How to make a logic probe
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- เผยแพร่เมื่อ 24 ต.ค. 2016
- One of the many workshop tasks that I cover with my students at Unitec during their Electrics course is to make their own Logic Probe. Sure, many already have test lights which, as many of you know, can be used to perform many initial diagnostic tests on a vehicle's electrical system. A Logic Probe is, I suppose, the next step & is extremely useful, and as it uses LED lights it's amperage draw is very very low making it ideal for testing ignition and fuel injection circuits.
Their construction is simple enough as there are few components, though it's easy to make mistakes on the assembly. This video covers the complete process of making your own Logic Probe, from the components needed, step by step construction, helpful hints along the way and of course, the testing and operation of the finished probe.
The cost of the making the tool is very low and all of the components are readily available from your local electronics store. The extra benefit of making your own is that, if it stops working you'll know exactly how to fix it too!
So, all you'll need to make this Logic Probe is:
A Brass rod 2.4mm diameter (approx) 150mm long
Red LED
Green LED
Length of BLACK and RED wire - min 1 meter but 2 meters is better
2x 1,000 ohm resistors
Red & Black alligator clips (insulated type is best)
Plastic tube with 7mm I.D.
Shrink tubing 2.4mm diameter - 300mm long
Shrink tubing 6.4mm diameter -175mm long
Shrink tubing 12.7mm diameter - 125mm long
Solder - flux core type
Emery cloth or sand paper
You'll also need a few standard workshop tools for the construction, a soldering iron, side cutters, long nose pliers and a mini heat gun or lighter to heat the shrink tubing.
If you have any questions just add them to the comments below...
Andy Mechanic - ยานยนต์และพาหนะ
Fantastic project so well explained at every stage great tuition.
Andy, I am a electronic engineer, I am impressed with your video, it was very informative for someone to do and to make the logic probe. Great job. I thought it was funny because your side of the world you call it crocodile clips and here in the US we call it alligator clips. LOL
+Randall Shular Thanks Randall, I appreciate your feedback. Yes, the students used to really enjoy making themselves a logic probe, good fun and extremely useful.
Cheers for now Andy
Good job, Andy....thanks!
Thank you!
Great video's Andy you have a great way of teaching.
keep them coming .
Far more interesting than the telly lol.
+Dennis Jones Thanks Dennis, yep, lots more videos on their way. In fact I'm editing up 4 that I filmed this afternoon :-)
Great Andy ! it helps a lot...thanks
Amazing vid mate!!! Using tools is great, but making them is more fun. And the best part is, like you said, if it breaks,,, you'll be able to fix it.
+chungaleta1234 Thanks for the feedback. Yes, I agree, much more rewarding to make something that will be useful in fault finding. Pleased you liked the video. Cheers Andy
Just built one . Got a cheap test light for £3 from a "pound shop "
Gutted it . Works great .
awesome job love that project thank you andy
Interesting, easy to make. 👍👍
Awesome 👍
Great really good!
Great demo of how to make this useful tool. I need to test an ignition control module of a Mazda engine and find that a normal test light with an incandescent bulb will not show voltage pulses arriving very clearly at all! I assume that is because the on/off response time to the 12v input to that bulb is too slow, whereas perhaps the LED light is much faster to respond and show clearly the pulses. Going to have to build one of these, that is for sure. Thanks Andy!
Thanks Stewart, yes, I use mine quite often & you are correct, LED type lights can react much faster to voltage switching, though I find less able to depict a wide voltage range, they tend to just go out as voltage drops whereas a normal bulb will just dim to a lower voltage.
Good luck with your build :-)
Cheers Andy
great job thank you friend
Good work, Andy - feel a trip to Maplins coming on. The 10 yr old may well get involved with this one! 👍
Great job
Good job amigo
Very nice tutorial. Really informative. And I like your hat too. :)
+TheBitchiness Hey Thanks, pleased you liked them :-) Cheers Andy
Awesome - thanks
Great tool thank you for the video! Do you have any tips on tracing down hard to find short to ground? I have a grizzly 700 EFI that shows a short to ground when battery cable is disconnected and the test light is put between the cable and the negative terminal. I removed all the wire loom and tape inspected every inch of the wire and unplugged every connecter I could find and still have a short. There is a black wire coming out of the ECU That has to faint yellow bands wrapping around it it shows 6 V I cut that wire in my short disappears, I traced it back to a distribution block and got lost at that point as there are approximately six wires just like it in that block.
The circuit can be greatly improved for pennies.
Simply twist the legs of FOUR 1k ohm resistors in such a way as to form a diamond shape.
The top corner of the diamond connects to the positive crocodile clip lead and the bottom corner to the negative crocodile clip lead.
Now take a green ultra bright LED, solder its anode(long lead) to the left hand corner of your resistor diamond shape then take the cathode(short lead) of an ultra bright red LED and solder it to the right hand corner of the diamond.
The two free ends of each LED can now be soldered together also forming the point to where the probe is the soldered.
This circuit gives far less ambiguous results than the original as both LEDs are completely extinguished until the probe is touched to a positive voltage or ground.
It worked .
thank you very much 😊
Great idea :-)
Great ! Very helpful, thanks. Also, could you PM the diagram ?
Hi Andy. I have this question, please assist. A logic probe is applied to the contact point on an IC that is operating in a system. The lamp on the probe flashing repeatedly. what does that indicate?
There's a constant signal on the line that's fluctuating between the two voltages that turn on the LEDs. If the flashing is very quick, you're probably looking at an injector control. More slowly indicates one of several other output lines that are not simply "on" or "off". Simply put, there are many components that constantly change voltage values somewhere between "on" (12V or 5 V, depending on the component) and "off" (0V), but swing between values anywhere between those two logic voltages, according to driving conditions, often in emission control circuits.
Good stuff. I'm sure you love using a gas blow torch for heat shrink tubing, and I'm sure you do a good job with it. That said, the 'proper' way is a hot air gun.
Hi Andy , i think you forgot to mention what resistor that should be used for this power probe. Can you also used this power probe to power up 12 volts motor or test relays by putting an extended negative wire with alligator clip just like the ones sold . You video was very informative and downloaded it si I can make one but will probably add a voltage meter on it.
[since Andy hasn't responded yet] At 4:50 he says " . . . one thousand ohm or 1K resistor . . ." He explains the color code, for those who don't know: four colored bands - in order Brown, Black, Red, Gold. The gold band is the tolerance (and you can use a silver colored band if you can't find a gold one). For folks who don't know the resistor color code, look it up (google, wiki . . .), the first band is the first digit, or number, the second, the second digit or number, the third one is the multiplier - how many zeros follow the first two digits. In this case brown means "1", black means "0", and red means "two zeros" or 1, 0, 00 = 1000 or 1K ohms. The resistor color code is a very useful thing to know in basic electronics. Color codes apply also to some older capacitors and inductors (coils). While color codes are not used much anymore, due to miniaturization and the use of silk screening or laser etching applied to surface mount devices, you may run across older components in older model cars - esp. back into the 60's and before.
As to your question about powering relays (or any other circuit in a car), you have to be careful with doing that in computer-controlled cars nowadays. The PCM/ECU (or whatever a computer is called by the car's manufacturer) has several very voltage-sensitive integrated circuits. Many if them are of CMOS technology, which can be blown simply by the static charge on your body if you handle them directly. The same can happen if you put an incorrect voltage (or current) on the signal lines going to the module from various sensors, relays, fuses or other devices. Testing the components with a DIGITAL volt/ohm/milliamp meter is almost required nowadays, because the meter input will be generally at least 1Mohm and not draw enough current to blow the components in the module, but unless you know what kind of current it normally handles and allow for that in your homemade device, you can very easily damage a $1K computer module. Even a momentary, if accidental, voltage on a line can do damage.
It's best to obtain the schematic for the car and study it to determine whether you can apply 12VDC directly to a relay without having to either disconnect the control module it's attached to or at least have some kind of current limiting device (a resistor at least) to protect the module's circuitry. The 1K ohm resistors, for example, serve to limit the current draw of the probe to somewhere under 20ma, depending on the LED used.
These things really do last. I've still got mine from 95.
can you give me the components required list and critr diagram ???
+manas ranjan nayak hi Manas, I'm sure I cover what's needed at the start of the video. If you send me your email address I'll send you the circuit diagram.
Pleased you found the video helpful. Cheers Andy
You do not need two resistors and its not important where resistor is placed in this circuit. ibb.co/L1HPyp7
Cool ... to learn a bit of electrical theory ... BUT!
- Consider most people will not have the tools to build this thing, much less the other pieces you scavenged in a mechanic's shop environment.
- you did not provide the information as to why the LEDs come on and off as they do, the electrical understanding as to why the LEDs go on and off in the circuit created.
- LEDs being always on... not so good (even if they behave appropriately when applying the probe
see Shellac Scrubber's post (8 months ago)
- and probably the worst, there are many logic probes available on the market that do much more than this bare-bone-concoction, as sound, etc... for a mere few dollars.
You could improve by maybe showing a graph of the circuit(static) and explain how it will behave when under load.
and then video yourself doing various tests on real equipment ... thar it be!
Very good comment. Yes, I agree. The video was made in the days when I was a lecturer. It was to assist the students in the physical build of the project - done by around 150 students a year. The theory as to how it works and the circuit diagram was covered during their theory class.
I should definitely do a follow up video covering the theory too. It's now on the list :-)
With regards the design, it is very basic & of course you can purchase much better probes these days for, like you say, just a few dollars. Some have found it very helpful & a few have even sent me pics of the probes they have built which was very cool.
Appreciate your feedback and ideas too. Thanks, Andy