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NE555 Control Voltage

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Suraj143

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I’m trying to understand what is the function of this control voltage pin (pin5)!!
So I plan to do a small experiment.

In NE555 monostable mode I created a maximum delay of 10mS.When active low pulse is present on trigger pin it will generate the delay.

I'm getting a low pulse on every 10mS.When I get this pulse 555 will start the delay.

What I want to do is I need to vary this delay from an external voltage feeding to control voltage pin5.
 
Just a note for clarity of reading in schematics it's best to use 10k instead of 10000 it's far too easy to misread a zero. LTSpice supports all standard metric notation. This is especially confusing on your capacitor notation.

The control voltage pin does not function in the manner you think it does, it's an output from the 555 timer that provides access to the internal voltage divider nothing more, it does not control the 555 at all.

I'm not particularly good with 555's so I can't suggest how to make the delay variable hopefully someone with 555 experience can assist.
 
The control voltage pin does not function in the manner you think it does, it's an output from the 555 timer that provides access to the internal voltage divider nothing more, it does not control the 555 at all.

I don't think that is accurate at all. The CV can be used to adjust the threshold voltage. It connects to the threshold comparator input that is connected to a voltage divider to Vcc. Thus, the CV can be used to make a VCO, among other things.

John
 
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Hi,

Scead:
I take it you dont use the 555 much at all :) When you say you are "not particularly good with 555's" you may want to note that they just contain a flip flop and two comparators really.

Suraj:
The "CV" terminal of the 555 is there to allow an external signal to modulate the pulse width of the output. This works in either astable or monostable circuit configurations.
The pin connects to the internal voltage divider that sets up the trip points for the internal comparators and is approximately Vcc*2/3 (two thirds of Vcc) without any external connection.
Because the divider voltage is internally set to Vcc*2/3, if you apply an external voltage of Vcc*2/3 then the pulse width is not changed, but if you increase the CV voltage the pulse width gets wider, and if you decrease the CV voltage the pulse width gets shorter. This is the pulse width directly out of the 555 so if you use a transistor inverter the output low state pulse width will change instead.

We could do a more exact analysis of this if you think you'd like to see more detail on how the control voltage really affects the output timing. The main idea is that instead of the cap charging between the two normal voltages of Vcc/3 and Vcc*2/3 it charges between two different levels depending on your CV input voltage.

One drawback to using the CV input voltage is that you loose the ratiometric timing inherent in the basic design of the 555. The 555 output timing is stable with changes to Vcc, but using the CV input changes that because the timing depends not only on Vcc but also the absolute CV voltage level which does not track Vcc, unless of course you care to design it that way. Most CV applications use the CV terminal directly with some modulating voltage that does not track Vcc though.
 
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Sorry folks! I should remind myself not to open my mouth about something I don't know enough about as much as I do other people! You are most definitely correct Mr RB, I remember enough that the CV was connected to the comparitor voltage divider but apparently too brain addled to remember it's an input! I started electronics learning about micro controllers, and as useful as 555's are I've just never had a need to use one.
 
Control Voltage vs Pulse Width

Hi again,

Here's a plot of output pulse width vs control voltage for a 555 with a 10v supply and RC time constant equal to 1 second (like a 100k resistor and 10uf capacitor).
Note how it is somewhat linear to start, but then shoots up rapidly. This means we'd have to be careful about the input range if we wanted to use this in an application that expected a circuit with a linear control law.

Scead:
I dont see a post by MrRB here :)
Take a look at the equivalent circuit of the 555. Two comparators and a flip flop, small drive transistor, resistive voltage divider of 3 resistors.
 
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Ugh.. apparently I can't type as well this morning <oy> Going to be one of those days!
 
Hi again,


Oh i just noticed where you are from...i used to have relatives that lived there. Havent been there myself though in ages (many years).
 
Hi MrAl again thanks for the explanation.Now I understood a bit.

I have attached the *.asc file.My simulator can adjust the delay (0- ~10mS) when I feed voltage 0V-5V to the CV pin.

Whats your ideas on this circuit?
 
Sceadwin I got your points reg LTspice.I changed labeling to standard format.But I have a problem how you guys uploading LTspice simulating images nicely.It doesn't have any export function on simulated results.

I just press print screen & crop with MSpaint.But when it zoom it is not clear to viewers.How other people uploading LTspice images nicely?
 
Hi,

Your .asc file does not load in LT Spice. Is that what you are using?
That should have been your save of the schematic right?
 
hi,
Change your LTS parameters to suit this image, you can see the change in frequency versus Vc.
 
Hi Mr AL sorry I have uploaded all the files that generated by LT spice.

Hi,

On yes that's much better. I was able to run the simulation too then.

As Eric posted you can sweep the pulse width if you like by changing the CV source voltage.

When the CV source voltage is Vcc*2/3 (in your case of Vcc=5 it will be 3.333333 volts) the pulse width is the same as with no CV voltage (except for the variation with power supply voltage).
If you change your CV source manually to say 4.333333 (one volt higher) you'll see the low state pulse width get wider. The high state pulse width stays constant.
 
Ok thank you guys now I understood.If I add a potentiometer to the CV pin it wont work for full range for 5V.

I must make sure to add another series resister to the pot, so it will vary between the effective range that is 0V-4V.
 
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