Introduction: Two Transistor Logic Probe

Picture of Two Transistor Logic Probe

This has to be the simplest practical logic probe you can build. It at the most consists of two transistors (one NPN one PNP) three resistors and two LED's. It can fit into the smallest of pens. There is no way to differentiate TTL from CMOS logic levels it just gives simple hi or low.

Step 1: Building the Curcuit

Picture of Building the Curcuit

It is a very simple construction unlike the dual comparator circuit in the second picture. The value of the resistors R must only be equal but, it is best to use low values (like the 180 ohms I used) because at 180 ohm 10% tolerance is ± 18 ohm, the virtual ground is very close to half the supply

LED's: any type will work great. I used two very bright 6V clear ones. 3V to 5V red and green ones would work best.
Red is logic hi and would be connected to the NPN so of course green is low and on the PNP.

Rop's value depends on the LED's you use. High power ones(6V 40mA???????) don not need this resistor but low power (3V 10mA) will need 100 or more ohms. It works by limiting base current of the transistors and thus limmits the current passed to the LED's.

The last photo is the prototype circuit.

Step 2: Bare Minimum

Picture of Bare Minimum

If you are truly going for simple it is possible to build this circuit with no resistors. The reason I put them there is so the LED's would be completely off.
With no voltage divider the two LED's are always dimly on. If this does not drive you nuts you could build a logic probe with four components. Keep in mind you need to use hardy LED's since there is no Rop to limit current

Step 3: Packaging and Other Ideas

Picture of Packaging and Other Ideas

As you can see this circuit is small enough to fit into any pen.( unlike one using an 8 pin DIP which needs a large diameter pen)
The second photo is the quick connect to a bread board.

Step 4: Suggested Construction

Picture of Suggested Construction

This is the way I constructed the circuit ( trying for as compact as possible ) they are CAD models because trying to photograph the circuit
was not working.
The first photo is the assembly and the second it the wire connections. It is possible to connect the LED's to the transistor collectors as opposed to the emitters for the sake of wire simplicity ( no crossing wires that might bridge )


hkuhlmann made it! (author)2016-04-12

A very good idea!

There is only one the problem, that the logicprobe will not run with supply voltages above 3.3V. One resistor and a zener diode solves the problem.

The layout for a breadboard is included. The blue lines are wire at the bottom red lines are free wires on the top.


you can get a detailed description of the circuit.

t1d (author)hkuhlmann2016-10-29

I have breadboarded the Praktische-Electronik design. I substituted a 2N3904 for the NPN Transistor, a 2N3906 for the PNP Transistor and a NTE519 for the Zener Diode. I measured its performance from >0.00 volts to 5 volts. The circuit performs predictably.

There is minor double lamp illumination in the middle voltages. With the green LED well illuminated on a rising voltage, the red LED first turns on, dimly, at 2.13v. Both colors are on, dimly, at 2.28v. With the red LED well illuminated on a falling voltage, the green LED first turns on, dimly, at 2.3v.

I will be building two of these probes, one for my brother and one for me. I have added the Fritzing files for the breadboard layout, schematic and PCB as an expression of my appreciation to the post's author and the P-E contributor. Thanks!

It does not appear that the files loaded properly. I tried fzz and zip. What is the trick to load fzz files?

t1d (author)t1d2016-10-29

Oh, I see that the files did load in the final post... Great!

hkuhlmann made it! (author)t1d2016-10-30

Yes you are right. Both LEDs will be illumination in the middle voltages.

But this is only the case if you do not use a 3.3V zener diode. The NTE519 is no zener diode (See A ZF3.3, BZX79-C3V3, 1N5226B or 1N4728A will work fine.

With a 3.3V zener diode the green LED will be off at about 0.6V.

The levels of 0V to 0.6V for green = LOW level and the 2.2 V to 5V for red = HIGH agree well with the levels for TTL.

By the way, I put my LogicTester in a transparent shrink tube.

t1d (author)hkuhlmann2016-10-30

Great catch on the NTE519! I will need to purchase the correct Zener. Thank you!

Ploopy (author)2015-07-23

Could you post a .stl file of the CAD so that we can get a better view of it?

Ploopy (author)2015-04-21


CookiE213 (author)2015-01-16

what kind of transistor did you use?

mathieu.b.collin (author)2014-09-19

How would you suggest conveting this to a 12vdc tester ?

jazzzzzz (author)2013-10-25

Sometimes going to the store and asking for a general purpose NPN transistor will get a perplexed response. (Asking for a 2N2222A though more specific will not usually help in this instance)

I do agree having a status indicator require more power than the actual circuit is bad practice, but a logic probe is a test instrument (and it is not like its drawing real amperage anyway). My oscilloscope uses more power than most of the oscillators I have made. A bit extreme an example, but carries a valid point.

This circuit will run on 3.3 volts, it’s a voltage divider, as long as it will run the LED any voltage should work. (As long as you do not exceed the max rating of any device in the circuit)

The point was to keep part count down, and keep it simple will easy to get parts. This will always sacrifice some quality, but not so much it becomes unusable.

manuka (author)2013-10-22

OK- but I still suggest you clarify your statement, as a novice asking at a parts store for LEDs that were "6V clear or 3v to 5V red and green" would be disheartened at the bemused response ... 

Modern circutry increasingly uses a 3.3V supply ( or less) & "runs on the smell of an oily electron" - check my single LiFePO4 14500 AA cell Instructable=>

FWIW low LED current draw is often increasingly crucial to prolonging battery life of course - it makes little sense to have a status LED drawing more than the entire circuitry!  I regularly now run status LEDs at well below 1mA - often to even 50µA for hi vis greens.   That's always on too -diverse tweaks into circuit snooze time, LED selection, pulse driving & best viewing colour etc can give  µA level average current drains..

manuka (author)2013-10-22

Ahem - your statement  " -two very bright 6V clear ones. 3V to 5V red and green ones would work best". is in error.  Classic red & green LEDs need only ~2V supply & even modern blue or white just ~3½V.  Higher supplies, if  used  without dropping resistors, will blow the LEDs.

Modern LEDs are now so bright that they've often clearly visible when drawing only  a few mA as well.

jazzzzzz (author)manuka2013-10-22

The operating voltage given for LED’s is not the forward voltage. An LED will not burn up the instant more than the forward voltage is applied, with or without a current limiting (the term dropping is ambiguous) resistor. The transistors should never be in full saturation, and thus limit current.

Modern LED’s are getting brighter and more efficient; however current draw is not a good measure. The wattage is the measure of power consumption. An LED operating at 3mA and 1000 V is no better than one at 600mA and 5 V.

omnibot (author)2012-09-07

Nice job.
I wouldn't build this without the resistors as suggested in Step 2 - Bare Minimum since this would cause a shortcurcuit between vcc and gnd.

omnibot (author)omnibot2012-09-07

BTW .. when I made mine I used two zener diodes instead of the resistors on the bases of the transistors (the voltage divider) since I like the way they tend to keep the LEDs floating when nothing is connected. The probetip was just a resistor.

Daniyal Khan (author)2012-03-16

Can we generate TTL and CMOS with pulse with it??

give me the names of some ICE'S on which it works

jazzzzzz (author)Daniyal Khan2012-04-24
and TTL or CMOS some ecamples are
  • SN74S85 4 BIT MAG COMP
In adition to all the 7400 and 5400 series I happen to have a lot of TI semiconductors but it will work on any logic
Daniyal Khan (author)jazzzzzz2012-04-24

thanx boy I have made this project and its working too... i have to show it in my university

jazzzzzz (author)Daniyal Khan2012-04-24

cool, glad I could help

jazzzzzz (author)Daniyal Khan2012-03-16

It cannot generate a pulse, it can only view them.
I probably could make a circuit to generate, but it would have more than two transistors (more than likely an IC).

Daniyal Khan (author)2012-03-04

And what is the result in the end to show that our project is working??

jazzzzzz (author)Daniyal Khan2012-03-05

Connect it up, it should be green on low (-) red on high (+).
You could test it on a cascade of inverters too.

Daniyal Khan (author)2012-03-04

Can i relate this project to digital system?

WWC (author)2012-02-24

Hey this looks like a great idea. It would help me very much. Is this your design?

jazzzzzz (author)WWC2012-02-24

Yes it is, I wanted somthing that would fit into a pen, and not use a SOIC.

solomonhorses (author)2012-02-22

Awesome! Im going to make it for presents for all my geek friends;) good work, great instructable, and excellent architecture of the cad drawing! Keep up the good work!!!

lukeD (author)2011-06-19

what can you do with it? is it usefull?

jazzzzzz (author)lukeD2011-06-19

It is useful for trouble shooting any circuit with logic gates, or any other type of TTL / CMOS driving circuit (like RS-232 serial signals, DTMF tone decoders, CTCSS encoder, EEPROM, MCU output, etc.) A handy circuit for the digital designer’s toolkit.

mwagner63 (author)2011-04-25

what program is that cad?

jazzzzzz (author)mwagner632011-04-26

It is SolidWorks 2007 best 3D CAD I know of.

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