lie detector test

[langging]

thank you very much sir!! its a big help to my project! thanks!
* and about your design sir, ill just wait for it!
i know your busy but thank you for the effort in answering my questions!! thank you sir!

 

[DerStrom8]

thank you very much sir!! its a big help to my project! thanks!
* and about your design sir, ill just wait for it!
i know your busy but thank you for the effort in answering my questions!! thank you sir!

I must say, thank YOU very much for your appreciation! Some members come here and ask for help, and when they receive it they just go on their way without so much as a "thank you". It is nice to know that there are still some people here who are grateful for what they are given

Best wishes,
Der Strom

 

[langging]

sir, why is it that the base current on tr1 is high even if the resistance on the two probes are low ?

* and whats the function of the 47k variable resistor?

 

[DerStrom8]

sir, why is it that the base current on tr1 is high even if the resistance on the two probes are low ?

* and whats the function of the 47k variable resistor?

The 1M resistors help to keep just a small amount of current flowing through the transistor. When a low resistance (lower than ~1M) is across the probes, it will change the amount of current that can flow, and it will trigger the red LED.

The 47K variable resistor is there to "balance" the LEDs. Its purpose is so that you can set the circuit to where the red LED is just barely off, so that when even a small change in resistance is across the probes, it will trigger. You can think of it like a sensitivity adjustment.

 

[colin55]

The only change I would make to :
**broken link removed**
is:
Take R5 to the positive rail.
It is pointless taking it to the collector of Tr2 as the voltage on the collector does not alter and the 47k provides a path to keep L1 illuminated, although it will be very dull.