The RFzero has an on-board ULN2803A device which is a 50 V, 500 mA Darlington transistor array. The device consists of eight NPN Darlington pairs, that feature high-voltage outputs with common-cathode clamp diodes for switching inductive loads. The collector-current rating of each Darlington pair is 500 mA. The Darlington pairs may be connected in parallel for higher current capability. The ULN2803A device has a 2,7 kΩ series base resistor for each Darlington pair for operation directly with the MCU.

The RFzero can make use of the ULN2803A for a variety of purposes such as controlling the relays in a low pass filter bank, antenna relays, dish actuators or transmitter and PA keying.

Hardware

As can be seen from the logic diagram the ULN2803A has built-in output clamp diodes to suppress the kick-back voltage from
inductive loads. The ULN2803A is very robust but is still mounted in a socket for easy replacement – just in case.

Logic diagram for the ULN2803A. Picture courtesy Texas Instruments.

Functional diagram for one channel of the ULN2803A. Picture courtesy Texas Instruments.

When the COM pin is tied to the coil supply voltage the ULN2803A is able to drive inductive loads and suppress the kick-back voltage through the internal free wheeling diodes.

Diagram for connecting inductive loads, e.g. relays. Picture courtesy Texas Instruments and own work.

When driving resistive loads, the COM pin on the ULN2803A can be left unconnected or connected to the load voltage supply. If multiple supplies are used, connect the COM pin to the highest voltage supply.

For more details and connection examples please see the datasheet for the ULN2803A.

Software

To control each of the channels in the ULN2803A by the RFzero is a matter of setting up the connected D0 to D7 pins high or low as required. Please remember that the ULN2803A inverts the input signal. So if you want to pull a relay to ground the input signal to the ULN2803A a has to be high.

// RFzero include and object creation
#include <RFzero.h>               // MUST ALWAYS be included for RFzero

void setup()
{
    // Make D0 to D7 output pins and set low
    for (int i = 0; i < 8; i++)
    {
        pinMode(i, OUTPUT);
        digitalWrite(i, LOW);
    }

    // Activate D0
    digitalWrite(D0, HIGH);

    // Don't activate D1
    digitalWrite(D1, LOW);

    // Activate D2
    digitalWrite(D2, HIGH);
}

void loop()
{
}

// RFzero include and object creation

#include <RFzero.h> // MUST ALWAYS be included for RFzero

void setup()

{

// Make D0 to D7 output pins and set low

for (int i = 0; i < 8; i++)

{

pinMode(i, OUTPUT);

digitalWrite(i, LOW);

}

// Activate D0

digitalWrite(D0, HIGH);

// Don't activate D1

digitalWrite(D1, LOW);

// Activate D2

digitalWrite(D2, HIGH);

}

void loop()

{

}

If you have an eight bits variable where each bit tells if the ULA2803A input channel is high (1) or low (0), then you must unwrap each bit and send it to the ULN2803A bit for bit. It doesn’t matter if the variable is in decimal or hex value. A digital bit is a digital bit.

// RFzero include and object creation
#include <RFzero.h>               // MUST ALWAYS be included for RFzero

int relayControl = 0x28;          // 0x28 = 40 = 0010 1000

void setup()
{
    // Make D0 to D7 output pins and set low
    for (int i = 0; i < 8; i++)
    {
        pinMode(i, OUTPUT);
        digitalWrite(i, LOW);
    }
}

void loop()
{
    // Get the value of each bit in the relayControl variable and set D0-D7
    for (int i = 0; i < 8; i++)
    {
        int theBit = bitRead(relayControl, i);
        digitalWrite(i, theBit);
    }

    // Get the value of each bit in the relayControl variable and set D0-D7 in reverse
    for (int i = 0; i < 8; i++)
    {
        int theBit = !bitRead(relayControl, i);
        digitalWrite(i, theBit);
    }
}

// RFzero include and object creation

#include <RFzero.h> // MUST ALWAYS be included for RFzero

int relayControl = 0x28; // 0x28 = 40 = 0010 1000

void setup()

{

// Make D0 to D7 output pins and set low

for (int i = 0; i < 8; i++)

{

pinMode(i, OUTPUT);

digitalWrite(i, LOW);

}

void loop()

{

// Get the value of each bit in the relayControl variable and set D0-D7

for (int i = 0; i < 8; i++)

{

int theBit = bitRead(relayControl, i);

digitalWrite(i, theBit);

}

// Get the value of each bit in the relayControl variable and set D0-D7 in reverse

for (int i = 0; i < 8; i++)

{

int theBit = !bitRead(relayControl, i);

digitalWrite(i, theBit);

}