LED displays are necessary to indicate audio levels, measurements and even for ornamental purposes. The simplest way to design an LED display is the circuit around the popular bar graph IC LM 3914. Only few discrete components are required to construct of Bar graph display. The note given here is useful to design the display driver using LM 3914 and its related versions.
LM 3914 / 3915 / 3916 versions ICs are used in display circuits to drive either individual LED or Matrix LED. These are mainly used in circuits where precision output display is needed. Its each output becomes low one by one with the increment of 125 milli volts in the input. These ICs are used in Audio displays, Temperature meters, Decibel meters etc.
The major difference between LM 3914, LM3915 and LM 3916 are
1. LM 3914 Internal resistors have equal value. Produce linear response. Used as volt meter.2. LM 3915 Scale Logarithmically and span 0dB to 30 dB in ten 3 dB steps. Used in signal strength measurements.3. LM 3916 Internal resistors related to semi-log fashion to simulate VU meter. Pin connections are same in all ICs.
These ICs have 10 outputs each capable of sinking current to light LEDs brightly. Up to 4 LEDs can be connected to each output serially if the supply voltage is more than 9 volts. LED does not require a series resistor since the IC can regulate output current according to the value of the Programme resistor in the pins 6 and 7.
Pin 3 -Positive (3-15 V)
Pin 2- Ground
Pin 4 -Low end of the internal resistor chain.
Pin 5- Signal input. Each LED from pin 18 to pin10 lights at the increment of 125 milli volts in the input.
Pin 6- High end of the internal resistor chain.
Pin 7- Voltage reference output. 125 milli volt if pin8 is connected to 0 rail.
Pin 9- Mode selector. Dot mode display if not connected. Bar mode display if connected to positive rail.
Pin1- First LED lights at power on. Represents 0 dB.
Pin-18 to 10 LED connectors.
To reduce signal to the input, a variable resistor is needed. Otherwise all LEDs will light (in bar mode) or turn off (in dot mode). If the input is not connected to the 0 rail through a preset or resistor, Lat LED at pin10 will lock on.
It can be as low as 3 volts. It must be at least 1.5 volts more than the reference voltage applied to the “high end” (pin6) of the resistor chain. Standby current is around 3 mA with 5V supply when all LEDs are off.
Reference Voltage Pin 7 and 8
Reference voltage of 125 milli volts is brought out from the IC at pin 7 and 8. The reference voltage at pin7 can be increased up to 12 volts by connecting the reference adjust pin 8 to 0 rail through a resistor .This gives the advantage of setting the sensitivity of IC over wider limits.
Fixing LED current
LED current is programmed by the resistor connected to pin7 and pin8. In LM 3914, total resistance of the internal resistor chain is 10K. So the LED current can be calculated as
LED current =10(1.25V / R + 1.25v / 10K) Where R is the value of programming resistor at pin7, 8 and ground. With 1.2K programming resistor, individual LED current will be
10 (1.25/1200+1.25/10,000) = 0.0116 Amps or 11.6 milli Amps.
To get more brightness to LED, value of the programming resistor should be below 1K to increase current. If the value of the programming resistor is 560 ohms, the current through the LED is 23.5 milli ampere which gives sufficient brightness. Current should be between 20 to 30 milli ampere.
DOT Mode and BAR Mode
If the pin 9 of IC is left free, it shows DOT mode display ( if one LED lights, the previous one turns off ) and if pin 9 is connected to positive supply, it shows BAR mode ( All LEDs remain lit) display.
Input voltage to pin5 required to turn on last LED (pin10) is equal to the reference voltage applied to pin6 (high end). The input voltage required to turn on first LED (pin1) depends on the voltage applied to pin4 (low end). But the limitations are pin4 cannot be taken below 0 volt and pin6 1.5 volts below the supply voltage. Basic sensitivity of the IC is set by connecting pin 6 and 7. So that the input voltage required to turn on last LED (pin 10) is 125 milli volts. The low end (pin4) of the resistor chain is connected to the negative terminal of the reference (pin8) pin through 0 rail. So that the input voltage required to turn on first LED (pin1) is 125 milli volts. In this state, each increment of 125 milli volts turns another LED on.
1. By feeding the input voltage through the wiper of a preset. One end of the preset must be connected to negative rail while the other end is used to feed the input signal.
2. By connecting pin8 to 0 rail. So that reference voltage at pin7 can be increased. If a preset (10K) is connected between pin8 and ground, the sensitivity can be adjusted.
1. If a preset is connected to pin7 and ground and its wiper is connected to pin6, the input voltage to turn on last LED(pin10) can be reduced from 125 milli volts to 100 mill volts.
Expanding the range
If the input voltage is low, a reset ant pin7 and its wiper to pin4 enables the IC to increase the range of display.
Protection from dissipation
Absolute maximum power dissipation of the IC is 1365 mW. So when using for bar mode display, total dissipation should not exceeds 600 mW when all the LEDs are on. To calculate the power dissipation, deduct the forward voltage drop (Vf) of the LED (2 v for Red, Yellow and Green and 3.6 volts for Blue and White LED) from the supply voltage and then multiplying the value by the total LED current. For example, if the supply voltage is 12V and each LED takes 10 mA current then,
Voltage across the IC is
Maximum current of LEDs is
10+ (10×10) = 110 mA.
Maximum power dissipation is
10×110 = 1100 mW
This is dangerous to IC. So it is necessary to increase the value of the resistor at pin7 and 8 if the supply voltage is 12 volts.
Dissipation can be reduced by
1. Using Dot mode display since only one LED lights at a time.
2. By reducing the supply voltage to 3 to 5 volts.
3. By using separate power supply (3-5 V) for the LEDs.
4. By connecting a 470 ohms resistor in series with each LED.
5. By using low current LEDs that consume 10 mA or less with maximum brightness.
Any one of the method can be adopted.
Circuit sometime becomes unstable if the LED leads are longer than 150 mm. A 10 uF capacitor across the power supply lines close to pin2 and 3 will solve the problem.
Use of LEDs
Light output of LEDs varies considerably.” High bright LEDs” certainly produce a vivid display, but this is achieved only when the current level is 20 mA or more. It is better to use ‘Low current’ (2mA) LEDs. 3 mm LED is better than 5 mm LED. Transparent LED gives better display than the ordinary types. If low current LEDs arte used in dot mode display at 3 volts, better display can be obtained by increasing the value of programming resistor to 10K.
LM 3914 Design Note