# measuring time interval signals using a watch

#### danny davis

May 9, 2012
306
I did a couple of things at work today that i don't understand

1.) I had to measure a signal from the microcontroller
To measure this signal , First i had to put a 1ohm resistor into the batterys and to the PCB and measure the voltage using the oscilloscope and Fluke meter which was across the 1 ohm resistor . The tech wanted me to use the oscilloscope in normal trigger mode to see the microscontrollers signal flashing at a interval rate.

It was just an DC voltage flashing at a time interval, but what is confusing for me is this
1.) The oscilloscope or multimeter wasn't fluctuating or pulsing using the 1 ohm resistor. It only worked when you put a 10ohm or 15ohm resistor not the 1ohm resistor, why is that?

2.) Once We got the oscilloscope and DVM meter to see the DC fluctuating and pulsing , the tech wanted me to count how many times in a minute did it pulse at. It was 30 pulses in a minute. The tech did some match by taking 30 pulses and 1 minute of time and used a formula to convert that into a time interval but i don't know how, do u guys know?

#### danny davis

May 9, 2012
306
block diagram

batterys 6 volts -----1 ohm resistor ------ PCB

I put the oscilloscope and DVM meter across the 1 ohm resistor

But didn't see any pulsating signal so we had to use a 10ohm , but why

#### davenn

Moderator
Sep 5, 2009
14,297
hey Danny,
as usual, your explanations are a bit incomplete

lets clarify ...
1) I have to assume the battery supplies power to the circuit with the microcontroller ?

2) I am assuming the 1 or 10 Ohm resistor was from one of the output pins of the micro to ground (0V) /

I put the oscilloscope and DVM meter across the 1 ohm resistor
But didn't see any pulsating signal so we had to use a 10ohm , but why

I would, at a good guess, say that the 1 Ohm was grounding ALL the signal from the micro output and increasing the resistor to ~ 10 to 15Ohms allowed a large enough voltage to be read by the scope or meter

Dave

#### danny davis

May 9, 2012
306
1) I have to assume the battery supplies power to the circuit with the microcontroller ?

YES

2) I am assuming the 1 or 10 Ohm resistor was from one of the output pins of the micro to ground (0V) /

No, the resistor was like the block diagram , the resistor is in series ,just inbetween the battery's and the circuit to measure the voltage

#### CocaCola

Apr 7, 2012
3,635
Can I again ask "Why don't you asked the tech your questions?" He obviously knows the circuit (total blank to us) and knows why he is doing what (again total blank to us), it's work related and thus you and him are both collecting a paycheck, so why do come here speaking riddles, giving us just a nibble of the facts and what not when you can go right to the horses mouth to get the answers?

Don't get me wrong, it's clear you can get A LOT of free advice here and many members are willing to give it, but history has shown that you will argue with what anyone says here, you won't listen to what anyone says here, and you won't or can't actually answer any of the questions people actually ask you here while they attempt to help you...

#### davenn

Moderator
Sep 5, 2009
14,297
No, the resistor was like the block diagram , the resistor is in series ,just inbetween the battery's and the circuit to measure the voltage

so why on earth is there a resistor in series with the load ?
its totally pointless you shouldnt see any pulsing of the DC supply from the battery unless the battery is faulty
the pulsing should be from the output of the microprocessor PCB

Dave

#### poor mystic

Apr 8, 2011
1,074

Maybe the tech wanted to measure current, and circuit activity, by observing the voltage dropped across a series resistor in the supply rail,.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,769
That would explain why you "don't see nothing" using 1 Ohm. The change in current may have been too small to give a noticeable change in voltage across 1 Ohm.
Using a larger resistor, however, will lead to a change in operating voltage of the controller circuit. Not something you usually are looking for.

Harald

#### danny davis

May 9, 2012
306
It was called wakeup signals from the microcontroller
The microcontroller is in sleep mode , but there is pulses still

yes you are measuring the current , but the resistor converts it to voltage to see on the oscilloscope and DVM

You do see the DVM meter fluctuating and on the oscilloscope it is pulsating at an interval

I counted 30 pulses per minute

the Tech converted 30 pulses/1min of time and used a formula which i don't know to calculate the time interval

do u guys know what he was doing?

#### BobK

Jan 5, 2010
7,682
I am sure you can answer this: If you counted 30 pulses in a minute, how many seconds between pulses? Now can you write down the equation to convert from pulses per minute to seconds per pulse?

Bob

#### poor mystic

Apr 8, 2011
1,074
Lots of people don't understand that the English word "per" is the same as the mathematical phrase "divided by".
So, if you have 30 pulse per minute, you have 30 pulses per 60 seconds, so use "30 divided by 60", and that is how many pulses per second, right?

#### BobK

Jan 5, 2010
7,682
Lots of people don't understand that the English word "per" is the same as the mathematical phrase "divided by".
So, if you have 30 pulse per minute, you have 30 pulses per 60 seconds, so use "30 divided by 60", and that is how many pulses per second, right?
Yet they all seem to understand miles per hour and miles per gallon.

Bob

#### danny davis

May 9, 2012
306
when i measured the 1ohm resistor , it measured 1.5 ohms
so i went and got a 1.5ohm resistor and measure it, it measure 1.5 ohms also

I guess the DVM meter can't measure 1 ohms

#### poor mystic

Apr 8, 2011
1,074

You also must take into account the resistance of the leads. If you first measure the apparent resistance of a 0 ohm resistor, by shorting the probes together, I bet you get 0.5 Ohms.

#### CocaCola

Apr 7, 2012
3,635
when i measured the 1ohm resistor , it measured 1.5 ohms
so i went and got a 1.5ohm resistor and measure it, it measure 1.5 ohms also

I guess the DVM meter can't measure 1 ohms

Even my cheap (\$3) multimeter measures down to 1/10 of an Ohm, if I factor in the probe wire error...

What do your meter probes measure when you touch them together? Now grind them a bit back and forth on each other, does it change when doing this? When you are measuring such low values even the slightest thing will change the value... That includes poor contact and even the probes own wire resistance...

#### poor mystic

Apr 8, 2011
1,074
Or, even better, drive a current of 1 Amp through the 1 Ohm resistor, and you should measure 1 Volt. This would show that the resistor was 1 Ohm.

Whenever a measurement is made, the system being measured is always disturbed by the device used for the measurement. Anticipating the disturbances, and knowing how to correct the apparent readings to discover what the reading would have been if the system had not been disturbed, is an important part of every technician's education.

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#### danny davis

May 9, 2012
306
I get .3ohms when i put the probes together

but its weird that 1ohm measure 1.5 ohms , it should have measure 1.3 ohms

and the 1.5ohm resistor , should have read 1.8 ohms since the .3 ohms is from the probe wires

#### CocaCola

Apr 7, 2012
3,635
I get .3ohms when i put the probes together

Is there no fluctuation in that measurement when you twist and grind them about each other? Take a pair of pliers and use that to hold them together

but its weird that 1ohm measure 1.5 ohms , it should have measure 1.3 ohms

What tolerance are the resistors?

If they are 10% then a + swing of 10% would be 1.1 + your probes measure at 0.3 that gets you to 1.4, add in non perfect continuity between your probes and the resistor leads and you are darn close to 1.5

and the 1.5ohm resistor , should have read 1.8 ohms since the .3 ohms is from the probe wires

Again at 10% tolerance a 1.5 ohm can be 1.4 (rounded to 10ths from 1.35) add in the 0.3 from your probes and you are only at 1.7 again not far from the 1.5 measured, and again due to non perfect continuity between your probes and the resistor leads you can easily guess there will be a +/- 1/10th Ohm fluctuation from a measurement to measurement... There is a reason high end "mechanical contact" stuff has gold and platinum surfaces to help reduce surface impurities that will cause extra resistance

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#### danny davis

May 9, 2012
306
so the 1ohm resistors tolerance + probe resistors makes it not 1 ohm

It has a gold band on it for the tolerance

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