# 555 Timer - basic Self Changing Frequency???

Discussion in 'General Electronics Chat' started by hydromagnet, Oct 12, 2011.

1. ### hydromagnet Thread Starter New Member

Aug 31, 2011
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0
Hello everyone,

I'm wanting to build circuit with 555 timer in A-stable mode.

I want the frequency to change within a couple seconds, automatically, instantly as the circuit is switched ON. And to repeat again, IF turned OFF/ON as a cycle.

Specifically, I need R2's resistance to self-change from 500K to 100K within a couple seconds...

could I just parallel 3 or 4 different charge-up caps (basic RC circuit??)

Thanks, Marty

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Rather than specifying changing resistance or capacitance values, why don't you tell us what the function of your overall circuit is supposed to be? There are lots of different ways to approach a problem; there may be a way to build yours that will be pretty simple.

What is the desired starting frequency, and what is the desired final frequency, and how long should it take to get from the 1st to the 2nd?

3. ### hydromagnet Thread Starter New Member

Aug 31, 2011
5
0
basically pulse charging a large coil/inductor...

with a high power fet,

tho, it won't pulse, if the circuit is turned on at the upper/highest frequency range (100K - R2), I have to adjust it all the way back down to 600K or so, then turn it on again.

simply using a 1M ohm Pot

(haven't calculated the frequencies yet)

thanks!

4. ### SgtWookie Expert

Jul 17, 2007
22,183
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Does this have anything to do with Bedini or Bearden's stuff?

Aug 31, 2011
5
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6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
I just don't want to waste your time/money (nor my time) on chasing some "free energy" or "over unity" scheme.

One possible valid use for the back-EMF would be a switching boost supply. However, you really need to give more details about what you are expecting for an output in order to develop a viable circuit.

I have used the CTRL input (pin 5) to control not only the frequency, but also the PWM% of a 555 timer. As the voltage on pin 5 is lowered, the frequency increases and the PWM% decreases. One can make a pretty nifty and fairly efficient boost converter that way.

But without a better idea on what your concept is, we'll just paddle in circles.

Just as an example, I'm attaching a schematic & simulation for a high-side buck current regulator for high-power LEDs, because I happen to have it handy, and it illustrates some interesting capabilities.

Very basically, Risense causes a voltage drop as the current through it rises; when that voltage reaches ~0.63v, Q2 starts sinking current from the base of Q1, causing the voltage at the CTRL input of the 555 to be decreased. C4 not only keeps the current through the LEDs stable, but passes the current pulses from the inductor to Risense to provide a "soft start" feature; preventing the current from overshooting the target during start-up.

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7. ### hydromagnet Thread Starter New Member

Aug 31, 2011
5
0
thanks!

i'll be able to take a look and study it later tonight...