Home > Circuit Blox > Doorway Monitor

UPDATED:.00:36 03 January 2019

This circuit activates a relay for a set duration when a sound is detected above a certain level.

The length of time the relay is activated can be adjusted with VR1 on the Monostable.

The sensitivity of the switch (level of sound it responds to) can be adjusted with VR1 on the Mic Amplifier.

Any device up to 24v at 3A can be connected to the contacts.

VR1 sets the sound level to trigger on - c/w increases sensitivity.

Adjusting VR1 will have a little effect on the response time, but not much - set midway.

Set to Range 2 (0.05s - 2.5s), VR1 midway,
or Range 3, (2.4s+)

The Microphone Amplifier increases the signal from the microphone which detects any sound. The output level is controlled by VR1. Increasing (c/w) the output level increases the sensitivity of the switch.

The audio signal is fed to a Diode Pump which removes the high frequency audio signal and produces the amplitude voltage of the signal at Vout.

When Vout from the Diode Pump reaches about 0.8v the Inverter inverts the signal producing a low output.

This signal triggers the Monostable which switches on its output for a set duration.

The output from the Monostable connects to a Relay with driver and activates.


Sound waves and audio frequencies - connect a frequency generator to a transistor amplifier and speaker and see what frequencies can be heard.

Converting AC sound to DC signals for monitoring

Monostables producing timed outputs

  • Calculating time using T=1.1CR


This can be used to activate a motor of a display or mechanised toy, such as a snake coming out of a basket when ever a loud noise is made.

This could also be used as an alarm system that picks up sound from a shed or garage door opening. However once activated, it would never stop since it would stay triggered by its own alarm sound.