Hi guys, I have a question regarding the 555 if anyone can help me out. I am using a 555 to generate a square wave, but need to be able to switch the HIGH with a toggle. Think a HIGH of 2V to a HIGH of 50mV. I have read some conflicting information regarding the output in the first place. Some places say it is near Vcc some say there is a drop of ~1.7V. Can anyone clear this up? Also, what do you guys think is the best way to regulate the output? Dividers on the output seem viable but not if the 1.7V thing is true considering I need a HIGH as low as 50mV (sorry if this high and low stuff is getting confusing).

I appreciate any help with this, hopefully im not missing something in the data sheet. Thanks!

 

The generic 555 cannot get close to the upper rail, and 50mV will be a problem at the low end. You need to read the data sheet for the exact device you are using.

How much current do you need at the two states, and how much precision in the 2V and 50mV specs? I mean, you could establish the upper voltage with a resistor voltage divider as long as the next step is high impedance (low or no current load). Getting the low end needs some thinking.

Oh wait, I missed that 50mV was an alternate high value. You should be able to hit with a divider if you don't need much current.

 

The datasheet for NE555 says that when you have Vcc=5 volts, the minimum output will be 2.75 volts, typical output will be 3.3 volts. So I think the 1.7 volt drop comes from NE555 datasheet (5-1.7=3.3 volts). However, there other makers and variations of 555, and they will have their own datasheets and values.

The thing is that this is data for TI NE555 for Vcc=5 volts. Pick a specific chip from your inventory, see EXACTLY what it does. Is your chip the typical one and output 3.3 volts or is it irregular one that outputs 2.75 volts or some other acceptable but not typical value? Once you have the data from your specific chip, you can experiment with your chip, reduce the Vcc until your output is 2 volts. After that you just need a 2 way switch and a voltage divider to drop 2 volts to 50 mV.

 

One more thing.

Interestingly enough, the datasheet for TI NE555 lists the absolute maximum Vcc. What it does not list the minimum Vcc. So you don't have to run the chip on 5 volts, you can try Vcc=3.7 volts, if you have typical chip, you drop 1.7 volts and your output is exactly (or almost) 2 volts that you actually want.

 

If you use a CMOS 555 you can get rail to rail, but there will be very little drive (1 ma max or so). At higher power supply voltage this goes up to 100 ma at 15 VDC.

Another scheme I have use is this, but it will invert the output.

Bill's Blog

 

The supply voltage is being supplied by a 4.2V battery. The NE555 TI chip is the one I am looking at, but my concern is that the 1.7 drop seems to be a +/- situation. Meaning if my ideal HIGH is in the mV range that could be a problem. I may have missed something, but there is no minimum supply voltage listed.. is 5V a minimum?

I could solve this experimentally, but this design is not being prototyped ideally, and straight to a PCB for testing purposes. I am hoping for a solid method to regulate the output HIGH voltage.

edit: not particularly worried about current, just need the square wave and voltage HIGHS

 

The zeners shown in my circuit (or the LEDs) is to prevent shoot through, where both transistors turn on at the same time. A 5VDC power supply can use red LEDs just fine.

A 7555 (one of several CMOS 555's) may fit your needs nicely. If memory serves a standard 555 can only go down to 4.5VDC on the power supply, while the CMOS will go to 2VDC.

 

Thanks Bill, that is interesting about the CMOS chips. I will look into them. By the way, there is a charging circuit that will be actively charging the battery, so Im not worried about the battery voltage dropping (in regards to the output dividers).

 

How about just using a 2V zener at the output of any 555, and then also divide down the regulated 2.0V to 50mV? You'll have both voltages at the same time and you can just switch between them.

 

I may be missing something, but where does the zener come in? My concern is not being able to nail down an output voltage accurately, so I am still worried about getting the divider wrong given a +/- scenario.

 

Start with a precision V supply like a LM336Z, 2.5 V, divide down to the 2 required V's & use 4066 to switch between them. 555 supplies square wave to the control input selected by your switch.

 

Thanks Bernard, that is a good idea, did not consider a switching IC. I will look into that.

 

I may be missing something, but where does the zener come in? My concern is not being able to nail down an output voltage accurately, so I am still worried about getting the divider wrong given a +/- scenario.

The idea was to use the zener (or other voltage reference) on the output of a 555, that would otherwise be something higher than 3V when high. The zener would control that down to 2.0V, with as much precision as the zener is spec'd for. A voltage divider off of that 2.0V reference could give you an accurate 50mV. You would want high precision resistors and you might want to hand pick them, to get the most accuracy you can.

 

I suspect it would be simpler and more accurate to make a variable power supply for the 555 and adjust it to where the 555 output is 2V. That would put the power supply somewhere around 3.3V.

A LM317 could do it, but it would need around 5V to drive it. You could also use a simple transistor common collector regulator.