Power Logic Probe




About: Why pay for stuff when you can make, build, hack, macgyver it. They are better and cheaper 100% DIY.

Having a logic probe on your toolbox it's a very useful tool to analyze and troubleshoot the logic state of a electronic circuit.

You can easily find out the High state or Logic 1 , "Positive", and Low state or Logic 0, "Negative".

But it's possible to improve the potencial of this tool, it would be nice having light on this probe to illuminate the area of work that we are probing.

When troubleshooting we can make mistakes so it's nice to have a probe that is protect against short circuit.

And also having the opportunity to give positive and negative to the circuit at will to power devices.

On this project i show you how to make a Power Logic Probe .


Step 1: Watch the Video

Watch the video and see how the Power Logic Probe works and how it can be a good asset to your toolbox.

Step 2: Gather the Material

You will need for this project:

- Enclosures Box

- Circuit Board Perforated

- Electric Wire

- Battery Alligator Clips

- Power Connectors

- ThreePosition Switch

- Momentary Switch

- Flat Screwdriver

- Silicone Nozzle Cover

- 22 Ohms Resistor

- 68 Ohms Resistor

- 2x 10K Ohms Resistor

- 2x 1K Ohms Resistor

- 5V Zener

- 2x 1N4001 Diode

- Bc 557

- Bc 547

- 12V Relay

- 5mm Red Led

- 5mm Green Led

- 2x 5mm White Led

Step 3: Get Started

To have a nice project in the end it's important to choose a nice enclosure to keep the circuit board, i chosen this one because its round on the edges so becomes comfortable to handel.

For the tip of the probe i use a old screwdriver that i didn't use, by changing the tip with a dremel to a needle tip, if you what you can use a tip from a multimeter leads.

Follow the circuit schematic and make the circuit on a perforated circuit board and keep in mind the components polarity.

With a drill make the holes for the switch, leds, and the probe tip.

I made also two cables one to connect the probe to the battery car and the other to the 12v cigarette lighter, because i designed this tool first to troubleshoot the electrical connections and devices in a car, so i have two cables to use, the alligator clips when i'm working on the engine and around the car and the cigarette lighter when i'm working inside the car.

I used a nozzle silicone cover to protect the tip when storage, also to increase the connection on the tip i painted with graphit 33 spray.

Step 4: How It Works

To power the probe connect the leads to 12v power supply and push the power and reset button to turn on the probe.

Now you can probe the wires to check the polarity, the red led will be positive or logic 1 and the green led will be negative or logic 0.

Thanks to the white leds you will always have a light work illuminating the wires under test.

Besides probing you can also power devices for exemple in a car, lights, horn, radiator fan, relays, etc , with the power button position 1 will give 12v positive on the probe tip and position 2 will gibe 12v negative to the probe tip.

The probe have also a circuit breaker to protect the probe and electronic circuit under test, to power back on just push the ower reset button.

Step 5: Final Thoughts

This is a nice and easy project to make and a very good tool to have for diagnostic the car wiring or a electronic circuit.

Tell me on the comments below what applications you will use this for.

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15 Discussions


6 months ago

Muy buena herramienta Mi pregunta es ¿Dónde comprar? el estuche


1 year ago

Replace zener for 6.8V 1Watt and resistor 180ohm 2watt


3 years ago

Can bc547 and bc557 be replace with 2n3904 and 2n3906 ???

1 reply

3 years ago

Hi, can the 2 transistor be replace with 2N3904 and 2N3906


3 years ago on Introduction

I have prototyped this circuit, and for some reason the 5 volt zener gets really hot and the relay turns on without the need to push the reset button. I have rechecked this part of the circuit acording to the schematic. I wonder if the is a problem with the drawing?. Can you check and advice please.

1 reply

3 years ago on Introduction

I measured the control side of the relay Im using. it measures 179 Ohms. I wonder if these could be the problem in having.


4 years ago on Introduction

Thanks for answering. Looking at the video i can see that you need to press the reset to start using the tool. Does this mean that when you press the reset , you cut power to the bobine and the relay closes the NC contacts?. is this how the protection circuit works?. I will apreciate your description of how this part of the circuit works, meanwhile I will try to complete my Power logic probe. Thanks!!

1 reply

Yes to start the Power Logic Probe you need to push the reset button to give positive to the relay coil, this will initialize the protection circuit, the relay will remain active with less then 12V when you have short circuit the relay will automatic cut the power, to turn on again just press the reset button, the reset button will not power down the circuit for that just plug out the power manually.

Makes sense?

Tell me how it turned out!


4 years ago on Introduction

Looks like an awsome proyect to build. I have been studing th diagram posted and i have some dificulty understanding the connection at the relay. perhaphs you could post a more clear diagram or sent it to my email. brsalegre@gmail.com. Thanks for this great proyect !!!

1 reply

Thanks, the diode 1N4001 are connected to the bobine off the relay on one end and the other to ground, this relay will give power to the rest off the circuit in this case 12V positive for this must be connected to the NC end off the relay.

Tell me if this helps in the meantime i will try to put another diagram just of this part.


Amazing, this is pretty much the same as a powerprobe, which costs like 700$. I will eventually, hopefully, get to making one of these. thx for sharing

1 reply

4 years ago

I actually have one of these that I bought called a "power probe". The functions are pretty much identical other than a LCD display. With that said it is a valuable tool to have and I have used it many times in the automotive world. Anyone who can, needs to build this!