# Measuring the Output of a 555

Discussion in 'General Electronics Chat' started by paul19811891, Apr 24, 2008.

1. ### paul19811891 Thread Starter New Member

Apr 24, 2008
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Hi
I would like to measure the o/p voltage of a 555 timer, would it be possible to measure the voltage in real time as the capacitor charges and discharges? I would like to see if my calulations are correct and compare the two for a college assignment. Any help woul be greatly appreciated.
Paul

2. ### mik3 Senior Member

Feb 4, 2008
4,846
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If you want to measure the voltage across the capacitor when it is charging i recommend using an oscilloscope because of its high input resistance and thus it wont affect the function of the circuit. By using a voltmeter you will change the time constant of the circuit a bit because of its internal resistance which is not very high. Also with the voltmeter you cant measure the voltage across the capacitor accurately because the voltmeter needs 1-2 seconds to take a measurement (depends how good it is).

You can measure the output of the 555 with a voltmeter when it is operating in a monostable or bistable mode otherwise use an oscilloscope.

3. ### paul19811891 Thread Starter New Member

Apr 24, 2008
7
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Thanks a lot, Sorry for being a bit of a thicko, but can I just connect the voltmeter to the output of the circuit, i.e. pin 3? or do I measure across the RC network, or just the capacitor? The timer circuit is monostable, and gives an output for about 1 minute, so during this time I should see a rise in voltage?
Thanks again

4. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Connect the positive lead (red) of the voltmeter to pin 3 and the negative lead (black) to ground (zero volts).

5. ### paul19811891 Thread Starter New Member

Apr 24, 2008
7
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Ok great, and this will allow me to see the voltage rise from 0-12V (using 12 volt supply) as the capacitor charges? what about when the capacitor discharges?

6. ### mik3 Senior Member

Feb 4, 2008
4,846
63
You will see the voltage on the output of the 555 (pin 3) to be 12 V when the capacitor is charging. When the capacitor will be discharging after the one minute time you calculated the output of the 555 will go to 0 V. So on the voltmeter you will see initially 12 V and after a while 0 V. Is that voltage you want to measure?

7. ### paul19811891 Thread Starter New Member

Apr 24, 2008
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Oh I see, Well really I wanted to see the voltage rise from 0-12V over the minute, and compare actual readings with calculated readings, i.e. the Vc= Vs(1-e^-t/RC) formula. Would this be possible using an o'scope? So for eg, at 25 secs the voltage would be 5V for eg?
Thanks again

8. ### mik3 Senior Member

Feb 4, 2008
4,846
63
yes you can do it by using an oscilloscope

9. ### paul19811891 Thread Starter New Member

Apr 24, 2008
7
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Thanks very much, I really appreciate your help.
Paul

10. ### Audioguru New Member

Dec 20, 2007
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896
The output of a 555 does not go to 12V when it has a 12V supply. It goes up to abput 10.8V without a load and up to about 9.5V with a 200mA load.

The voltage across the timing capacitor in a 555 circuit does not go anywhere near 12V. It goes to 2/3rds of the supply voltage which is 8.0V when the supply is 12V.

The voltages and currents for a Cmos 555 are different.

11. ### mik3 Senior Member

Feb 4, 2008
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You are right, i know that but in an approximation it goes to 12 V

12. ### Audioguru New Member

Dec 20, 2007
9,411
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In technical language, "an approximation" is just a wild guess.

If the supply to a 555 is 5V then the output goes as high as 3.8V without a load and as high as only 2.5V with a 200ma load. This is actual, not an approximation.