Introduction: LED Distance Indicator Dog Harness

About: Electrical Engineering Instructor at University of Santiago de Chile.

I usually take my dog Rusio for a walk when the sun goes down so he can play without getting too hot. The problem is that when he's off the leash sometimes he gets too excited and runs further than he should and with the low light and the other dogs it's not easy to spot him right away.

To solve this I had the idea of making a dog harness that will glow red when he goes to far from me, as an added requirement I wanted to make it without using any microcontroller or programming to make it as accessible as posible for other makers.


This project keeps a walkie talkie constantly "talking", this actually jams the frequency band that it's using in the near range of the emitter which might be illegal in your country (and even if its not illegal it's not a nice thing to do). This was overlooked during the design and will be addressed in a future version.


  • cheapest pair of walkie talkies you can find
  • 2x 555 ICs
  • 1x 2n2222A transistor
  • 2x TIP120 transistor
  • 7x 1Kohm 1/4W resistor
  • 1x 5.6Kohm 1/4W resistor
  • 1x 6.5 ohm 1/4W resistor
  • 2x 220ohm 1/2W resistor
  • 1x 470nF 10V capacitor
  • 1x 10nF 10V capacitor
  • 3x general use diodes (I used 1n4004)
  • LEDs of your color preference

Step 1: Walkie Talkie Disassembly

The plan is to use the communication range of a cheap walkie talkie as a makeshift distance indicator, for this the first part is disassembling it and checking how it works!

Most cheap walkie talkies use the speaker as a mic too, the function changes when pressing the button to talk and we are going to use this for our advantage. One walkie talkie will be continuously "talking" and the other one "listening", if the listening end can't hear the talking one it means that they are to far away and we can use that as a indication to turn the lights on.

Step 2: "Talking" Walkie Talkie

To achieve a continuously talking transmitter first we need to keep the "talk" button pressed, for this the button contacts can be soldered together.

The need to be talking all the time will get tiresome really fast so instead we are going to use a signal generator hooked to where our mic/speaker is supposed to go. The generator will be a 555 astable circuit powered from the battery pack of the walkie talkie (6V on my case), the circuit can be seen in the image and the component values selected should generate something close to a 1KHz square wave on the output.

It's important to check which end of the speaker goes to the negative of the battery pack as this will be soldered to the ground of the generator, the other end is connected to the wire labeled as "out". Also the output of the 555 is clipped with a diode to have a voltage in a range similar to the voltages generated when talking to the mic.

Finally we add a LED to know when it's on and we close the walkie talkie with all the new circuits inside.

*The falstad txt export is provided if you want to tinker with the component values.

Step 3: "Listening" Walkie Talkie

Then we proceed to disassemble the second walkie talkie, this time the talk button is not pressed and the wires that goes to the speaker will be used as our input to check if the lights should be turn on.

The first part of the circuit is a half wave rectifier that charges a capacitor when a signal is present (we are close enough to hear the transmitter) and a parallel resistor to discharge the capacitor when the input signal is not present (too far from the transmitter). Also a resistor of similar value to the speaker is placed in its place to trick the original circuitry into working as intended.

The output voltage of the rectifier is used as input for a schmitt trigger (comparator with different on and off thresholds) made with a 555 IC, we use a diode as voltage reference giving a ON voltage threshold near 0.45V and a OFF on voltage threshold of around 0.23V. The output of the schmitt trigger drives the TIP120 that powers on the red LEDs when our circuit can't hear the other walkie talkie.

As an optional feature I added a logic inverter (by using the 2n2222a) to drive a second TIP120 that will turn on green lights when the dog is in range of the "speaking" walkie talkie.

Finally everything was soldered with the exception of the LEDs and put into a box (original walkie talkie was too small), the connection ends for the LEDs were left accesible for connection later and a power on LED indicator was added as a final touch.

*Falstad txt import is attached as well, the changes in the input takes about a second in real life to switch between lights but in the simulation it takes forever because of the simulation step size.

Step 4: Harness and LEDs

The final step is to put the LEDs on the harness (I made my own test harness from a couple of belts bolted together), I went with 3 parallel groups of 2 series LEDs for both red and green leds. The total voltage of the supply should be greater than the forward drop of the series LEDs plus the drop in the transistor that is about 1.5V for the TIP120. In our case we have:

red: (2.2V)*2 + 1.5V = 5.9V < 6.0V

green: (2.0V)*2 + 1.5V = 5.5V < 6.0V

With the calculations done we go forward and poke some small holes for the legs of each LED and they are put in place and soldered on the other side with small wires (I salvaged some from an old ethernet cable). Afterwards some padding is added to protect the dog from any pointy bit of the LEDs legs or solder joints. Finally both ends of each LED group are soldered to the terminals that were previously prepared on the "Listening" walkie talkie.

Step 5: Test Walk

With all the previous steps we are ready for a test walk! I went to the park with Rusio at dusk as usually. The whole setup worked as intended with a change to red lights when he goes 6 to 8 meters away from me, after that he need to get closer to return to the green lights.

As a final suggestion for the people trying this instructable it seems to be a better idea to just go with the red LEDs as the green ones serve no actual purpose by being on when the dog is near. I added them mainly to show clearly the two states that can be detected, but the problem is that they dry out the battery pack quickly as they are almost always on.

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