Logic Probe
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Logic Probe
The Logic Probe can be considered a physical implementation of a comparator. However, it differs in that it takes only one value as input, rather than two values like A and B, and compares it with the existing value it holds. Also, due to its measurement capabilities, it’s difficult to expect an A = B state, as the measurement device value doesn’t fluctuate significantly. Therefore, it produces different signals in three ranges (not all values can be represented due to device constraints) depending on the measured value.
The image above represents the actual appearance of a Logic Probe.
The LEDs in the picture represent the output signals.
In other words, if it’s in the High state (1.8 ~ 5 V), the red LED lights up; if it’s in the Low state (0 ~ 1.2 V), the green LED lights up; if it’s in the Floating state (1.2 ~ 1.8 V), the light is off.
With this information, users can determine whether the system is functioning properly.
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**Logic Probe Circuit
The part labeled TP LOGIC IN is the probing section.
In other words, it’s where you want to measure the voltage.
First, let’s examine the circuit on the left.
The portions labeled 5.5 V, 1.8 V, 1.5 V, 1.2 V from top to bottom in blue are present.
Firstly, 5.5 V is the supplied power value.
But what are 1.8 V, 1.5 V, and 1.2 V?
These values are obtained by considering the left side as a single series connection, with the total current being the same.
However, what about the right-side wires labeled 1.8 V, 1.5 V, and 1.2 V? Firstly, note that there’s an Op Amp on the right side of the parts labeled 1.8 V and 1.2 V.
Even with just an Ideal Op Amp, its resistance is ∞!
Furthermore, the part labeled 1.5 V on the right side has a 1 MΩ resistor waiting, which is over 100 times greater than 6.8 kΩ.
So, the current flowing through the right-side wires of the parts labeled 1.8 V, 1.5 V, and 1.2 V would be very small.
This results in very low voltage drop, making this approach valid, considering the vibrating effect as mentioned earlier!
This same approach can be applied to TP LOGIC IN.
Compared to R6 = 10 kΩ, the input resistances of 1 MΩ and the Op Amp are much larger.
Hence, the voltage drop effect due to 10 kΩ is negligible.
So, the 2nd voltage and the 5th voltage are the values of TP LOGIC IN.
Now, familiarize yourself with Op Amp.
Note that all these Op Amps don’t have Feedback circuits.
In other words, the amplification range of the Op Amp is [VLow/A, VHigh/A], and due to the large value of A, this range is very small.
Therefore, even considering the mentioned fluctuation effect, the output voltage of the Op Amp would be VLow or VHigh.
Judging by the circuit layout, the value of VHigh would be 5.5 V.
Usually, VLow = - VHigh is set (or it could be supplying VLow = 0 and VHigh = 5.5 V).
Remember that these Op Amps are not Ideal Op Amps; they are Practical Op Amps.
That means the Inverting input voltage and Noninverting input voltage of the Op Amp can be different.
One more thing to remember is that the output voltage of the Op Amp cannot exceed 5.5 V.
The rightmost part labeled 5.5 V forms the highest potential, so the current passing through R7 goes from right to left.
Therefore, the portion between the two diodes is certainly at a voltage lower than 5.5 V!
(And R7’s value would have been adjusted appropriately to ensure that the voltage between the two diodes is greater than -5.5 V.)
Case 1 . When the voltage of TP LOGIC IN is measured to be between 0 and 1.2 V
The Inverting input voltage of the upper Op Amp = vTP LOG IN
In the case of the upper Op Amp, the 3rd voltage (1.8 V) is greater than the 2nd voltage (vTP LOG IN), so it outputs VHigh (= 5.5 V).
Therefore, current should flow from the output terminal of the upper Op Amp to the portion between the two diodes, but this is impossible.
(Diodes allow current to flow in only one direction.)
Therefore, the red LED is off.
The Noninverting input voltage of the lower Op Amp = vTP LOG IN
In the case of the lower Op Amp, the 5th voltage (vTP LOG IN) is smaller than the 6th voltage (1.2 V), so it outputs VLow (= - 5.5 V).
Therefore, current definitely flows from the output terminal of the lower Op Amp to the portion between the two diodes; the direction of the diodes is appropriate. Hence, the green LED is on. __
Case 2 . When the voltage of TP LOGIC IN is measured to be between 1.2 and 1.8 V
The Inverting input voltage of the upper Op Amp = vTP LOG IN
In the case of the upper Op Amp, the 3rd voltage (1.8 V) is greater than the 2nd voltage (vTP LOG IN), so it outputs VHigh (= 5.5 V).
Therefore, current should flow from the output terminal of the upper Op Amp to the portion between the two diodes, but this is impossible.
(Diodes allow current to flow in only one direction.)
Therefore, the red LED is off.
The Noninverting input voltage of the lower Op Amp = vTP LOG IN
In the case of the lower Op Amp, the 5th voltage (vTP LOG IN) is smaller than the 6th voltage (1.2 V), so it outputs VHigh (= 5.5 V).
Therefore, current should flow from the output terminal of the lower Op Amp to the portion between the two diodes, but this is impossible.
Therefore, the green LED is off.
Case 3 . When the voltage of TP LOGIC IN is measured to be between 1.8 and 5 V
The Inverting input voltage of the upper Op Amp = vTP LOG IN
In the case of the upper Op Amp, the 3rd voltage (1.8 V) is smaller than the 2nd voltage (vTP LOG IN), so it outputs VLow (= 5.5 V).
Therefore, current should definitely flow from the output terminal of the
upper Op Amp.
Hence, the red LED is on.
The Noninverting input voltage of the lower Op Amp = vTP LOG IN
In the case of the lower Op Amp, the 5th voltage (vTP LOG IN) is greater than the 6th voltage (1.2 V), so it outputs VHigh (= 5.5 V).
Therefore, current should flow from the output terminal of the lower Op Amp to the portion between the two diodes, but this is impossible.
Therefore, the green LED is off.
One question may arise.
Why not simply make R5 Open? This would make interpreting the circuit much cleaner.
However, the reason for not doing so can be found in device-related factors.
When the Logic Probe is applied to classic objects, the device can easily be damaged.
Therefore, in that case, a significant current is directed through the R5 part.
This part has a ground connection, making it easy to discharge high voltage.
Input: 2016.01.10 21:45