Home > Circuit Bricks > Demodulator (Rectifier & Filter)
UPDATED:.21:05 21 October 2013

  • Rectifies an AC signal and filters out high frequencies
  • Ideal for monitoring audio signals on a meter or LED display.

An AC signal from 1 to 12v


A DC voltage matching the peak of the higher frequency input signals.

RectifierLow Pass Filter


The circuit works in 2 parts:

First the negative siganls are removed by the rectifier D1 & D2

Then the higher frequencies are removed by C2 & R1 acting as a low pass filter - so the output follows only slow changes in the input signal

Circuit Simulation

To change the response of the output, adjust R1 & C2.

  • Increasing R1/C2 slows down the output response
  • Decreasing R1/C2 speeds up the output.

Blue Trace = Input
Red Trace = Output

This circuit is useful for monitoring peak audio signals from a music source and driving a meter or dot/bar display driver. The circuit bricks for an audio meter are shown below.

Note the diodes used are germanium types which have a voltage drop of around 0.1-0.2 volts rather than 0.7 volts for a silicon diode. This reduces the loss in signal. Take care not to overheat these when soldering them.
Written by Phil Townshend - 2010
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