An ultrasonic range finder using an SRF05 and an ATtiny85

Atmel ATtiny85

An ultrasonic range finder is a useful tool in a variety of real-life and robotic applications, such as in obstacle avoidance and distance measurement systems. The ultrasonic range finder measures the distance by emitting a 40 kHz pulse of ultrasonic sound that travels through the air until it hits an object, then it measures the delay of the reflected signal and sends proper commands to other units.

I used an SRF05 ultrasonic sensor and an ATtiny85 microcontroller. The distance data is displayed on a 128×64 OLED screen, both in centimeters and inches. Also, a horizontal bar graph provides a visual estimation of the distance. The MCU code was developed using the Arduino IDE.

To design the schematic and PCB, I used Altium Designer 22 and SamacSys component libraries (Altium plugin). To get high-quality PCB boards, I sent the Gerbers to PCBWay and purchased original components using componentsearchengine.com. To examine the current consumption of the circuit, I used the Siglent SDM3045X multimeter.

Isn’t cool?! So let’s get started.

Specifications

Input voltage: 6-24 VDC
Current consumption: 24 mA
Detection range: 2-400 cm (see text)
Distance data: centimeters, inches, bar graph
Display: 128×64-yellow blue OLED

Circuit analysis

Figure 1 shows the schematic diagram of the ultrasonic range finder device. As it is clear, the circuit consists of four main parts: sensor, power supply, MCU, and display. I explain each part separately.


Figure 1.	*Schematic diagram of the ultrasonic range finder.*

SRF05 ultrasonic sensor

I used an SRF05 ultrasonic module for the circuit. There are a variety of SRF05 modules in the market, I used the one that is shown in figure 2.


Figure 2.	*An SRF05 ultrasonic module.*

The quality of the modules might vary, so the maximum detection range can not be guaranteed. Some of them have blue solder-mask and some are green, also different manufacturers provide such modules.

According to the SRF05 module datasheet:”The SRF05 is an evolutionary step from the SRF04, and has been designed to increase flexibility, increase range, and to reduce costs still further. As such, the SRF05 is fully compatible with the SRF04. The range is increased from 3 meters to 4 meters. A new operating mode (tying the mode pin to the ground) allows the SRF05 to use a single pin for both trigger and echo, thereby saving valuable pins on your controller. When the mode pin is left unconnected, the SRF05 operates with separate trigger and echo pins, like the SRF04. The SRF05 includes a small delay before the echo pulse to give slower controllers such as the Basic Stamp and Picaxe time, to execute their pulse in commands.”

Power supply

The main component of the power supply is TS2937CW50 regulator (REG1). It is a +5 V SOT-223 LDO regulator. According to the TS2937 datasheet: “TS2937 of fixed-voltage monolithic micro-power voltage regulators is designed for a wide range of applications. This device excellent choice of use in battery-power applications.

Furthermore, the quiescent current increases slightly at dropout, which prolongs battery life. This series of fixed-voltage regulators feature a very low ground current (200 µA typ.) and very low drop output voltage (Typ. 60 mV at light load and 600 mV at 500 mA). This includes a tight initial tolerance of 2%, extremely good line regulation of 0.05% typ., and very low output temperature coefficient.”

FB1 and C5 reduce input voltage noises. D1 is a blue 0805 LED to indicate a proper supply connection and R2 limits the current of D1. C4 and C6 are used to reduce the noises of the +5 V supply rail. P1 is an XH-2P female connector to connect the supply wires to the board.

Microcontroller

IC1 is an ATtiny85 MCU, which is the heart of the circuit. I selected the SMD package of this chip. According to the ATtiny85 datasheet: “The ATtiny25/45/85 provides the following features: 2/4/8K bytes of In-System Programmable Flash, 128/256/512 bytes EEPROM, 128/256/256 bytes SRAM, 6 general-purpose I/O lines, 32 general purpose working registers, one 8-bit Timer/Counter with compare modes, one 8-bit high-speed Timer/Counter, Universal Serial Interface, Internal and External Interrupts, a 4-channel, 10-bit ADC, a programmable Watchdog Timer with Internal Oscillator, and three software selectable power saving modes. Idle mode stops the CPU while allowing the SRAM, Timer/Counter, ADC, Analog Comparator, and Interrupt system to continue functioning. The power-down mode saves the register contents, disabling all chip functions until the next Interrupt or Hardware Reset. ADC Noise Reduction mode stops the CPU and all I/O modules except ADC, to minimize switching noise during ADC conversions. The device is manufactured using Atmel’s high-density nonvolatile memory technology. The On-chip ISP Flash allows the Program memory to be re-programmed In-System through an SPI serial interface, by a conventional non-volatile memory programmer, or by an On-chip boot code running on the AVR core.” C1, C2, and C3 are decoupling capacitors and are used to reduce noises. R1 is a pullup resistor to avoid the unwanted triggering of the MCU’s RESET pin.

OLED display

The display consists of a 0.96” 128×64 OLED module with an SSD1306 controller chip. The data/command interface of the module is I²C. Figure 3 shows a picture of this module. I²C consists of SDA and SCL lines that need to be pulled up using two resistors. This module has already implemented the pullups, so there is no need to add more resistors on the line.


Figure 3.	*OLED Display of the device (0.96”, 128×64, I²C).*

PCB layout


Figure 4.	*The PCB layout of the ultrasonic range finder.*

Figure 4 shows the PCB layout of the circuit. It is a two layers PCB board and all components are SMD. You can download Gerber files from Downloads section. The display should stand a few millimeters above the PCB to maintain a distance between the components and the backside of the display to avoid and short circuit… etc. The PCB is designed to be compact and easy to use. Table 1 shows the bill of materials.

Table 1.

Bill of materials

Designator	Description	Package/ Footprint	Type
C1, C2, C3	100 nF-50 V ceramic	0805	SMD
C4	10 µF 25 V ceramic	0805	SMD
C5	100 µF 25 V 6.3 × 7.7 mm aluminum		SMD
C6	4.7 µF-25V ceramic	0805	SMD
R1	4.7 kΩ 5% 125 mW	0805	SMD
R2	470 Ω ±1% 125 mW	0805	SMD
D1	Blue	0805	SMD
P1	XH-2A		В отв.
REG1	TS2937CW50	SOT223-3	SMD
IC1	ATtiny85-20SU	SOIC-8	SMD
LCD1	SSD1306, 128 × 64		TH
SRF05	HY-SRF05 (5 Pins)		TH
FB1	Ferrite bead FBG3216-301Y	1206	SMD

When I decided to design the schematic and PCB for this project, I realized that I don’t have the component libraries of REG1 and IC2 in my component libraries storage. So as usual, I selected IPC-rated SamacSys component libraries and installed the missing libraries (schematic symbol, PCB footprint, 3D model) using the free SamacSys tools and services. There are two ways to import the libraries: You can visit the Electronic Component Search Engine [1] and download and import the libraries manually, or you can use the SamacSys CAD plugins [2] and automatically import/install the libraries into the design environment. I use Altium Designer, so I installed the missing libraries using the SamacSys Altium plugin [3]. Figure 5 shows a 3D view of the board and two assembly drawings of the PCB board.


Figure 5.	*A 3D view and two assembly drawings of the PCB board.*

Code

The code of the MCU is written as below. You need to install the ATtinyCore Board Manager [4] and select ATtiny25/45/85 (no bootloader) from the menu. Then select the Chip as ATtiny85 and Select 8 MHz (Internal) for the clock source.

Then you need to install the NewPing [5] and Tiny4KOLED [6] libraries. After this just go to the “Sketch” menu and select “Export compiled binary”. That’s it. You can find the compiled HEX file in the same folder as your code exists. Just use an AVR ISP programmer (such as USBasp or whatever) to program the chip using the available pins on the backside of the PCB (GND, RESET, MISO, MOSI, SCK). Follow the procedure and program the fuse bits as shown in figure 6. Disconnect the programmer and wires from the board and that’s it.


Figure 6.	*How to program the board using an AVR ISP Programmer.*

The firmware can be downloaded from the link in the Downloads section.

Assembly and test

Figure 7 shows the assembled PCB board. I put a piece of double-sided sticky tape between the display and PCB to avoid any possible short circuit. As it is clear in the picture, the characters on the OLED screen are quite bright and vivid.


Figure 7.	*Assembled PCB board of the ultrasonic range finder.*

If you plan to power the board using a battery, then the current consumption of the device is important. So I used the Siglent SDM3045X benchtop multimeter and measured the current as 24 mA. Figure 8 shows the multimeter screen.


Figure 8.	*The current consumption of the ultrasonic range finder circuit.*