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Key Finder or Pet Tracker Circuit

Key Finder or Pet 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.

Key Finder using 433MHz RF Modules

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 sing 433MHz RF modules..the idea is when the receiver receives signal from the transmitter, it can produce sound if it detects the object..

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

The 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 anΒ  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.

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

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.

Receiver Schematic

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.



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!

63 thoughts on “Key Finder or Pet Tracker Circuit”

  1. sir when the object is moving out of range buzzer is ON but again suddenly when the object is inside the range buzzer sound is not OFF again we are switching off the power supply and then buzzer is OFF. we don’t want that we need to reset it automatically

    • Pranitha, to solve this you can operate the RX with a oscillating DC supply instead of a constant DC, use a IC 555 base astable circuit having a frequency of arounf 2Hz, and connect the Rx circuit across it pin#3 and ground for powering it.

  2. sir thank you its working but we want to reset the circuit automatically so can u please tell us about that and will u please say briefly about the main principal of dis project?

    • I am glad it is working, What kind of reset are you referring to? The circuit does not require any resetting, the buzzer is supposed to switch ON when the Tx is out of range and switch OFF when it comes back within range

  3. thanku so much sir fr clr xplanation.nd one mre doubt.
    v have connectd as per ur ckt diagrm so nw v want to test our ckt whether it is crct or not how do v check that our ckt is wrkng properly r not ? plzz guide us sir since v r dng our projct.

    • Pranitha, in the receiver schematic, connect an LED in series with the 330 ohm resistance at pin#10. Cathode will be towards pin#10, anode towards BC557….you can connect this LED on right side of the 330 ohm or at the left side, it doesn’t matter.

      when you switch ON the transmitter, you must see this LED glowing, and the buzzer should be silent.

      however this will only happen if the transistor oscillator circuit connected with the transmitter is working correctly

        • as long as the Tx is within the range, Rx will be receiving the signals and this will keep the connected buzzer switched OFF, but the moment the Tx is taken out of the range of Rx, Rx will stop getting the signals and the buzzer will start ringing or buzzing…

  4. Can we use 434mHz RF module ?And how can we vary the range ?
    We are not getting buzzer at collector of right side transistor ? Sir can u plzz help us

    • Pranitha, you can use any RF module for creating this circuit, the range can be roughly tweaked by changing the antenna wire length, there’s no other way of varying it as far as I know.

      Sorry I did not understand what you meant by: we are not getting buzzer at the right side of the transistor? please clarify

  5. 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.

  6. 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 πŸ™‚

    • 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.

  7. 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?

  8. hello sir i am going to build one, can you please give me the specific names of components to buy, thank you, sorry newbie here

  9. 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

  10. Hello sir, I am just curious if you have a key finder circuit similar to this, that would be in need of manual programming?

    • Hello Portgaz, I am sorry presently I do not have a programmable circuit, if I find one I'll surely share it with you.

  11. 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??

  12. 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??

  13. 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??

  14. 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?

  15. 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

  16. 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.

    • the range could be well over 60 meters and there's no way to make it shorter,,breadboard will not give proper results as it's an RF circuit and requires high precision PCB design and assembly.

  17. 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.

  18. 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! πŸ™‚ sorry for troubling you again..my lecturer also confused when I asked them..

  19. 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.

  20. As usual, you did a great job sir!..Thank you for helping me..I can't wait to try this out..:D Have a nice day~

  21. Hi Swagatam

    Interesting circuit!

    I expect a circuit to trace a misplaced mobile phone (within house) which is in silent mode.

    • 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.

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