555 Timer Min Max Expected Output Voltage Value

Question

Could someone please tell me what are the min and max output values that should be expected from a 555 timer Output (pin 3) given X volts as the voltage source?

For example, assuming that the 555 timer is powered by 9 volts, what should the pin 3 voltage be when the pin is low and when the pin is high? I was under the impression that the voltage should be 0 when the pin is low and 9 when the pin is high but instead I get 0 volts when the pin is low but only 7.7 volts when it is high (Note that I measured the voltage with no load on pin 3).

I tried this with a couple of timers that I have laying around here and they all did the same thing. At this point I am not sure if I have a bad batch of timers, if I am connecting things the wrong way of if this is the way is should be.

If possible, could someone also point me to the place in the timer 555 specs document where it specifies what the output should be? I tried looking at the specs but I got lost, I am still pretty new to electronics and spec sheets overwhelm me a little.

The specific timer I am using is NE555P.

Thanks.

Answer 1

The question you ask is not easily answered. The problem is that the chip really doesn't have any specs for your condition. If you look at http://www.ti.com/lit/ds/symlink/ne555.pdf, section 7.4, you can find the high output voltage numbers, but these are only given for fairly large (100 mA) currents. There is simply no guidance in the data sheet for performance with very low currents.

With that said, you'll notice that (for instance) at a 15 volt supply the high level is guaranteed to be 13 volts, or 2 volts less than the supply. For 5 volts, it is once more spec'ed for about 2 volts under the supply.

So, anything above 7 volts is about as good as you should hope for, and 7.7 volts seems a pretty good number.

Answer 2

Well the answer will be in the data sheet. It will give you output values at nominal voltages. eg 5-10-15 etc so if you are supplying something in between that you can estimate or the datasheet may graph it.

In your case look for the "output high voltage".

To be honest if it is a real concern I would breadboard the basic 555 circuit and test the output values at your specific input voltages. Which I know you have done. The 7.7v value looks fairly good to me I dont think you have a faulty batch.