Building a simple Circuit Probe




The Goals

Sometimes you need to check one circuit and test some of its nodes. Usually a tester in voltage mode is a good solution, but it has a pair of problems. First, it measures about zero both when the node is driven at zero volts and when the node is floating (not driven at all). Second, it gives the information on the tester display, so you need to take the view from the circuit to the tester to check the voltage.
Building a simple Circuit Probe




The proposed circuit somewhat qualifies as a logic probe. It should give no indication when the node is not being driven and it should give a different indication when the node is driven at high or a low voltage.
A lot of logic probes are not self powered. They rely on the circuit supply to operate. In my case I would like the probe to be usable also a as a continuity tester. If we set ground in one point of the circuit and we probe another point, the continuity can be detected between both points because a low level will be driven even if the circuit is not powered at all.

The Circuit

The following figure shows the schematic of the circuit.
The circuit is powered with a 3V CR2032 battery. When the probe is not connected to anything, the probe voltage, as set with R1 and R2 will be 1.5V. As both Operational Amplifiers (MCP6002) have very low bias current (1pA), no voltage drops on R3 and 1.5V is also seen on the non inverting input of U2 and on the inverting input of U1.
Resistors R4, R5 and R6 define a voltage divider that sets 1V on the inverting input of U2 and 2V on the non inverting input of U1. With 1.5V on the probe, both operationals saturate at high level (3V) so both LEDs are OFF.
If we set the probe tip to a voltage below 1V (The trip point of U2), U2 will saturate at low level and the Blue LED will turn ON. In a similar way, if the probe tip voltage is set above 2V (The trip point of U1), U1 will saturate at low level and the White LED will turn ON. I know white is a strange color for high level. A red color will be better buy I had no red LEDs similar to the blue one at hand.
Operational Amplifiers are not the best suited components for this project. As they are used as comparators, a dual comparator could be better in this case. The choice of the MCP6002 dual opamp was only because I had them available.

Circuit operation

In the end, the circuit puts a 1.5V voltage on the probe tip, provided by the two 100k resistors, so it behaves like a 1.5V source with a 50k series resistance. If the probe voltage is kept between 1V and 2V, no LED is lit. If the probe voltage is below 1V, the blue LED turns ON. And if the probe voltage is above 2V, the white LED will turn ON.
When you probe a node in the circuit, you can sense its voltage using a high impedance voltmeter. That will give you the open circuit voltage (Voc). This voltage can be driven with different strengths. The drive strength is related to the equivalent resistance at the node. For a linear non reactive circuit it will be the equivalent Thevenin resistance. In the end, the equivalent circuit will be something like that:

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