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
70
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
70
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
70
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
70
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 Expert

Dec 20, 2007
11,085
1,286
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
4,846
70
You are right, i know that but in an approximation it goes to 12 V

12. Audioguru Expert

Dec 20, 2007
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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.