555 timer VCO

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suzuki

Joined Aug 10, 2011
119
hi,

i am trying to use a 555 timer to build a voltage controlled oscillator. I am finding that by increasing the control voltage i send to the 555, the lower the frequency, so i do know how to generate for example, 50kHz or 500kHz. this seems to be verified after doing a couple google searches.

My problem is how can i increase the range of frequencies that the 555 can output? for example, if i can generate 50kHz, the range of frequencies is from 50kHz to 100kHz, or if i generate 500kHz, the range is from 500k to 550k. I have seen the formula

f = 1.44/(C*R)

which i believe gives the center frequency, but i dont really know what parameters i can change to increase the range of possible frequencies. e.g. i want my 555 timer to be able to output between 50kHz and 500kHz, which i suppose means increasing the bandwidth of my vco?.

thx
 
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SgtWookie

Joined Jul 17, 2007
22,210
You won't be able to get that wide of a bandwidth. You might increase or decrease the frequency by 100%, but not 1000%.

That's because pin 5, CTRL only gives access to the threshold of the internal voltage divider, not the trigger.
 

Thread Starter

suzuki

Joined Aug 10, 2011
119
You won't be able to get that wide of a bandwidth. You might increase or decrease the frequency by 100%, but not 1000%.

.
Hmm ok, could you give me more details about this?. I would still be interested to know how i could get 100% of the frequency. I.e. if my center was 200kHz then how I would be able to get +/- 200kHz or 0 to 400k.

I was finding that i could only get about 20% of bandwidth ie for 100k about 20kHz of possible range.

Thanks for all replies
 

Yako

Joined Nov 24, 2011
245
That's because pin 5, CTRL only gives access to the threshold of the internal voltage divider, not the trigger.
Yeah 2/3 of VCC.

The duty cycle for an NE555-based VCO is terrible too.

Very, very narrow. Try interfacing it to some other parts.
 

Thread Starter

suzuki

Joined Aug 10, 2011
119
Yeah 2/3 of VCC.

The duty cycle for an NE555-based VCO is terrible too.

Very, very narrow. Try interfacing it to some other parts.
does this mean that the CTRL voltage can only go up to 2/3 of Vcc? im not sure if this is a problem, you would just get a broader step between frequencies. i.e instead of each volt going up 10kHz, each control voltage step goes up 50kHz...so less resolution, but a more broad range is still achievable, i think.

still awaiting sgtwookie's response here
 

SgtWookie

Joined Jul 17, 2007
22,210
You can pull pin 5 nearly to Vcc, but as you do so, the time will be very susceptible to triggering by noise on the power rails.

You can also pull pin 5 down to ground. [eta] When pin 5 is at ground, the ON duty cycle % will be the lowest, but the output frequency starts decreasing again once you get below a certain point; somewhere around 1/10th Vcc, but it depends on your Vcc and the particular timer as well.

Whatever voltage pin 5 is set to (threshold), the trigger will be 1/2 of that voltage.

The duty cycle will change very significantly as the CTRL input is varied from nearly Vcc down to ground.

In a bjt 555, there are three 5k resistors in series from Vcc to GND. The threshold and pin 5 are connected to the upper junction. The trigger is connected to the lower junction; and there is no external access to this junction.

If you want a frequency of zero Hz, turn the power off, short pins 2 & 6 to ground, short pins 2 & 6 to Vcc, or short pin 4 to ground, as those are about the only ways you'll get there.
 
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HF94

Joined Feb 10, 2014
9
I really hope there's a complete theory with right formula to control the frequency range and duty cycle for this 555 VCO :(
 

Wendy

Joined Mar 24, 2008
22,130
Necroposting and Hijacking are not allowed on AAC.

Necroposting, posting on a long dead thread, where the TS has moved on and no body care.

Hijacking, trying to take over someone elses thread. A thread belongs to the one who started it, so feel free to start your own.
 
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