This is the second of three articles that will examine the LM391x series of LED driver ICs. The first covered the LM3914, this will cover the LM3915 and the LM3916 will follow. The goal of these is to have you using the parts in a small amount of time and experiment with your driver ICs, from which point you can research further into their theory and application.
Although these parts have been around for many years, the LM3915 isn’t used that much however for the sake of completeness we’re writing the tutorial. The LM3915 offers a simple way to display a logarithmic voltage level using one or more groups of ten LEDs with a minimum of fuss. If you’re wanting to make a VU meter, you should use the LM3916 which we will cover in the final instalment of this trilogy.
Instead of having each LED represent a voltage level as with the LM3914, each LED connected to the LM3915 represents a 3 dB (decibel) change in the power level of the signal. For more on decibels, check out Wikipedia.
To display these power level changes we’ll run through a couple of examples that you can use in your own projects and hopefully give you some ideas for the future. Originally by National Semiconductor, the LM391X series is now handled by Texas Instruments.
Getting Started
You will need the LM3915 data sheet, so please download that and keep it as a reference. First – back to basics. The LM3915 controls ten LEDs. It controls the current through the LEDs with the use of only one resistor, and the LEDs can appear in a bar graph or single ‘dot’ when in use. The LM3915 contains a ten-stage voltage divider, each stage when reached will illuminate the matching LED (and those below it in level meter mode).
Let’s consider the most basic of examples (from page two of the data sheet) – a simple logarithmic display of voltage between 0 and 10V:
After building the circuit you can connect a signal to measure via pin 5, and the GND to pin 2. We’ve built the circuit exactly as above on some stripboard for demonstration purposes, with the only difference being the use of an 8.2kΩ resistor for R2:
To show this in action we use a signal of varying AC voltage – a sine wave at around 2 kHz. In the following video, you can see the comparison of the signal’s voltage against the LEDs being illuminated, and you will see the logarithmic voltage increase represented by the LEDs:
We used the bar display mode for the voltage increase, and the dot display mode for the voltage decrease. Did you notice that during the voltage decrease, the LEDs below the maximum level being displayed were dim?
As the signal’s voltage was varying very quickly, the change in the LED’s location is a blur due to the speed of change. In the video below, we’ve slowed the frequency right down but kept the same maximum voltage.
Well that was a lot of fun, and gives you an idea of what is possible with the LM3915.
Displaying weaker signals
In non-theoretical situations your input signal won’t conveniently be between 0 and 10 V. For example the line level on audio equipment can vary between 1 and 3V peak to peak. For example, here’s a random DSO image from measuring the headphone output on my computer whilst playing some typical music:
Although it’s an AC signal we’ll treat it as DC for simplicity. So to display this random low DC voltage signal we’ll reduce the range of the display to 0~3V DC. This is done using the same method as with the LM3914 – with maths and different resistors.
Consider the following formulae:
As you can see the LED current (Iled) is simple, however we’ll need to solve for R1 and R2 with the first formula to get our required Vref of 3V. For our example circuit I use 2.2kΩ for R2 which gives a value of 1.8kΩ for R1. However putting those values in the ILED formula gives a pretty low current for the LEDs, about 8.3 mA.
Live and learn – so spend time experimenting with values so you can match the required Vref and ILED.
Nevertheless in this video below we have the Vref of 3V and some music in from the computer as a sample source of low-voltage DC. This is not a VU meter!Wait for the LM3916 article to do that.
Again due to the rapid rate of change of the voltage, there is the blue between the maximum level at the time and 0V.
Chaining multiple LM3915s
This is covered well in the data sheet, so read it for more on using two LM3915s. Plus there are some great example circuits in the data sheet, for example the 100W audio power meter on page 26 and the vibration meter (using a piezo) on page 18.
Conclusion
As always we hope you found this useful. Don’t forget to stay tuned for the final instalment about the LM3916.
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This is the first of three tutorials that will examine the LM391x series of LED driver ICs. In this first tutorial we cover the LM3914, then the LM3915 and LM3916 will follow. The goal of these tutorials is to have you using the parts in a small amount of time and experiment with your driver ICs, from which point you can research further into their theory and application.
Although these parts have been around for many years, the LM3914 in particular is still quite popular. It offers a simple way to display a linear voltage level using one or more groups of ten LEDs with a minimum of fuss.
With a variety of external parts or circuitry these LEDs can then represent all sorts of data, or just blink for your amusement. We’ll run through a few example circuits that you can use in your own projects and hopefully give you some ideas for the future. Originally by National Semiconductor, the LM391X series is now handled by Texas Instruments.
Getting Started
You will need the LM3914 data sheet, so please download that and keep it as a reference. So – back to basics. The LM3914 controls ten LEDs. It controls the current through the LEDs with the use of only one resistor, and the LEDs can appear in a bar graph or single ‘dot’ when in use. The LM3914 contains a ten-stage voltage divider, each stage when reached will illuminate the matching LED (and those below it in level meter mode).
Let’s consider the most basic of examples (from page two of the data sheet) – a voltmeter with a range of 0~5V.
The Vled rail is also connected to the supply voltage in our example. Pin 9 controls the bar/dot display mode – with it connected to pin 3 the LEDs will operate in bar graph mode, leave it open for dot mode.
The 2.2uF capacitor is required only when “leads to the LED supply are 6″ or longer”. We’ve hooked up the circuit above, and created a 0~5V DC source via a 10kΩ potentiometer with a multimeter to show the voltage – in the following video you can see the results of this circuit in action, in both dot and bar graph mode:
Customising the upper range and LED current
Well that was exciting, however what if you want a different reference voltage? That is you want your display to have a range of 0~3 V DC? And how do you control the current flow through each LED? With maths and resistors. Consider the following formulae:
As you can see the LED current (Iled) is simple, our example is 12.5/1210 which returned 10.3 mA – and in real life 12.7 mA (resistor tolerance is going to affect the value of the calculations).
Now to calculate a new Ref Out voltage – for example we’ll shoot for a 3 V meter, and keep the same current for the LEDs. This requires solving for R2 in the equation above, which results with R2 = -R1 + 0.8R1V. Substituting the values – R2 = -1210 + 0.8 x 1210 x 3 gives a value of 1694Ω for R2. Not everyone will have the E48 resistor range, so try and get something as close as possible. We found a 1.8 kΩ for R2 and show the results in the following video:
You can of course have larger display range values, but a supply voltage of no more than 25 V will need to be equal to or greater than that value. E.g. if you want a 0~10 V display, the supply voltage must be >= 10V DC.
Creating custom ranges
Now we’ll look at how to create a lower range limit, so you can have displays that (for example) can range from a non-zero positive value. For example, you want to display levels between 3 and 5V DC. From the previous section, you know how to set the upper limit, and setting the lower limit is simple – just apply the lower voltage to pin 4 (Rlo).
You can derive this using a resistor divider or other form of supply with a common GND. When creating such circuits, remember that the tolerance of the resistors used in the voltage dividers will have an affect on the accuracy. Some may wish to fit trimpots, which after alignment can be set permanently with a blob of glue.
Finally, for more reading on this topic – download and review the TI application note.
Chaining multiple LM3914s
Two or more LM3914s can be chained together to increase the number of LEDs used to display the levels over an expanded range. The circuitry is similar to using two independent units, except the REFout (pin 7) from the first LM3914 is fed to the REFlo (pin 4) of the second LM3914 – whose REFout is set as required for the upper range limit. Consider the following example schematic which gave a real-world range of 0~3.8V DC:
The 20~22kΩ resistor is required if you’re using dot mode (see “Dot mode carry” in page ten of the data sheet). Moving on, the circuit above results with the following:
Where to from here?
Now you can visually represent all sorts of low voltages for many purposes. There’s more example circuits and notes in the LM3914 data sheet, so have a read through and delve deeper into the operation of the LM3914.
Furthermore Dave Jones from eevblog.com has made a great video whcih describes a practical application of the LM3914:
Conclusion
As always we hope you found this useful. Don’t forget to stay tuned for the second and third instalments using the LM3915 and LM3916.
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Have you ever wanted more analogue input pins on your Arduino project, but not wanted to fork out for a Mega? Or would you like to generate analogue signals? Then check out the subject of our tutorial – the NXP PCF8591 IC.
It solves both these problems as it has a single DAC (digital to analogue) converter as well as four ADCs (analogue to digital converters) – all accessible via the I2C bus. If the I2C bus is new to you, please familiarise yourself with the readings here before moving forward.
Before moving on, download the data sheet. The PCF8591 can operate on both 5V and 3.3V so if you’re using an Arduino Due, Raspberry Pi or other 3.3 V development board you’re fine. Now we’ll first explain the DAC, then the ADCs.
Using the DAC (digital-to-analogue converter)
The DAC on the PCF8591 has a resolution of 8-bits – so it can generate a theoretical signal of between zero volts and the reference voltage (Vref) in 255 steps. For demonstration purposes we’ll use a Vref of 5V, and you can use a lower Vref such as 3.3V or whatever you wish the maximum value to be … however it must be less than the supply voltage.
Note that when there is a load on the analogue output (a real-world situation), the maximum output voltage will drop – the data sheet (which you downloaded) shows a 10% drop for a 10kΩ load. Now for our demonstration circuit:
Note the use of 10kΩ pull-up resistors on the I2C bus, and the 10μF capacitor between 5V and GND. The I2C bus address is set by a combination of pins A0~A2, and with them all to GND the address is 0x90. The analogue output can be taken from pin 15 (and there’s a seperate analogue GND on pin 13. Also, connect pin 13 to GND, and circuit GND to Arduino GND.
To control the DAC we need to send two bytes of data. The first is the control byte, which simply activates the DAC and is 1000000 (or 0x40) and the next byte is the value between 0 and 255 (the output level). This is demonstrated in the following sketch:
// Example 52.1 PCF8591 DAC demo
// https://tronixstuff.com/tutorials Chapter 52
// John Boxall June 2013
#include "Wire.h"
#define PCF8591 (0x90 >> 1) // I2C bus address
void setup()
{
Wire.begin();
}
void loop()
{
for (int i=0; i<256; i++)
{
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(0x40); // control byte - turn on DAC (binary 1000000)
Wire.write(i); // value to send to DAC
Wire.endTransmission(); // end tranmission
}
for (int i=255; i>=0; --i)
{
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(0x40); // control byte - turn on DAC (binary 1000000)
Wire.write(i); // value to send to DAC
Wire.endTransmission(); // end tranmission
}
}
Did you notice the bit shift of the bus address in the #define statement? Arduino sends 7-bit addresses but the PCF8591 wants an 8-bit, so we shift the byte over by one bit.
The results of the sketch are shown below, we’ve connected the Vref to 5V and the oscilloscope probe and GND to the analogue output and GND respectively:
If you like curves you can generate sine waves with the sketch below. It uses a lookup table in an array which contains the necessary pre-calculated data points:
For the following DSO image dump, we changed the Vref to 3.3V – note the change in the maxima on the sine wave:
Now you can experiment with the DAC to make sound effects, signals or control other analogue circuits.
Using the ADCs (analogue-to-digital converters)
If you’ve used the analogRead() function on your Arduino (way back in Chapter One) then you’re already familiar with an ADC. With out PCF8591 we can read a voltage between zero and the Vref and it will return a value of between zero and 255 which is directly proportional to zero and the Vref.
For example, measuring 3.3V should return 168. The resolution (8-bit) of the ADC is lower than the onboard Arduino (10-bit) however the PCF8591 can do something the Arduino’s ADC cannot. But we’ll get to that in a moment.
First, to simply read the values of each ADC pin we send a control byte to tell the PCF8591 which ADC we want to read. For ADCs zero to three the control byte is 0x00, 0x01, ox02 and 0x03 respectively. Then we ask for two bytes of data back from the ADC, and store the second byte for use.
Why two bytes? The PCF8591 returns the previously measured value first – then the current byte. (See Figure 8 in the data sheet). Finally, if you’re not using all the ADC pins, connect the unused ones to GND.
The following example sketch simply retrieves values from each ADC pin one at a time, then displays them in the serial monitor:
// Example 52.3 PCF8591 ADC demo
// https://tronixstuff.com/tutorials Chapter 52
// John Boxall June 2013
#include "Wire.h"
#define PCF8591 (0x90 >> 1) // I2C bus address
#define ADC0 0x00 // control bytes for reading individual ADCs
#define ADC1 0x01
#define ADC2 0x02
#define ADC3 0x03
byte value0, value1, value2, value3;
void setup()
{
Wire.begin();
Serial.begin(9600);
}
void loop()
{
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(ADC0); // control byte - read ADC0
Wire.endTransmission(); // end tranmission
Wire.requestFrom(PCF8591, 2);
value0=Wire.read();
value0=Wire.read();
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(ADC1); // control byte - read ADC1
Wire.endTransmission(); // end tranmission
Wire.requestFrom(PCF8591, 2);
value1=Wire.read();
value1=Wire.read();
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(ADC2); // control byte - read ADC2
Wire.endTransmission(); // end tranmission
Wire.requestFrom(PCF8591, 2);
value2=Wire.read();
value2=Wire.read();
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(ADC3); // control byte - read ADC3
Wire.endTransmission(); // end tranmission
Wire.requestFrom(PCF8591, 2);
value3=Wire.read();
value3=Wire.read();
Serial.print(value0); Serial.print(" ");
Serial.print(value1); Serial.print(" ");
Serial.print(value2); Serial.print(" ");
Serial.print(value3); Serial.print(" ");
Serial.println();
}
Upon running the sketch you’ll be presented with the values of each ADC in the serial monitor. Although it was a simple demonstration to show you how to individually read each ADC, it is a cumbersome method of getting more than one byte at a time from a particular ADC.
To do this, change the control byte to request auto-increment, which is done by setting bit 2 of the control byte to 1. So to start from ADC0 we use a new control byte of binary 00000100 or hexadecimal 0x04. Then request five bytes of data (once again we ignore the first byte) which will cause the PCF8591 to return all values in one chain of bytes. This process is demonstrated in the following sketch:
// Example 52.4 PCF8591 ADC demo
// https://tronixstuff.com/tutorials Chapter 52
// John Boxall June 2013
#include "Wire.h"
#define PCF8591 (0x90 >> 1) // I2C bus address
byte value0, value1, value2, value3;
void setup()
{
Wire.begin();
Serial.begin(9600);
}
void loop()
{
Wire.beginTransmission(PCF8591); // wake up PCF8591
Wire.write(0x04); // control byte - read ADC0 then auto-increment
Wire.endTransmission(); // end tranmission
Wire.requestFrom(PCF8591, 5);
value0=Wire.read();
value0=Wire.read();
value1=Wire.read();
value2=Wire.read();
value3=Wire.read();
Serial.print(value0); Serial.print(" ");
Serial.print(value1); Serial.print(" ");
Serial.print(value2); Serial.print(" ");
Serial.print(value3); Serial.print(" ");
Serial.println();
}
Previously we mentioned that the PCF8591 can do something that the Arduino’s ADC cannot, and this is offer a differential ADC. As opposed to the Arduino’s single-ended (i.e. it returns the difference between the positive signal voltage and GND, the differential ADC accepts two signals (that don’t necessarily have to be referenced to ground), and returns the difference between the two signals. This can be convenient for measuring small changes in voltages for load cells and so on.
Setting up the PCF8591 for differential ADC is a simple matter of changing the control byte. If you turn to page seven of the data sheet, then consider the different types of analogue input programming. Previously we used mode ’00’ for four inputs, however you can select the others which are clearly illustrated, for example:
So to set the control byte for two differential inputs, use binary 00110000 or 0x30. Then it’s a simple matter of requesting the bytes of data and working with them. As you can see there’s also combination single/differential and a complex three-differential input. However we’ll leave them for the time being.
Conclusion
Hopefully you found this of interest, whether adding a DAC to your experiments or learning a bit more about ADCs. Please consider ordering your PCF8591 from PMD Way.
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In this post we would like to share a series of interviews conducted by Dave Jones from eevblog.com. Dave interviews Colin Mitchell from Talking Electronics. Throughout the 1980s and onwards, Colin published a range of electronics magazines, tutorials and a plethora of electronics kits – of which many are still available today.
Tens of thousands of Australians were great fans of the TE products, and longed for their range of kits and products. We hope you enjoy these interviews, and if not – stay tuned for upcoming articles.
Once again, thanks to Dave Jones and of course Colin Mitchell from Talking Electronics for their interview and various insights.
This post brought to you by pmdway.com everything for makers and electronics enthusiasts, with free delivery worldwide.
To keep up to date with new posts at tronixstuff.com, please subscribe to the mailing list in the box on the right, or follow us on twitter @tronixstuff.
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