Home » Sensors and Detectors » 3 Sound Activated Switch Circuits Explained
3 Sound Activated Switch Circuits Explained

3 Sound Activated Switch Circuits Explained

The post details 3 simple sound activated relay switch circuits which can used as a module for any system that might be assigned to trigger by detecting some kind of sound pressure level.Or simply applications  such as a  voice activated alarm security circuit.

1) Circuit Objective

Utilizing this basic sound activated switch design, toggling a system by sound pulse could be very effective, not simply on a robot but as well as for some kind of home automation. As an illustration this could be a sound-activated light bulb responding to a knock on the front door.

The lighting is going to be promptly switched off after several seconds. An optional implementation is security protection system when someone aspires to break open the front door or ruin a thing, the light bulb may be expected to come ON on, indicating that someone uninvited is at your home.

The circuit could work from any 5-12 VDC controlled power source as long as a relay with the appropriate coil voltage is employed.

Video Demonstration

How it Works

As soon as you first associate the source voltage to the sound activated switch circuit, the relay will likely be energized on account of the impact of capacitor C2.

You must permit a couple of seconds for the relay to be flipped off. It is possible to maximize or minimize the ‘on’ time frame by modifying the uF C2.

A larger uF contributes to an extended ‘on’ span, and the opposite way round. However, you should not employ a value exceeding beyond 47μF.

Biasing resistor R1 establishes to a significant degree the microphone level of responsiveness. An electret microphone commonly possesses just one central FET inside which demands a bias voltage to run. The best possible bias degree for response to audio or noise level must have to be discovered by experimentation.

All of the related and useful electronic protection precautionary measures is required to be recognized each time while connecting mains powered loads to the relay contacts.

Parts List

  • R1 = 5k6
  • R2 = 47k
  • R3 = 3M3
  • R4 = 33K
  • R5 = 330 OHMS
  • R6 = 2K2
  • C1 = 0.1uF
  • C2 = 4.7uF/25V
  • T1, T2 = BC547
  • T3 = 2N2907
  • D1 = 1N4007
  • Relay = coil voltage as per the supply voltage, and contact rating as per the load specs
  • Mic = electret condenser MIC.


The concept can be used as a vibration activated LED lighting, for sound triggered recording systems. It can also used as a sound toggled night bed room light circuit

2) Sound Activated Switch with Customized Sound Frequency

The next project below explains a simple, accurate remote control system through sound vibration that will work on a particular sound frequency. Therefore it's perfectly foolproof since it won't be disturbed through other unwanted sound or noise.

The idea was requested by Mr. Sharoj Alhasn.

The Sound Sensor Circuit

The figure shows the circuit of a sound detector circuit which can be effectively converted into a remote control, triggered using a sound generator handset.

We have already learned a lot regarding this wonderful frequency decoder LM567 IC. The IC will lock-on into any frequency that's fed across its input and which exactly matches the frequency fixed across its pin5 and pin6 via the relevant R/C components.

The formula for determining the latching frequency across pin5/6 may be calculated using the following formula:

F = 1/R3xC2,

where C is in farads, R is in Ohms while F is in Hz.

Here it's set to around 2kHz.

Pin3 is the input of the IC which tracks, responds and locks on an frequency which may be reaching the 2kHz figure.

Once the IC detects this, it produces a zero logic or an instant low at its output pin8.

This low at pin8 sustains as long as the frequency at the input pin stays active, and becomes high as soon as it's removed.

Circuit Diagram

In the discussed sound triggered remote control circuit, a MiC is configured across pin3 of the IC.

An external matching frequency (2kHz) in the form of an audible sound or whistle is pointed toward the mic such that the sound hits the mic starighton.

The mic converts the sound into electrical pulses corresponding to the received frequency at the relevant input pin of the IC.

The IC immediately acknowledges the matching data and reverts the output into a low for the necessary actions.

The output may be directly connected with a relay if only a momentary toggling is required or only for the time the input is active.

For an ON/OFF switching the same may be configured with a FLIP-FLOP circuit.

Sound Activated Remote Transmitter Circuit

The following circuit may be utilized for generating an audible frequency for the above described sound remote receiver circuit.

The circuit is based on a simple AMV concept using a few ordinary transistors and some other passive parts.

The frequency of this transmitter circuit must be first set to the receivers matching frequency which is calculated to be 2kHz. This may be done by suitably adjusting the 47k preset and monitoring a latching response from the receiver simultaneously.


The above explained project which uses foolproof unique frequency for sound triggering can be specifically for remote locks in cars, house doors or safes for jeweler's shops and office entrances etc

3) Alarm Trigger with Sound using Piezo

Sp far have learn regarding ON/OFF application using noise generation, now let's see how the same could be used for triggering an alarm, whenever a noise or a sound is detected.

A simple sound triggered alarm circuit is a device which is used for triggering an alarm on detection of a sound vibration. The sensitivity of the unit is set externally according to the requirement of the user.

The circuit discussed in this article can be implemented for the above purpose or simply as a security device for detecting an intrusion. For example it can be fitted in a car for detecting a possible intrusion or a break-in.

Looking at the circuit diagram we see that the circuit uses only transistors and therefore becomes very easy even for a new hobbyist to understand and make the system at home.

How it Works

Basically the whole circuit is made up of two small signal amplifiers which are connected in series for doubling the sensing power.

T1, T2 along with the associated resistors becomes the first small signal amplifier stage.

The introduction of the 100K resistor across the emitter of T2 and the base of T1 plays an important role in making the amplifier stage very stable due to the feedback loop connected from the output to the input of the stage.

The input of T2 is connected to a piezo transducer element, which is used as a sensor here.

Sound signals hitting the piezo transducer surface is effectively converted to tiny electrical pulses which are amplified by the amplifiers made from T1 and T2 to a certain higher level.

This amplified signal which becomes available at the collector of T2, is fed to the base of a high gain PNP transistor T3 via the 47uF coupling capacitor.

T3 further amplifiers the signals to yet higher levels.

However, the signals are still not strong enough and won't detect the minute sound vibrations, probably which might be emitted by human physical contacts over a particular body.

The next stage which is a replica of the first stage, consists of the transistor T4 and T5.

The amplified signals generated at the collector of T3 is further coupled to the above stage for the final processing.

T4 and T5 makes sure that the signals are amplified to the required limits as per the units expectations.

If the piezo is attached to, say for example a door, even a slight knock over the door will be easily sensed and the alarm connected to T5 will become active.

The 10uF capacitor across the 10K preset keeps the alarm activated for a few seconds of time, its value may be increased for increasing the above delay of the alarm sound.

The discussed sound activated alarm circuit will work with any supply in between 6 and 12, however if the alarm is a powerful one, the current might have to be selected accordingly.

The preset may be used for setting the sensitivity of the circuit.

Circuit Diagram

For the sensor, a 27mm piezo transducer will work the best, the following figure shows the image of this device:


The sound vibration operated switch as explained above looks suitable for creating alarm or siren alarms in response to sound vibrations and therefore could be installed under mats or fixed on doors as safety alarm units.

Whenever a intruder or thief tries to trespass the area by stepping on the mat or opening the door, the sound activates the alarm allowing the user and the neighboring people to get warned about the break-in.


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

54 thoughts on “3 Sound Activated Switch Circuits Explained”

  1. Hi Swagatam, I bread boarded the “alarm trigger with sound using piezo” and the buzzer sounds when power is applied and continues. Have you checked this circuit? When I look at the circuit I understand that if T1 is triggered, then T2 will be off and T3 will be off. Will T4 be on and therefore T5 will be on. If T1 is off, then T2 will be on and T3 will be on and T4 will be off and therefore T5 will be off. I really don’t understand capacitors used this way, so I am at a disadvantage. If you have time, please explain the circuit for me.

    • Hi Norman, it is a thoroughly tested circuit, but looks like an overkill….why don’t you use the first circuit with a piezo?

      You can replace the MIC with a piezo and check the response…

      for the last circuit you can try connecting an LED in series with the 1K or T3…this LED should remain shut off unless the piezo is knocked.

      This will give you a clear idea regarding which stage is wrongly configure or is faulty.

  2. Swagtam ji ,I am in need of a circuit of digital indicator of number of whistle produced by a pressure cooker , i.e. digital indictor shows 1 for the first whistle , 2 for second whistle and so on . The circuit should have a button to reset counter. It can avoid surrounding noise so as to show exat number of whistle. As I am 65 yrs of age some times it becomes difficult to count the exact number of whistle due to doing other house hold work .Thanking u in anticipation……………..P K Bajpai

    • Hello Pradeep ji, that’s a very good project idea, I’ll surely try to design it soon for you! The challenge is to make the the circuit “listen” to the cooker whistles and not any other sound disturbances…I’ll try to design and post it, and let you know once it’s done!

      • Thanx a lot Swayam ji I am very much obiliged towards such a prompt attention . Pl. design a circuit without incorporating Arduino or Programmable ICs as I do not have any practical experience on them. Thanking u again ……..P. K Bajpai , Lucknow ,UP

  3. Hi Swagatam,
    I think I figured it out. I increased the resistance of R1 and it is now more sensitive to sound. Your comments please.

    • By changing the R1 value you have changed the MIC’s internal FET’s biasing level, so that might have caused the MIC to become a bit more responsive…you can also try increasing the C1 value and see if that helps to further improve the performance.

  4. Hi Swagatam,
    I would like to make this circuit more sensitive. I removed the 4.7uF cap so it would follow music and flash a LED. It works but misses some of the sound, so I would like to make it more sensitive. Suggestions please. Thanks! Norman

    • Hi Norman,

      yes removing the 4.7uF cap will remove the latching function of the circuit and will allow instantaneous detection of the input vibrations.

  5. Sir,

    I want to make a Circuit which Can Active BY getting a siren Sound since 3 second Continiously …

    And relay will Active For 30 Second….. sir please help me easily
    It will very helpful for me

    • then please do exactly as I have suggested in the previous comment…make the two referred deigns separately first and check them separately…

  6. Sir it will Very hard for me . i am basic Hobbist . Sir I want make a nice device with your kind help. i have a good idea . i want to tell you details then i think you will must help me.
    So Sir how can i contact with you . and also i can discribe my idea to you.

  7. Sir please help me
    i want to modify this circuit. i want like this when, get a sound Continuously 3 seconds Then the relay will energized for 35 seconds . After 35 seconds relay will turn off Automatically. Everything will Reset

  8. Sir Actually I Want To tell you that
    I want Make this System when This Circuit get a Sound Continuesly 2 Second Only then The will Start Working.

  9. Thank You Sir For Your Kind Reply.

    When This Circuit Get Sound Since 2 or 3 Second Then The Relay Will On.
    Sir please Help me

  10. Dear Sir
    I want to Make A System.
    I Want To Connect My Alarm Circuit Speaker Out Cable To Your Sound Active Relay On Circuit. When Get A Sound This Circuit Will Get On. My Mobile Speed Dail Number 1 is Connect With This Circuit relay. So when This Circuit get any alarm Then Relay will on. Thn My mobile genate a call . Now

    Sir I Want To Remove My Mobile Microphone and Replace Mp3 out line on the point of my mobile Microphone
    So Sir How Can i do this ???

    Can u help me sir

    • Dear Sharoj,

      I am not sure about how to replace mobile mic with MP3 feed, you will have to consult this with an expert mobile phone technician…

  11. Hello Sir If Get Any Sound Relay Will Active. But For How many Second ??
    How Can i Adjust This As my Wish??

    And Sir Another Question Is
    How Can I Remove My Phone Microphone And Replace 2 Mp3 out Cable Inthe place of My Mobile microphone

    Thank You Sir

    • Sharoj, yes it will be activated with any form of signal input…you can alter C2 value for the setting the time duration as per your requirement.

      I am sure what exactly you are trying to do with your MP3 and cellphone mic….please clarify more for a better understanding.

  12. Yes Sir I Need mp3 sound active relay on Circuits. Sir basically I Want To Make An Circuit When this Circuit Get a signal From my Alarm Circuit this Circuit relay will on an i Want To Connect A phone Speed dail i mean 1 button with relay point. So when get a Sound I Will get a Call

    • OK, then you can connect the junction of C1/R2 and the ground wires with the MP3 input…and the relay contacts configured with the phone speed dial button internally

  13. Sir,

    How can I modify the circuit in such way that another similar sound or clap is required to turn off output (return relay NC position )

    regarding your previous reply…
    Shorting R6 with 10uF capacitor not behaving to eliminate Infrequent chattering effect.

  14. Thanks for posting such interesting circuit..
    How to cancel Relay chattering effect ( some time happening) while returning NC position ?
    How to increase or decrease sensitivity ( louder or low sound) triggering NO position?

    • to stop chattering, try using a 10uF capacitor parallel to R6….

      sensitivity can be increased to some extent by modifying (increasing) the value of C2

Leave a Comment