Wireless Baby Monitor

Pradeep G. He is B.Sc. (Physics) and a regular contributor to international magazines. He is also a small-business owner in south India.

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This wireless baby monitor circuit using FM is designed to operate at a frequency of 100MHzefy tested. Its range of transmission is more than 100 metres with a 75cm wire antenna. The circuit is fully transistorised and so even a beginner can easily assemble it on a veroboard or the PCB whose pattern is given below for convenience.

Wireless Baby Monitor circuit

fig 1
Fig. 1: Circuit of the wireless baby monitor

Circuit diagram of the wireless baby monitor circuit is shown in Fig. 1. It is built around a condenser microphone (MIC1), transistors BC549 (T1 and T3) and BC557 (T2), along with a few other components.

parts listT1 and T2 provide high-gain audio to the VHF oscillator wired around T3. BC549 can oscillate well in VHF range. Frequency-modulated (FM) signals are transmitted through antenna ANT.1.

By adjusting trimmer capacitor VC1, frequency can be set within 88MHz -108MHz band. Using a good FM radio (analogue type) receiver, transmitted signals can be heard. Do not use a mobile phone as receiver.

fig 2
Fig. 2: Inductor details

This unit with 9-volt battery can be kept near a baby’s bed. Any sound generated by the baby including a cry can be monitored wirelessly from another room. Readily available 1-micro Henry inductor can be used as L1. It can also be home brewed as shown in Fig. 2.

Construction and testing

fig 3
Fig. 3: PCB pattern of the wireless baby monitor

A single side PCB pattern of the wireless baby monitor circuit is shown in Fig. 3 and its component layout in Fig. 4. After assembling the circuit on a PCB, enclose it in a plastic case and keep it near the baby’s bed.

fig 4
Fig. 4: Component layout of the PCB

Download PCB and component layout PDFs: click here

The circuit works well off a 9V battery.


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13 COMMENTS

  1. Hello my name is Varun and I m making an Advanced Baby Bassinet and monitoring system.
    If i m not mistaken, the circuit shown above is to be kept near a baby. If so, then what will be used as a receiver ?
    If u have the circuit of that too please mail that photo or pdf to [email protected].
    Or u can mention about that in next months EFY issue as i have the subscription and i can read it from it.
    Please this info will be really helpful for my project.

    • In the above write up, its mentioned that you can use a normal FM radio receiver to receive the signals
      I copy the relevant para below:
      By adjusting trimmer capacitor VC1, frequency can be set within 88MHz -108MHz band. Using a good FM radio (analogue type) receiver, transmitted signals can be heard. Do not use a mobile phone as receiver.

  2. Analog FM receivers are easily available in electronics shops and online such as ebay.in. Mic1 is an electret condenser microphone available in electronic component shops

  3. My Name is venkat.I had tried this circuit but I can hear only Air blown sound sound on the Mic.Please explain how to increase the Audible sound in the circuit
    You can drop me mail on [email protected]

  4. Vary VC1 slowly until you get clear audible sound in your FM receiver. Also make sure you tune to the same frequency in the FM receiver.

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