# 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
22,220
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.

#### MrChips

Joined Oct 2, 2009
22,090
Try an XR2209.

#### 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%.

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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.

#### Wendy

Joined Mar 24, 2008
22,172
Actually the duty cycle problem can be solved by using pin 5 as a VCO and the 555 Hysteretic Oscillator, though it would work better with a CMOS 555.

#### Yako

Joined Nov 24, 2011
245
Actually the duty cycle problem can be solved by using pin 5 as a VCO and the 555 Hysteretic Oscillator, though it would work better with a CMOS 555.
I am going to take an educated guess and say that you've read most of Forrest M. Mims III 's Radio Shack books.

#### Wendy

Joined Mar 24, 2008
22,172
Yep, then gave them away while I was in college to a high school student. He visits this site occasionally.

Forrest M. Mims III

#### 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
22,220
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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#### Bernard

Joined Aug 7, 2008
5,686
The 4046, phase -loched loop, contains a wideband,self contained VCO, requiring 2 ext Rs & one C.

#### 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
22,172
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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