How to Design Schematic Circuit of LED Bargraph

 This circuit drives ten LEDs in response to an analog voltage applied to its

input and can be used to provide a visual indication of the instantaneous

power being developed by an audio amplifier. lt can also be used to provide

visual indication of variable events in science experiments.

You can extend the project by using two chips and you'll be able to continuously monitor the audio output power of both channels of your stereo system.

The circuit can be used with amplifiers ranging from 0.1 to 100 watts.

In some cases you should use a small transformer, wired as in Figurc 1, to

isolate the input and match impedances. This ls necessary when the amplifier has a low-output impedance. Resistor R, is chosen according to the output power of the amplifier. Values are given in Table 1.

Table 1

Amplifier Output Power      Rx

0.1 to 1 watt

1 to 5 watts                               10 ohms, l/2 watts

5 to 20 watts                             22 ohms, 1 watt

20 to 50 watts                            47 ohms, 1 watt

50 to 100 watts                         100 ohms,2 watts

A schematic diagram of the LED bargraph is shownin Figure 2.The circuit

uses an LM3914 lC to drive the LEDs, converting voltages into one of ten

outputs.

The components are placed on a printed-circuit board as shown in Figure 3.

lf you d like to build your circuit the same way, you can etch your own board

using the solder and component-side templates shown in this figure.

P2 adjusts the dynamic range in which the circuit operates. P1 adjusts sensitivity and depends on the application. You can set the value to a fixed point by

using a trimmer potentiometer.

Audio comes from the loudspeaker output of the amplifier. Any small transformer with a high-impedance coil and a low-impedance coil can be used.

Any small transformer with a 117 VAC primary, secondary rated from 5 to

12.6 volts, and current ratings between 100 and 500 mA can be used.

Position of the polarized cornponents, such as LEDs, diode, and electrolytic

capacitor, should be observed.

Bass response is provided by C1. You can alter this component within the

range given in the schematic diagram to achieve best performance.

Remember that the highest applied voltage to the input shouldn't exceed the

power supply voltage.

Parts List - LED Bargraph

lC1 - LM3914 Integrated circuit (National)

LEDs - Common red, yellow or green

D1 - 1N914 or 1N4148 general purpose silicon diode

Rl - 10,000 ohm, 114W,5% resistol

R2 - 470 ohm, 114W,5% resistor

P1 - 10,000 ohm potentiometer

P2 - 4,700 ohm trimmer potentiometer

C1 - 1 to 10 uF, 12 WVDC electrolytic capacitor