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For a hobby project, I want to connect my own circuitry to a PCB which I have not developed. On the PCB is a linear hall sensor which outputs a voltage at about VDD/2 when a magnetic field is not detected and about 0 when a certain pole of the magnetic field is detected. I have posted below a very simplified version of the circuit in order to provide a reference for my questions.

The goal of this part of the project is to occasionally drive the linear output line from "My UC" regardless of the output of the linear hall sensor. By occasionally, I mean that most of the time P1 will be floating.

  1. If the linear hall sensor is driving the line to VDD/2 or 0 what will happen if "My UC" drives the line to 0 or VDD/2?

  2. What sort of issues will this cause with the sensor? (Potentially kill it?)

  3. Is there a way to drive the line from "My UC" without destroying the sensor or without cutting the PCB traces?

enter image description here

EDIT:

Extra information:

What is the actual voltage range of the sensor linear output? Is your MyUC able to output the same type of signal range as that from the hall sensor device? How does the voltage range from the MyUC compare with that of the sensor output? Are you intending to also interconnect the GNDs? The "My UC" microcontroller will be operating on the same voltage source as all the other components. It will share the same 3.3V VDD and the same ground connection as all other components.

Does the sensor truly output a linear level that changes with magnet position? The sensor does truly output an analog signal and not a digital value as might be assumed from the limited information in the OP.

If the magnet poles are interchanged does the sensor output start to swing from VDD/2 up to VDD? If the magnet poles are interchanged the output will range from ~1.5 to ~3.3.

Is the override control you intend to inject going to be digital in nature? The signal I am planning on using to drive the line is also analog in nature. It will be the output of a DAC in the same range (~1.5V to ~0V).

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A bit more information is required before any attempt can be made to answer. What is the actual voltage range of the sensor linear output? Does the sensor truly output a linear level that changes with magnet position? If the magnet poles are interchanged does the sensor output start to swing from VDD/2 up to VDD? Is your MyUC able to output the same type of signal range as that from the hall sensor device? How does the voltage range from the MyUC compare with that of the sensor output? Are you intending to also interconnect the GNDs? Is the override control you intend to (continued) –  Michael Karas May 26 at 7:06
    
(continuation from above) inject going to be digital in nature? –  Michael Karas May 26 at 7:08
    
As you can see there are a whole slew of things that need answering before trying to suggest what to do. Keep in mind that it is highly unlikely that you can simply tap in the wires from your MyUC. But until more is known there is no one that can come close to suggesting what might be possible. –  Michael Karas May 26 at 7:12
    
Sorry about that. I'll post it here and also update the OP. The "My UC" microcontroller will be operating on the same voltage source as all the other components. It will share the same 3.3V VDD and the same ground connection as all other components. The sensor does truly output an analog signal and not a digital value as might be assumed from the limited information in the OP. If the magnet poles are interchanged the output will range from ~1.5 to ~3.3. The signal I am planning on using to drive the line is also analog in nature. It will be the output of a DAC in the same range (~1.5V to ~0V). –  James May 26 at 7:27

1 Answer 1

up vote 2 down vote accepted

From the additional information provided by the original questioner it is clear that there will be a necessity to cut the trace between the existing hall sensor and the ApplicationUC. Two signals from the MyUC will be required, one of them being the signal to inject and one to control an added analogue MUX chip that gets added in to the circuitry roughly as follows:

enter image description here

Select an analogue MUX that can operate on the 3.3V that you said is your VDD. Make sure to be sure that the MUX can pass signals all the way from GND to the VDD range. The select digital output from MyUC will determine which entity can drive the analogue input at the AppUC.

As indicated this is an rough idea diagram. There are some other things to work out here. One of those will be how low the ON switch resistance of the MUX has to be. Another thing will be to determine if the D/A converter on the MyUC has a low enough source impedance to drive the load at the AppUC. If the load impedance is fairly high (in the multi K ohm range) then it may be possible to use a low cost CMOS '4066 type chip for the MUX. If lower ON switch resistance is needed and/or smaller packages then take a look at the many analogue MUX parts from vendors such as Maxim, TI and Analog Devices.

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What sort of problems could arise if the trace is not cut and an attempt is made to pull the voltage high even though the hall sensor is attempting to drive it low? –  James May 26 at 13:27
    
@James - It is hard to provide an exact answer as to what would happen if you directly short the sensor output with the output of your DAC. The reason for this is that nothing is known about the output circuit structure of either device. In the most unfortunate case one device would source a lot of current whilst the other would sink a corresponding amount of current leading to extreme stress of the outputs in both devices or even outright unrecoverable damage. In another possible scenario one or the other device may have internal current limiting and thus this keep the (continued) –  Michael Karas May 26 at 14:01
    
(continued from above) two parts from burning themselves out. In this latter case it is extremely unlikely that the DAC output would produce the voltage level that you would desire. Much more likely current flows between the two devices, as they fight, would produce some other voltage level based in part upon the output impedances of the two parts. –  Michael Karas May 26 at 14:06
    
@Micheal Ah... I see it now. If the output of the hall sensor is ~0V for example and I attempt to pull the line to 1.5V from "My UC" the potential difference there would cause the sensor to sink the current being sourced by the microcontroller (which is max 25ma, but still enough to kill the sensor). Is this correct? In any case, there is a RC low pass filter after the actual sensor. So, I'll remove the R and C to simulate cutting the trace, solder to the sensor side of the pads and include the filter in my own circuitry before the MUX. You have been very helpful, thanks for your time. –  James May 26 at 14:52

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