What laws of physics? voltage dropping out of existence

Thread Starter

KevinEamon

Joined Apr 9, 2017
272
Hey guys

I have this configuration on a breadboard.
I'm trying to get 3.3v out to stimulate a GPIO on the raspberry pi.

You'd think this should be the easiest thing in the world... you'd think.

Now when I measure across both resistors I'm getting the 9v.

When I measure across R1 I'm getting a drop of 1.79v
And across R2 a drop of 1.1v.

Wt actual heck? Seriously...

1574251436003.png
 

SteveSh

Joined Nov 5, 2019
105
The 9V you measure across the two resistors makes sense because, if your sketch is correct, those two resistors are across the 9V power supply.

I assume the dotted lines are wires?

There must me something else between the two resistors.
 

Thread Starter

KevinEamon

Joined Apr 9, 2017
272
The 9V you measure across the two resistors makes sense because, if your sketch is correct, those two resistors are across the 9V power supply.
I stated that just to say that the 9v was showing across the circuit

I assume the dotted lines are wires?
The dotted lines are the line to a voltmeter

There must me something else between the two resistors.
Resistance from the board itself maybe... mmm cheap ass crap probably from the uni... sigh...
 

BobTPH

Joined Jun 5, 2013
2,260
well, something is wrong. I am not sure of the meaning of your drawing. How about a photo showing the breadboard connections ckearly?

Bob
 

Thread Starter

KevinEamon

Joined Apr 9, 2017
272
No, the series of black dots. Are they a connection, or are they supposed to represent the holes in a breadboard?
This is the configuration Steve and in turn the power terminals are connected to a 9v battery.

I've tried the battery straight into the power rails avoiding these terminals as well
1574257959175.png
 
Last edited by a moderator:

BobaMosfet

Joined Jul 1, 2009
1,010
Learn how to draw a schematic. Please. Hand-drawn is fine. If you are serious about electronics in any way, this is a requirement and you should already be practicing making them for any circuit you design before you breadboard it.

Here's a working simulator example, showing all necessary values, if your circuit is wired this way:
1574259033751.png
 
Last edited:

WBahn

Joined Mar 31, 2012
25,249
12.png

Measure the voltage indicated above.

Your measurements appear reasonably consistent with there being ~34 kΩ of resistance between the resistor leads.

Be aware that this can be very unstable if it is do to oxidation or other cause of a poor connection -- just moving the leads to clip the probe onto them could completely change it.
 

Thread Starter

KevinEamon

Joined Apr 9, 2017
272
View attachment 192017

Measure the voltage indicated above.

Your measurements appear reasonably consistent with there being ~34 kΩ of resistance between the resistor leads.

Be aware that this can be very unstable if it is do to oxidation or other cause of a poor connection -- just moving the leads to clip the probe onto them could completely change it.
I dunno what the hell you did Wbahn but it seems to have started working.
I'm getting about 3.11V at the measuring point you've indicated. Do you think this will be enough to trigger the GPIO on the pi which is marked as 3.3V ?
 

MrSoftware

Joined Oct 29, 2013
1,676
Probably you had a bad connection at some point between your resistors, as suspected by @WBahn , and the act of taking the measurement moved things around and improved the connection. Breadboards can be very handy, but they are imperfect and bad connections are not uncommon.
 

panic mode

Joined Oct 10, 2011
1,821
abused breadboard? spots can get deformed is jamming wrong size wire. try to connect your circuit to a different part of breadboard and make sure leads are clean.
 

TeeKay6

Joined Apr 20, 2019
572
abused breadboard? spots can get deformed is jamming wrong size wire. try to connect your circuit to a different part of breadboard and make sure leads are clean.
I dunno what the hell you did Wbahn but it seems to have started working.
I'm getting about 3.11V at the measuring point you've indicated. Do you think this will be enough to trigger the GPIO on the pi which is marked as 3.3V ?
Am I the only one who sees the resistors as having different values than stated? From the color codes I read: 12K (5%) and 6.2K (5%). If the battery is indeed 9.0V, then the voltage across the 6.2K resistor would be 3.07V...pretty close to what TS reads.
 

Thread Starter

KevinEamon

Joined Apr 9, 2017
272
Thanks guys...
I've taken a look into what Wbahn was saying. I just need it to work for the mo till I finish programming the pi.
I ended up going on a rabbit hole journey and apparently - solid state batteries are the way forward! And if you think that's not Goodenough just ask John :D

Anywho... I probably do need a better solution to this. One of the conditions of the project - it that it be low powered - but I was thinking along the lines of a normally open pressure switch that would close; when the door is open... if such a thing exists..
The battery won't always be in use in that case, and so potential battery problems are not so much of an issue... maybe.

You're possibly right about the resistor Teekay, but that was the measurement from the multimeter.

Cheers for the info Jony... I'm waiting on decent wires from Amazon. Then I'll plug that bad boy in and hopefully not fry my pi. Cause fried pis are never a good thing!
 
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