LED level indicator

1. The LED level indicator introduced in this example has two display modes: spot type and light type. It can be used as a playback level indicator or an audio power output level indicator.
Circuit working principle
The LED level indicator circuit consists of four operational amplifier integrated circuits IC (N1-N4), resistors R1-R5, light-emitting diodes VLI-V14, capacitors C and diodes VD1-V D7, as shown in Figure 6-135.
When the input audio signal level is less than 0.7 V, N1 outputs a high level and lights up VLI; when the input signal level is between 0.7 and 1.4 V, N2 outputs a high level, which lights up V L2 on the one hand, and on the other hand, the inverting input terminal of N1 becomes a high level through V D6, so that N1 outputs a low level and VLI goes out. Similarly, if the input signal level becomes higher, VU and V L4 will light up respectively.
By removing diodes VD4 - VD7, the circuit becomes a four-level LED level indicator. As the input signal level changes, VLI-VIA will light up step by step.

Component selection
R1-R5 use 1/4W carbon film resistors or metal film resistors.
C uses an aluminum electrolytic capacitor with a withstand voltage of 16V.
VD1-V D7 use 1N4148 silicon switching diodes.
VL1-VIA use Φ5mm light-emitting diodes.
IC uses LM324 quad op amp integrated circuit.

2. The LED level indicator introduced in this example can be connected to the output end of the audio power amplifier circuit as an output level indicator of the power amplifier, or it can be connected after the audio preamplifier circuit (before the volume potentiometer) as a playback or recording level indicator
.

The LED level indicator circuit consists of an adjustable gain amplifier and an LED drive circuit, as shown in Figure 6-136.
In the circuit, the adjustable gain amplifier consists of an operational amplifier IC, resistors R01-R05, a potentiometer RP, and capacitors C1 and C2; the LED drive circuit consists of a transistor V, a capacitor C3, a voltage-stabilizing diode VS, resistors R1-Rn, light-emitting diodes VL1-VLn, and diodes VD1-VDn.
The audio input signal from the power amplifier or preamplifier is coupled to pin 5 of IC through C2. After being amplified by IC and V, the signal voltage is output from the emitter of V to light up VL1-VLn step by step. The stronger the audio input signal, the more light-emitting diodes are lit.

Component selection
RO1-R05 and R1-Rn use 1/4W carbon film resistors or metal film resistors. RP uses ultra-small potentiometers or vertical variable resistors.
C1-C3 use aluminum electrolytic capacitors with a withstand voltage of 16V. VD1-VDn use 1N4148 silicon switching diodes or 2AP5VS use 1/2W, 3.6V silicon voltage regulator diodes.
VU-VLn use 5mm red high-brightness light-emitting diodes.
V uses C8050 or 58050, 3DG8050 silicon NPN transistors.
IC uses LM324 operational amplifier integrated circuit.