Rx - Key Finder Circuit - Pet or Kid Tracker Circuit

Key Finder Circuit – Pet or Kid Tracker Circuit

Pets and kids (under 4) have one thing in common, they are curious by nature and love venturing into unknown zones ending up in some sort of mess or trouble. Pet or kid tracker equipment which are quite similar to a key finder circuit were specifically designed for solving this issue, here we learn how these work and how to build one at home. The idea was requested by Mr. Akmar.

The Circuit Request

Hello sir,

thank you for posting this article..:) I really like it.

For your information, I want to build an object finder (ex:key finder) similar to this circuit and I will modify it a bit..the idea is when the receiver receive signal from the transmitter, it can produce sound if it detect the object..

if possible, can I substitute the load with a buzzer? thank you and I hope you can help me with this.

The Circuit Design

The requested application of a keyfinder is similar to what we normally do when one of our cell phones become untraceable in our home, we then resort to calling it through another phone so that it rings and identifies its location.

However when it comes to some other equally important item such as a key which has no calling feature to respond, locating it becomes hugely difficult and frustrating.

A simple remedy for this may be to attach some kind of a wireless device to the key chain so that whenever a misplacement of the thing occurs, the owner is able to quickly find it by connecting it through a matching transmitting handset.

The handset transmits the matching frequency forcing the attached key receiver to beep or produce the programmed sound for identifying itself.

The above concept may be also applied for tracking or monitoring purpose, for example on pets or small kids. Here the transmitting device could be attached to the member so that whenever the member tends to move out of a predetermined safe premise, the owner or the parent is instantly notified about it through an  alarm over a nearby or a pocket receiving device

Keeping a continuous watch over these elements may not be always feasible for all and therefore resorting to some kind of hi-fi device becomes a favorite alternative.

Below we see two diagrams which can be tried for the proposed pet finder or tracker and also for monitoring and restricting kids in parks or in building safe premise.

Using 315 MHz RF Modules

The heart of the circuits are the standard 315MHz RF modules that come in compatible packages as Tx/encoder and Rx/decoder.

The first diagram shows a Tx (transmitter) and its encoder module. The HT12E is the encoder chip while the upper small chip is the RF transmitter.

The Transmitter Circuit (Tx)

The function of the Tx chip is to generate the 315 MHz carrier signals and modulate it with the applied digital or an analogue data via its relevant pinouts.

This data must first go through a processing phase in order to make the data recognizable to the Tx chip. The digital or analogue input could be from an external source such as a PC USB, a sensor device or simply from an oscillator as used here.

The data is fed to one of the inputs of the encoder (since we have four inputs/outputs for this model, we may choose any one of these for the intended actions).

The diagram shows D0 being used as the input for the data feed, the data feed here is nothing but an ordinary square wave oscillator using a standard transistorized oscillator circuit consisting of Q1, Q2 and the associated parts.

The frequency from the astable constantly switches the encoder which in turn processes this signal and feeds it to the data input of the Tx.

Tx now makes sure that this data rides over the 315MHz carries and is transmitted into the atmosphere for enabling the Rx module in the vicinity to capture and reprocess these signals into the required tracking info.

An oscillating frequency is used as the trigger instead of a fixed switching to minimize current consumption of the Tx unit and ensure a much longer battery life.

The battery here could be a small 3V button cell, the entire circuit could be built using SMD to make the configuration as small as possible.

Transmitter Schematic

Rx 1 - Key Finder Circuit - Pet or Kid Tracker Circuit



The above system would be rather incomplete if it's not complemented by an Rx (receiver) circuit. The following circuit highlights the receiver system which works exactly in the same way as its Tx counterpart but in an opposite manner.

Here, the Rx chip is stationed for capturing the data sent by the Tx module. The captured data is mingled with the 315MHz carrier waves and is in an encoded form, therefore it must go through a reprocessing, which is done by the HT12D chip via its pin14 (data feed).

The Receiver Circuit (Rx)

The processed signal is anticipated across the relevant output pin of the decoder and terminated to produce a high or a low logic signal or as per the original data content.

In the discussed pet finder/tracker or the key finder circuit we have used D0 as the I/O pins for both the modules, therefore in the following Rx module we find D0 being hooked up with an external transistor buzzer driver stage.

The processed square wave output at D0 is now used for triggering the first BC557 transistor, which responds to these signals by staying continuously ON due to the inclusion of the base 330 ohm resistor and the filter capacitor 4.7uF stabilizing components.

As long as the first PNP stays ON, the second PNP is restrained from activating and keeps shut off but only for so long as the Tx module is within the transmitting range.

Once the Tx module which may be fixed on the kid or the pet drifts out of the predetermined safe zone, the RX module is inhibited from the signals. When this happens the second PNP gets a chance to trigger ON activating the attached buzzer.

The alarm sound instantly notifies the owner or the parent regarding the situation.

Tx 1 - Key Finder Circuit - Pet or Kid Tracker Circuit

In order to use the above circuit as a key finder, the right side BC557 will need to be removed and the buzzer replaced with the shown collector resistor of the left BC557.

Also Tx circuit will now need a switch for the required toggling and detection of a buzzing sound from the above Rx attached key circuit.


45 thoughts on “Key Finder Circuit – Pet or Kid Tracker Circuit

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
    • Hi Abu-Hafss,

      Thanks for the amusing yet technically correct question:) but I have no idea how to do it, probably you can throw some light on it and suggest an appropriate idea.

    • Hi Swagatam

      You might have noticed interference in speakers of radio, cassette or cd player, or even in land telephone, whenever a call is received at a nearby mobile phone. That is because of the strong signals.

      Similarly, I have seen Chinese tiny gadgets which are sticked to mobile phones as decoration. They have some colorful LEDs, which light up or blink in pre-arranged manner whenever a call is received.

      A small circuit can be designed to detect those signal waves and some additional circuit may be added to show the intensity of the signal.

  2. Hello Sir,
    I really need your help again..I just want to know what software did you use to make this schematic circuit? I used the Proteus and OrCAD to make circuit simulation based on this circuit but it seems that the ht12d and ht12e with the rf transmitter and receiver are not available in its library..so what software can I use to make the circuit simulation? can you suggest to me? fyi, I've also tried the eagle software but only the ht12e and ht12d are available..the rf transmitter and receiver was not..thank you for your help and waiting for your reply soon..:) have a nice day!

    • Hello Akmar, I use my mind and knowledge to simulate a particular circuit design, I have never used a simulator software, so i am sorry won't be able to provide any help on this.

  3. Hello sir, it's nice to meet you again..I've tried built this circuit..the receiver circuit works well as long as there is a power supply to the cct..for the tx cct, actually, I've done some modification, like put a push button connected to vcc and gnd so that it will act as a remote control..however, it works vice versa..for example, when I push the button, both buzzer and led will turn off whereas when I didn't push the button, as long there is a power supply to the cct, both led and buzzer still on..may I know which part that I've done wrong? Because what I wanted for my keyfinder is that when I push the button, then the led and buzzer will turn on..not turn off..Thank you very much sir,..have a nice day! 🙂 sorry for troubling you again..my lecturer also confused when I asked them..

  4. Hello sir,may i ask u.if i want to reverse the circuit function?is it possible?i want the buzzer ON when within range and OFF when out of range?hope u can help me sir.thank you

    • Hello Farid, yes you can do it by simply removing the extreme right hand side BC557 in the Rx circuit, and by replacing the collector resistor of the left hand side BC557 with the buzzer.

  5. Oh sir.what about the range?i want to make it a shorter range.but im using 433Mhz rf module.i already build the circuit on breadboard but it didnt work.i dont know why.

  6. Oh. I see.because i want to make it short range.if i cannot test it using breadboard how do i know the circuit is working?im working on my project that requires a short range sensor that will activates a solenoid.however im not going to use any microcontroller.sorry i wrote too much.need ur opinion.thanks sir

  7. Hi I am new to this.. and I have a project to make based on a key finder so I was wondering if what micro controller that was used for this, and also by any chance did you use coding to program this? and if so what was the codes you used?

  8. Hello sir. Iam shylaja, btech ece 2nd year student. Me and my friends hav decided to make this key finder circuit as our mini project. Can u please tell us what ics should we use for the circuits. Also what is the value of the resistor near BC557 in the receiver circuit??

  9. Hello sir. Me n my friends hav decided to make this key finder circuit as our mini project. Can u please tell us what ics should we use for the circuits. Also what is the value of the resistor near BC557 in the receiver circuit??

  10. Hello sir! Sorry to disturb u again. In one of ur replies to others, u hav said tat for the led n buzzer to turn ON the extreme bc557 is removed and buzzer must be connected in the place of collector resistor. Does tat mean, the 10k resistor u asked me to take must be replaced with buzzer?? Also how the antenna must be chosen??

  11. Hello sir! I have some queries about this circuit. What is the approximate range of distance for which this circuit works? What kind of oscillator is being used in transmitter circuit?
    Is there any speciation on the rf tx and rx module that we have to use here?

    • Hello Prabhakar,

      the range is within 50 meters, the oscillator is RF based, there are many versions of these Rx/Tx modules, each having its own unique range and specifications…any of those can be used for the mentioned application

  12. Good day sir. May i know if this circuit is working? And, can i make 3 pairs of transmitters and receivers? if yes, do i need to change some components? Because i think that the components in this circuit share the same frequencies right?

  13. Sir, here you have used a RF module, so instead of that will i be able to use a bluetooth based module as Tx/Rx ? if so may i know the changes that will occur in the circuit? and what are the new components that i have to buy.
    thank you 🙂

    • Shashini, You will just need to replace the Q1/Q2 stgae from the first circuit and associate it with your Bluetooth headset Mic wires.
      for the receiver you won't have to do anything, since the tone from the above modified Tx could be directly heard on the Bluetooth headset speakers, however this tone would be in the form of a continuous tone.

  14. dear sir,
    how should the antenna be chosen ?
    ive tried this circuit on a bread board..but doesnt work.
    is the led supposed to turn on when supply is given??

    • Maria, the antenna can be made by using a small piece of flexible wire.

      On breadboard you can make mistakes, so it is never recommended for high frequency RF circuits, you should build it by soldering on a well designed PCB or a veroboard.

      the LED will blink on signal reception from the Tx circuit.

      build it stagewise, first confirm the Rx/Tx circuits using only with LEDs, once you succeed with that then you can proceed with the external BJT circuits.

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