Analyzing pushbutton switch ADC circuit edge cases

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Discussion Overview

The discussion revolves around the behavior of an ADC circuit when multiple pushbutton switches are pressed simultaneously. Participants explore the implications of this scenario on current flow and voltage levels at the ADC pin, addressing both theoretical and practical aspects of circuit design.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that when multiple switches are pressed, current seems to only flow through the first switch, questioning why this occurs.
  • Another participant explains that pressing the top switch causes the voltage at all other switches to drop to zero, suggesting that this renders the other switches ineffective in affecting the circuit.
  • A follow-up question is raised regarding the potential difference at the ADC node after the first switch is pressed, seeking clarification on the relationship between voltage and current flow in this context.
  • Further clarification is provided about the series resistors in the circuit, indicating that with zero voltage across them, no current flows through the lower switches, regardless of their state.

Areas of Agreement / Disagreement

Participants generally agree on the behavior of the circuit when multiple switches are pressed, particularly regarding the impact on voltage and current flow. However, there is some uncertainty expressed in the follow-up questions about the underlying principles, indicating that not all aspects are fully resolved.

Contextual Notes

Limitations include assumptions about the circuit configuration and the specific roles of the resistors and switches, which may not be fully detailed in the discussion.

bremenfallturm
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TL;DR
Hi, I'm trying to build a circuit with a varying voltage on the ADC pin depending on which switch is held down. In the circuit below, all 4 switches are held down. I would assume that current flows through all of the different switches, but it only flows through the first switch. Why is that so?
Hi, I'm trying to build an ADC circuit with a varying voltage on the ADC pin depending on which switch is held down.
I have been thinking about the edge case where multiple switches are held down, and what would happen. In the circuit below, all 4 switches are held down. I would assume that current flows through all of the different switches, but it only flows through the first switch. Why is that so? Wouldn't some current flow through all of the different switches?
(please apologize for my very rudimentary electronics knowledge. please don't come at me for not trying to think about this on my own, I have been doing so)

1744483083409.png
 
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bremenfallturm said:
I would assume that current flows through all of the different switches, but it only flows through the first switch. Why is that so? Wouldn't some current flow through all of the different switches?
When you press the top switch, the voltage at all other switches become zero, so pressing other switches makes no difference.
 
Baluncore said:
When you press the top switch, the voltage at all other switches become zero, so pressing other switches makes no difference.
Sorry for the rookie follow-up question then, but why is that so?
Is it because the potential at the point where the first switch is connected is 0V after it has been pulled to ground, and there is no potential difference between that point and concequtive points after pulling to ground?
1744570740695.png
 
Last edited:
When you press the top switch, the voltage at the ADC node goes to zero, the ground potential.

The three series resistors, 220R + 500R + 700R = 1420R, then have zero voltage across them, so by Ohms law, the current through them, is 0 / 1420 = 0 amp.

Therefor, there is no resistor current to flow through the three lower switches, even if they were closed. There is no voltage across any of those switches, so their state is not important to the circuit analysis.
 
Thank you! I understand now.
 
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