Two Transistor Logic Probe




Introduction: 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

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

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

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

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 )

2 People Made This Project!


  • Water Contest

    Water Contest
  • Fix It! Contest

    Fix It! Contest
  • Creative Misuse Contest

    Creative Misuse Contest

27 Discussions

I built one like the transistor version of this over 30 years ago, though I think it had some current limiting resistors for the LED's. Very useful little tool! You have inspired me to make another...

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

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.

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..

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.

1 reply

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.

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.

1 reply

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.

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

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).

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