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Wireless Silent Notifier

Please see the attached Wireless-Silent-Notifier-master.zip for code files.

This is a perfect project for new mums and dads or anyone thatneeds to be alerted without making a sound. Included is a simple base station that picks up wireless signals, a sensor (in this case a sound level sensor) and a strobe light. As soon as the baby cries the strobe light will go off alerting you. Whilst 1 detector is included in this project you can use any number of different types, depending on what you want to measure and be alerted for (temperature, moisture, door entry, ph levels etc.).

Bill of Materials

Base Station

1MP305812V power supply
1XC4430Leonardo main board
1ZW3102433Mhz receiver
1LA5328LED Strobe Light
1ZT2116BC338 NPN Transistor
A small resistor and some wire to act as antenna


Use any style of transmitter you want, this is just an example

1ZW3100433Mhz Transmitter
1PH92802AA switched battery compartment
1XC4438Sound sensor module
Some spare wire to act as antenna

Connection schematics

Base Station

The 12V strobe light is a very simple circuit, we are simply using the Vin pin to syphon the 12V from the DC jack into the strobe light, using the NPN transistor as a "low side switch"

We connect Pin 8 to drive the circuit; when pin 8 is high, current is applied to the base of the transistor and switches on, allowing the strobe to connect to ground and complete the circuit.

We've used a 1k resistor but you've got playing room for different values depending on how much current you want to drive through the transistor.

Use the falstad app to play around with different set-ups and see if your resistors work and how much current they provide.

The receiver module is easy enough to install, one side has VCC, 2x GND, and ANT (for antenna) and the other side has VCC,GND, and 2x DATA

Connect VCC and GND to where they've got to go (on both sides), then DATA to pin13 on the Arduino, and connect a 17mm wire to ANT

ArduinoReceiver ZW3102TransistorDescription
5VVCCPower supply
GNDGNDCollectorGround connections
13DATAData from wireless receiver to the Arduino
8Base ( with resistor)through the resistor to the base of the transistor
ANT17mm wire to act as antenna
VinPositive lead of LED Strobe light

The Emitter of the Transistor should be connected to the negative (black) lead of the strobe light.

  • ●   You should be able to test the code now and find that you get random data on pin13 and turning pin8 on/off will turn on/off the strobe light.


The transmitter is a simple set up, as ZW3100 only has 4 connections ( with an extra ground):

DATAData input

DATA is the only pin that we're concerned about here, after you've connected the power rails and 17mm antenna. Whenever DATA is activated, the transmitter will send a signal burst to be picked up by the receiver.

For our purposes, we're going to be using the XC4438 sound module, that has a digital output when the sound level goes beyond the threshold value, which is set with the potentiometer.

Sound Module providing a digital output (indicated with the second LED in the top picture) - when the LED is on, it is transmitting a digital signal (not audio) to be picked up by our base station.

Connections are simple:

Sound ModuleTransmitterBattery Pack
GGNDBlack lead
+VCCRed Lead

With the antenna on ANT. You can probably do much better design than what we have in the image above.


This is a very simple project and only requires a few lines of code. Dig into the code and have a read of the comments to make sure you understand what is going on.

One of the main points about the code is figuring out what signal is from our sensor. In our unit, we are simply checking for over 200 points of the same data signal, which signifies that there's something transmitting that continuous stream of either 0's or 1's.

While the transmitter is off, our receiver picks up something similar to this:

You could even try and see it in the Serial monitor, and it'd give the same garbled information:

However once our transmitter is on, we get something like this:

Our code simply counts the number of repeated signals, and checks that it's more than 400 samples of the same signal (40milliseconds worth), before turning on the strobe light for 4 seconds.


Simply connect 12v power to the Arduino, and turn the transmitter on. Adjust the potentiometer on the transmitter so that the second LED only lights up when it can detect sound (speak into it a few times to get the value just right) - then place where you need it to be.

Back at the base station, you'll be notified any time the sensor picks up some audio.

Future Improvements

Our default code just checks for a long chain of FSK data, if you want to, you can use another Arduino to encode information, such as "ID" or certain values, and then decode it on the base station.