Building a Functional Lie Detector: Troubleshooting Tips and Breadboard Basics

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Discussion Overview

The discussion revolves around troubleshooting a DIY lie detector circuit built on a breadboard. Participants are sharing their experiences, challenges, and seeking assistance with the circuit's functionality, particularly regarding LED behavior and component placement.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant expresses uncertainty about their breadboard setup, noting that LEDs are getting destroyed when applying 9V, despite requiring only around 3V to turn on.
  • Another participant suggests that the circuit "should" work, indicating a level of confidence without providing detailed reasoning.
  • A participant reports that after testing, their setup does not turn on the LEDs at all and requests a schematic check.
  • A participant shares a circuit picture not on a breadboard, which may assist in troubleshooting.
  • One participant identifies a potential error in the schematic regarding the placement of VR1 and the labeling of resistors, suggesting that correcting this may resolve issues.
  • After making adjustments, a participant notes that one LED (green) now works, but another LED (red) still does not, prompting further questions about the transistor placements in the schematic.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the circuit's functionality, as multiple issues are raised, and uncertainty remains regarding the correct configuration of components.

Contextual Notes

Participants mention potential errors in schematic representation and component placement, but the discussion does not resolve these issues or clarify assumptions about the circuit's design.

Who May Find This Useful

Individuals interested in electronics, DIY projects, or those seeking to understand breadboard setups and troubleshooting techniques may find this discussion relevant.

maverick99
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I'm trying to build this simple lie detector (picture below). I'm 90% sure that i have it built correct on my breadboard, but for some reason my LEDS are getting destroyed if i try applying the full 9V. It takes around 3V to make the LEDS to turn on about halfway. For some reason, there is a lot of current going through this thing. If someone could draw the components on a breadboard picture for me that would be awesome. I just need to see if I'm building this right. Also, if anyone knows a good site that has a tutorial of the basics of breadboards i would appreciate that too.
Thanks
 

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This "should" work.
 

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  • bread_brd.jpg
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faust9 said:
This "should" work.


Alright thx! I owe you!
I will be shore to reply once i test it out on monday!
 
Last edited:
I tried it in class today and I'm 90% sure i set it up right. It doesn't turn the leds on at all. Can you double check the schematic?
 
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here is a picture of the circuit itself not on a breadboard if it helps
 

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  • lie_detector.jpg
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http://www.iguanalabs.com/breadboard.htm

You're right though. It appears I drew VR1 to the collector of T3 but it should go to the b ase of T2. R1 and R2 are labeled backwards but that's inconsequential.

The VR change should fix everything.
 
Last edited by a moderator:
alright cool thx
 
I switched the VR1 today and now my green LED comes on, but for some reason my red LED isn't coming on. I have tried applying lower resistances to the probes, but the red LED isn't coming on. Are all of the transistors in the right spots on the schematic you drew? If you could double check, i would greatly appreciate it.
 

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