6 LED VU meter using one transistor

This simple 6 LED VU meter using one transistor, allows us to visualize the level of an audio signal. Due to its few components, it is a cheap circuit and easy to build. The components are very common and very easy to obtain.

The interesting thing about this circuit is that it uses a single transistor as an active component and its operation affects the behavior of the entire circuit.

The progression of the illumination of the LEDs is linear, it is not logarithmic.

How does the 6 LED VU meter using a transistor works?

The audio signal is applied directly to diode D6. Diode D6 rectifies the incoming audio signal which is then delivered to the capacitor C1. C1 flattens the signal obtaining in its terminals a DC voltage proportional to the audio signal.

The R1, C2 set helps to eliminate the small voltage variations that may exist before applying the signal to the base of the bipolar transistor Q1.

The diodes D1 to D5 create a voltage difference between each LED of approximately 0.6 volts. Thus, a LED will light up after the 0.6 volt increase to the whole set of LEDs. The transistor Q1 works in its active zone and ….

• The closer the collector-emitter voltage (VCE) is to the saturation region, the more LEDs will light up.
• The closer the collector-emitter voltage (VCE) is to the cutting region, the fewer LEDs will light up.

6 LED VU meter using one transistor

The collector-emitter voltage of the transistor that controls how many LEDs light up depends on the level of the audio signal applied to the base of the transistor.

List of 6 LED VU meter components

• 1  2N2222 NPN transistor or similar (Q1)
• 6  1N4148 diodes or similar (D1, D2, D3, D4, D5, D6)
• 1  1N4148 silicon diode / 1N60 germanium diode or similar (D6) (see Note below)
• 6  LEDs, 2 green, 2 yellow or amber, 2 red (L1, L2, L3, L4, L5, L6)
• 1  82K resistor (R1)
• 4  330 ohms resistors (R2, R3, R4, R5)
• 2  390 ohms resistors (R6, R7)
• 1  1 uF (microfarad) capacitor (C1)
• 1  10 uF capacitor (C2)
• 1  9-volt battery or a 9 volt power source (BT1)
• 1  audio signal input connector (J1)

Note: To make this vu meter more sensitive and activate with weaker audio signals, you can change the 1N4148 silicon diode (D6) 1N4148  with the germanium diode 1N60.

If you make this modification, change the R1 resistor value to 2.2K and eliminate capacitor C1.