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Wheatstone click

‍Wheatstone Click is a measurement Click board™ which utilizes a Wheatstone bridge circuit onboard, in order to precisely measure the resistance of an external element. Besides the wheatstone bridge circuit, this Click board™ also utilizes MAX4208 – an ultra-low offset/drift, precision instrumentation amplifier, from Maxim Integrated.

click Product page

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Click library

• Author : MikroE Team
• Date : Jun 2020.
• Type : SPI type

Software Support

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

Package can be downloaded/installed directly form compilers IDE(recommended way), or downloaded from our LibStock, or found on mikroE github account.

Library Description

‍This library contains API for Wheatstone Click driver.

Standard key functions :

• wheatstone_cfg_setup Config Object Initialization function.

  void wheatstone_cfg_setup ( wheatstone_cfg_t *cfg ); 

• wheatstone_init Initialization function.

  err_t wheatstone_init ( wheatstone_t *ctx, wheatstone_cfg_t *cfg );

Example key functions :

• wheatstone_set_potentiometer Set potentiometer ( 0 - 100k )

  void wheatstone_set_potentiometer ( wheatstone_t *ctx, uint8_t pot_value );

• wheatstone_read_an_pin_voltage This function reads results of AD conversion of the AN pin and converts them to proportional voltage level.

  err_t wheatstone_read_an_pin_voltage ( wheatstone_t *ctx, float *data_out );

Examples Description

‍This example demonstrates the use of Wheatstone click board by measuring the input resistance.

The demo application is composed of two sections :

Application Init

‍Initializes the driver and logger and sets the default potentiometer (gain) level.

void application_init ( void )
{
    log_cfg_t log_cfg;
    wheatstone_cfg_t cfg;
 
    LOG_MAP_USB_UART( log_cfg );
    log_init( &logger, &log_cfg );
    log_info( &logger, " Application Init " );
 
    // Click initialization.
    wheatstone_cfg_setup( &cfg );
    WHEATSTONE_MAP_MIKROBUS( cfg, MIKROBUS_1 );
    wheatstone_init( &wheatstone, &cfg );
 
    wheatstone_set_potentiometer ( &wheatstone, WHEATSTONE_POT_MAX );
 
    log_info( &logger, " Application Task " );
}

Application Task

‍Reads the AN pin voltage and calculates the input resistance from it. All data are being displayed on the USB UART where you can track their changes.

void application_task ( void )
{
    float an_pin_v = 0;
    float vout = 0;
    float r_kohm = 0;
    if ( WHEATSTONE_OK == wheatstone_read_an_pin_voltage ( &wheatstone, &an_pin_v ) ) 
    {
        vout = an_pin_v / wheatstone.gain;
        if ( 0 != vout )
        {
            r_kohm = ( WHEATSTONE_VCC_5V - 2 * vout ) / ( 4 * vout );
        }
        log_printf( &logger, " VCC     : %.3f V\r\n", WHEATSTONE_VCC_5V );
        log_printf( &logger, " GAIN    : %.3f\r\n", wheatstone.gain );
        log_printf( &logger, " AN_PIN  : %.3f V\r\n", an_pin_v );
        log_printf( &logger, " VOUT    : %.3f V\r\n", vout );
        log_printf( &logger, " R_INPUT : %.3f kOhm\r\n\n", r_kohm );
        Delay_ms ( 1000 );
    }
}  

Note

‍The following formulas you may find useful:

• AN_PIN(V) = ( ( 1kOhm + R_INPUT(kOhm) ) / ( 1kOhm + 2*R_INPUT(kOhm) ) - 1/2 ) * VCC(V) * GAIN
• VOUT(V) = AN_PIN(V) / GAIN
• R_INPUT(kOhm) = ( VCC(V) * GAIN - 2*AN_PIN(V) ) / ( 4*AN_PIN(V) )
• R_INPUT(kOhm) = ( VCC(V) - 2*VOUT(V) ) / ( 4*VOUT(V) )

The full application code, and ready to use projects can be installed directly form compilers IDE(recommneded) or found on LibStock page or mikroE GitHub accaunt.

Other mikroE Libraries used in the example:

• MikroSDK.Board
• MikroSDK.Log
• Click.Wheatstone

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. The terminal available in all Mikroelektronika compilers, or any other terminal application of your choice, can be used to read the message.

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