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 )