I have a few NE555 timer chips and I want to generate a 5V-0V clock signal using external RC frequency control. I was referring to the datasheet found here which stated that the typical value of OUTPUT HIGH VOLTAGE under VCC=5V is 3.3V (under the ELECTRICAL CHARACTERISTICS section):
Aren't these two things contradictory (I am fairly inexperienced when it comes to reading datasheets so I could be wrong). Is it not possible to obtain a 5V output then?
On a TTL input, anything higher than 2V is read is high, and a TTL output-high must be 2.4V or more. Input voltage less than 0.8V is read as low, and a TTL output-low must be 0.4V or less. (in-between values re undefined.)
On the ATmega chip anything 3V or more is read as high and anything less than 1.5V is read as low.
Fig-1 says that (Vcc = 5V) the high level output voltage (VOH) will be about 3.3V (minimum 2.75) while delivering 100 mA current to load (the driven circuit).
If the load draws lesser current, the output voltage will increase proportionately.
If you can manage the load to draw 10 mA current, then the output voltage will remain at about 4.83V (5.0 - (5.0-3.3)/100*10).
I think you will obtain a 5V output, or at least something close enough to 5V that it works.
The 3.3V figure you found in the data sheet is what you would expect when the supply voltage is 5V and the output is supplying (sourcing) 100mA. But 100mA is a pretty high current! For comparison, an Arduino pin would be damaged if it sourced more than 40mA.
Yes, they are. But I think what the data sheet is saying is that the chip can either sink/source up to 200mA or produce TTL voltage levels, but not both at the same time!
As @GolamMostafa says, when the current supplied is lower, the output voltage will be closer to 5V. If you are connecting that output to a digital input (e.g. an Arduino input pin or the input pin of most other logic chips) the current will be very, very low, perhaps only a few microamps or even less. So the output voltage will be pretty close to 5V, I suspect.
Here is an important question: are you using a 555 timer in an Arduino project? Because that is almost certainly a bad idea. The Arduino can generate clock signals that are far more accurate, over a wider range of frequencies, than a 555 timer, without the need for any extra circuitry.
Indeed, I found the output voltage to be around 4.7 V when connected into the analog input pin of the Arduino (I am yet to use a multimeter to measure the current though).
Thanks for the info. No, I was simply testing out an astable timer to generate a clock for a simple digital circuit which uses a shift register for clocking bits. My intention was to replace the arduino completely and use as many basic chips (7400, etc) as possible. I got it working satisfactorily although the actual time period is off from the typical calculated values (T = 0.639*(Ra + Rb)*C). Any help on that? Maybe I should start a separate thread. Thanks.
