Stretch Click

How does it work?

Stretch Click is based on the circuitry that allows measuring the stretch forces of the 2mm diameter conductive rubber cord. In a “relaxed” state, the resistance is about 130 ohms per centimeter. The resistance increases as you stretch it out (the particles get further apart); for example, a 15cm piece is proportional to 2.1k ohms (25cm long stretch is 26/15*2.1K = 3.5k ohms). You can stretch the rubber about 50-70% longer than the resting length, so a 15cm piece shouldn’t be stretched more than 25cm. Once the force is released, the rubber will shrink back, although it’s not very “fast” and it takes a minute or two to revert to its original length.

The resistance of the cord increases when stretched, impacting the reverse voltage on the voltage divider, which can be measured. The Stretch Click allows stretch force readings to be available on an analog AN pin of the mikroBUS™ socket. It’s not a true linear sensor, and the resistance may vary from batch to batch, so we consider it a way to measure stretching motion, but it isn’t really precise. In addition, this Click board™ features a user-configurable LED1 light-emitting diode that can visually represent the measured force. This LED1 can be controlled over the PWM pin of the mikroBUS™ socket.

This Click board™ can operate with either 3.3V or 5V logic voltage levels selected via the J1 jumper. This way, both 3.3V and 5V capable MCUs can use the communication lines properly. However, the Click board™ comes equipped with a library containing easy-to-use functions and an example code that can be used, as a reference, for further development.

Specifications

Pinout diagram

This table shows how the pinout on Stretch Click corresponds to the pinout on the mikroBUS™ socket (the latter shown in the two middle columns).

Notes	Pin					Pin	Notes
Analog Output	AN	1	AN	PWM	16	SYN	External sync
	NC	2	RST	INT	15	INT	Interrupt
SPI Chip Select	CS	3	CS	RX	14	NC
SPI Clock	SCK	4	SCK	TX	13	NC
SPI Data OUT	SDO	5	MISO	SCL	12	NC
SPI Data IN	SDI	6	MOSI	SDA	11	NC
Power Supply	3.3V	7	3.3V	5V	10	NC
Ground	GND	8	GND	GND	9	GND	Ground

Onboard settings and indicators

Stretch Click electrical specifications

Software Support

We provide a library for the Stretch Click as well as a demo application (example), developed using MIKROE compilers. The demo can run on all the main MIKROE development boards.

Package can be downloaded/installed directly from NECTO Studio Package Manager (recommended), downloaded from our LibStock™ or found on MIKROE github account.

Library Description

This library contains API for Stretch Click driver.

Key functions

• Config Object Initialization function.

• Turn on the LED function

• Turn off the LED function

Example Description

The application is for stretch measuring

 void application_task ( void ) 
 { 
     stretch_data_t tmp; 

     //  Task implementation. 

     tmp = stretch_generic_read ( &stretch ); 
     log_printf( &logger, "** ADC value : [DEC]- %d, [HEX]- 0x%x rn", tmp, tmp ); 
     Delay_ms( 1000 ); 

     Delay_100ms( ); 

     if ( tmp < 500 ) 
     { 
         stretch_turn_on_led( &stretch ); 
     } 

     else 
     { 
         stretch_turn_off_led( &stretch ); 
     } 

     log_printf( &logger, " Resistance : %d rn", tmp ); 
     log_printf( &logger, "-------------------" ); 
     Delay_1sec( ); 
 }

The full application code, and ready to use projects can be installed directly from NECTO Studio Package Manager (recommended), downloaded from our LibStock™ or found on MIKROE github account.

Other MIKROE Libraries used in the example:

• MikroSDK.Board
• MikroSDK.Log
• Click.Stretch

Additional notes and informations

Depending on the development board you are using, you may need USB UART click, USB UART 2 Click or RS232 Click to connect to your PC, for development systems with no UART to USB interface available on the board. UART terminal is available in all MIKROE compilers.

mikroSDK

This Click board™ is supported with mikroSDK – MIKROE Software Development Kit. To ensure proper operation of mikroSDK compliant Click board™ demo applications, mikroSDK should be downloaded from the LibStock and installed for the compiler you are using.

For more information about mikroSDK, visit the official page.