Hello!
I need a circuit in which the frequency of LM555 astable multivibrator can be controlled externally I mean by the help of transistor etc. So, please guide what modification may be done in the attached circuit

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Have you read the LM555 data sheet? It shows a crude VCO application. There are much better ICs available to create a VCO.

 

I do not need a precise VCO, actually I want to control the frequency by the help of R2R (ladder network), where the digital input value will increase/decrease the DC voltage of R2R network, so the frequency may be varied from 1Hz to 5Khz (i.e. My desired frequency). So what should I do?

 

hi,
Do you require a 1:1 mark/space ratio on the Vco square wave output.?

Also whats the range of Vcontrol of the modulation.?

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I misunderstood. Varying the frequency of a simple 555 astable by varying voltage doesn't work very well, but varying frequency by changing resistance in the timing network can work quite well.

If you are planning to use a R/2R ladder, how many different discrete frequencies would you like to get?

Also, it is possible to build a voltage-controlled current-source, which in turn drives the timing capacitor in the 555 astable, which does give a reasonable F vs V curve.

I attached a sim of the 555 driven from a current source. I played with the values to get 5KHz and about 5Hz as the control input goes from 0 to 8V. If you change the supply voltage, you will have to re-optimize the other resistor values. Temperature stability will not be very good.

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Go here: 555 Timer/Oscillator Tutorial

Scroll down until you find fig. 10. It's a little complex, but it allows wide freq variation and maintains 50% duty cycle, or 1:1 mark/space as our British friends say

 

Fig 10 in the subject link has nothing to do with a VCO

 

Sorry, I was responding to the orginal post that said he wanted to use a transistor, etc., and wanted to modify the attached ckt.

EDIT: Getting back to fig. 10, it would be possible, I think, to replace the var. resistor with a FET biased in the linear region as a controlling device. I've often thought about that, but never tried it.

 

I think you could do it easy enough with a 2 transistor multivibrator.

Since the "threshold" voltage is only 0.6v to trigger the base of the transistor(s) your incoming voltage from your R2R network would cause the base cap(s) to charge quicker and give you frequency control, which would be inverted.

It's not that elegant but then you don't need a precise VCO.

It is simpler than a 555+transistor etc and if you apply your control voltage to BOTH transistor bases of the multivibrator you should be able to maintain a decent 50:50 duty cycle while varying the frequency, although that may cost an extra couple of resistors.

 

Hi,

Did you try simply using the 'control voltage' input? That's what it does,
it varies the frequency as the voltage changes.

 

The best I could get was about a 10:1 frequency range, varying the "CV" pin from near ground to near Vss. The duty cycle does not stay the same.