.
1. generate ultrasound at 40kHz (a few pulses of a square wave) to the TX
2. Turn on receiver.
3. Count time from end of 40kHz pulses to start of reception of reflected ultrasound
4. Ultrasound takes a set time to travel through air (varies mainly with temperature)
5. Apply D=SxT (Distance, Speed, Time)
You know the time taken for the round trip – the capture module is started at the end of the TX pulse – this just counts pulses of timer1 until it receives an input – when it does -the capture interrupt makes the program jump the interrupt routine where the current captured time is stored in:
t_capL = CCPR1L;t_capH = CCPR1H;
variable gCapInt = 1; // signal that a capture occurred.indicates to the main program that an echo was captured
calc = ((s1)<<8)+s2;change to 16 bit number
Multiply by 340 m/sto get cm divide by 100.
so calc * (340/100)/2
i.e. calc * 340/200
Final calculation is
calc * (((340/100)/2)/10) the same as 340/2000
But you don't want to do this calculation all at once if using onlyintegers as it will overflow so its split into 2 pieces
calc = calc * 34;
calc = calc / 2000;
i.e. its the same as calc *
340/2000 but does not
overflow the calc integer store.
The variable val is assigned
the calculated value
and then automatically displayed on the 7segments.
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