Key Features of the LM3914

Here are some of the top features and specs of the LM3914:

• Senses analog voltage levels and drives 10 LEDs
• Provides linear and logarithmic 3 dB/step analog display
• Operates from single supply of 3V to 25V
• Internal voltage reference from 1.2V to 12V
• Programmable LED current up to 30 mA
• Input impedance of 10 kΩ
• Adjustable analog threshold

These features make the LM3914 well-suited for creating simple analog meters, battery voltage monitors, audio level meters, and other LED bar graph displays. It requires minimal external components and is easy to interface with.

LM3914 Pinout and Pin Functions

The LM3914 comes in an 18-pin dual-inline package (DIP). Here is the pinout and a description of what each pin does:

The LED pins (1-11) connect directly to the cathodes of the 10 LEDs in the display. The REF OUT and REF ADJ pins allow you to set the internal voltage reference. SIG IN is where you apply the input voltage signal to be measured.

RHI and RLO set the LED currents and are tied together for bar mode. V+ is the supply voltage input and GND is ground. The MODE pin is left open for bar mode and pulled to V+ for dot mode.

How the LM3914 Works

Internally, the LM3914 has a 10-stage voltage divider that sets the switching thresholds for driving the LEDs. It compares the input signal voltage to these threshold voltages and turns on the appropriate number of LEDs.

In dot mode, only one LED is illuminated at a time, corresponding to the level of the input signal. As the input voltage increases, the lit LED moves from LED1 to LED10.

In bar mode, the LEDs are turned on sequentially from LED1 up to the LED that indicates the input signal level. So multiple LEDs can be lit at the same time, forming a bar graph.

The LED current is set by the resistor values connected to RHI and RLO. A reference voltage applied to REF IN determines the switching threshold voltages. This makes it easy to calibrate the display for the desired input signal range.

Typical LM3914 Circuits

Let’s look at some common example circuits using the LM3914.

Basic Bar Graph Display

Here is a basic LM3914 circuit that creates a 10-segment LED bar graph display:

The 10 LEDs connect to pins 1-11 with current limiting resistors. A potentiometer on the REF ADJ pin sets the reference voltage and calibrates the display. The input signal connects to SIG IN. RLO and RHI are tied together and a resistor from RHI to V+ sets the LED current.

Adjustable Range Voltmeter

This circuit uses the LM3914 to create an adjustable range voltmeter:

A multi-turn potentiometer on REF ADJ allows you to fine-tune the reference voltage and scale the display for different input voltage ranges. The input signal is scaled down by a voltage divider on SIG IN. The LED currents are set as before.

Audio Level Meter

An LM3914 can be used to display audio signal levels, as shown here:

The audio signal is AC coupled and fed to SIG IN. The reference is set up as a simple voltage divider. Capacitors filter the LED currents and the supply voltage for a cleaner display.

Important Usage Notes

When using the LM3914 in your projects, keep these tips in mind:

• Always use current limiting resistors on the LED pins to protect the LEDs and IC from damage. Values of 220Ω to 1kΩ are typical.

• Size the resistor from RHI to V+ to set the LED current between 2-30mA. 10-20mA is a good LED brightness for most applications.

• The reference voltage should be very stable for an accurate display. Use a voltage reference IC for REF IN if needed, rather than deriving it from the supply voltage.

• Be mindful that the LM3914 can only sink LED current, not source it. Make sure your LEDs are oriented correctly. Add a buffer if you need to drive higher currents.

• The LED currents in dot mode are 10x higher since only one LED is on at a time. Use a higher resistor value on pin 15 to compensate.

• In noisy environments, bypass the supply voltage with a 0.1uF ceramic capacitor close to the IC.

• The LM3914 has an absolute maximum supply voltage of 25V. For higher voltages, add an external regulator.

Frequently Asked Questions

What is the difference between the LM3914 and LM3915?

The LM3914 provides a linear display, while the LM3915 has a logarithmic 3dB/step response. This makes the LM3915 better suited for audio applications like VU meters. Otherwise, they are functionally the same.

How do you make the LM3914 display in dot mode instead of bar mode?

To put the LM3914 in dot mode, pull the MODE pin high to V+. For bar mode, leave MODE disconnected.

Can the LM3914 drive more than 10 LEDs?

The LM3914 is limited to driving 10 LEDs directly. However, you can use multiple LM3914s to create displays with more than 10 steps. It’s also possible to multiplex the outputs for larger displays.

What happens if the input signal goes above the reference voltage?

If the input signal exceeds the reference voltage, all 10 LEDs will be lit. The display “rails” at the top and won’t go any higher. To avoid this, make sure to set the reference voltage higher than the maximum expected input voltage.

Can I use the LM3914 to measure negative voltages?

The LM3914 can only measure positive voltages out of the box. To handle negative voltages, you need to add a DC offset to shift them into a positive range. Alternatively, consider using the LM3916 dot/bar display driver which has a built-in precision half-wave rectifier for measuring AC signals.

Conclusion

The LM3914 LED bar graph driver is a handy and easy-to-use IC for adding a visual analog display to your electronic projects. Its 10-step resolution and linear/logarithmic output options make it suitable for a wide variety of applications, from simple battery meters to professional audio gear.

In this LM3914 Guide, we’ve covered the key features, pinout, internal operation, example circuits, usage tips, and answers to common questions. Hopefully you now have a solid understanding of how to put this versatile chip to work in your own designs!

With its minimal external component count and single-supply operation, the LM3914 is a great choice for quickly adding an eye-catching bar graph or dot/point display. So consider picking up a few LM3914s and start experimenting – I think you’ll find it a useful addition to your electronic toolbox.