The logic probe is powered from the device under test (DUT) – it may be any binary logic, which can be powered in the range +2 V to +6 V. This may be a microcontroller or 74/54 series logic chips, including HC/HCT chips.
The probe determines 3 conditions:
- Logical 0
- Logical 1
- Undefined (this may be a Z-condition, or bad contact).
It also features a counter, which is very handy when you want to count impulses, to estimate the value of frequency or to test an interface. (This part is shown as a sketch.)
The probe in Figure 1 consists of two Schmitt triggers, the upper trigger on the figure determines the logical 0, and the lower trigger determines the logical 1.
 |
|
| Figure 1. | The logic probe with two Schmitt triggers where the upper determines logical 0 and the lower determines logical 1. |
Two different colors were selected:
- Blue for logical 0
- Red for logical 1
Since the blue LED demands more than 2 V, a slightly modified “joule-thief” circuit on Q2 is used to increase the voltage. The transformer has 2 windings with an inductance ranging from 80 to 200 µH, if the windings are not equal, the greater one should be connected to the collector. (The author used a tiny transformer from an old ferrite memory, but any coil with an added winding over it can do.)
If you choose a green or red LED instead of blue, the “joule-thief” circuit can be eliminated, and the LED connected between the upper terminal of R5 and “+A”.
Due to the wide supply voltage range, the current through the LEDs can increase 100% or more. Since the LEDs are quite bright, some control of brightness is desirable. It’s performed by the circuit’s U3, Q3, and two diodes, which can decrease the LEDs supply by 1.4 V.
Note, the 74HC14 can be used instead of the 74HC132 almost everywhere in the circuit.
