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Timer 1 Trigger Timer 2 ?

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Chilly

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Using 2 555's is this possible? Set up Timer 1 with RC network to run at say, 10khz. Then connect pin 3 of Timer 1 to pin 2 of Timer 2 and pin 3 of Timer 2 back to pin 2 of Timer 1.
Both would need to trigger on the falling edges.
 

MikeMl

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I think you will be better off with this circuit. It provides separate adjustment of the frequency and pulse width, and self starts on power up. Yours doesn't start without adding a push-button or a forth RC network

Double555.png
 
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Chilly

New Member
Thanks MikeMI,
That's exactly what I've been searching for. Thanks!! I'm assuming R3 & C5 determines the delay. I'm also assuming I could replace R1,R2 & R3 with a pot and be fully adjustable ? What speed is this setup for?


After looking at this a bit, if you put delay between U1 & U2 or widened the pulse width on U2 wouldn't that overlap the output on U1?

Can this be modified to trigger U1 after U2 is finished, regardless of delay or pulse width.
 
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MikeMl

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...That's exactly what I've been searching for. Thanks!! I'm assuming R3 & C5 determines the delay.
R3*C5 is just a differentiator which triggers the second 555. It doesn't change as you vary the Frequency or the PulseWidth.

I'm also assuming I could replace R1,R2 & R3 with a pot and be fully adjustable ? What speed is this setup for?
The product R1*C1 sets the Frequency. The product R2*C2 sets the Pulse Width. You can make periods from a hundreds of micro-seconds up to 10s of seconds (Frequencies of 10s of kHz to a fraction of a Hz). Obviously, the PulseWidth must be shorter than the period of oscillation, otherwise the second 555 would be triggered 100% of the time.

When using a pot for either R1 or R2 remember to put about 2K or more fixed resistance in-series with the pot to prevent from blowing up the 555 in-case the pot gets turned to minimum resistance. Per the 555 data sheet; 2K <= R <= ~10meg; 50pF <= C <= hundreds of uF (OTTOMH, read the data sheet to refine these numbers).

In the example I posted, the Frequency = 1/period = 1/28ms = 35.7Hz
 

Chilly

New Member
Thanks again Mike. There's something I don't understand. If you adjusted R2/C2 to widen the pulse width on U2
wouldn't that take longer to complete and therefore get interrupted with a reset before it completed? Or would that widen pulses on both?

Ok, I wasn't taking into account the time for U1 to pulse. But that would still mean you couldn't widen pulse on U2 more than double?
Am I right thinking that to reset pin 2 requires voltage less than 2/3 rail? If so, why couldn't I change R3 with a 100K trimmer and pull up slightly to create a delay?
 
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MikeMl

Well-Known Member
Most Helpful Member
Thanks again Mike. There's something I don't understand. If you adjusted R2/C2 to widen the pulse width on U2
wouldn't that take longer to complete and therefore get interrupted with a reset before it completed? Or would that widen pulses on both?
...
You seem to be hung up on R3/C5. Here it is again, without them. It occurred to me that they are not needed, so it is easier to explain what is going on with them gone... This is because the low period at V(out1) is determined only by the discharge pin current sink, and the resulting very-narrow pulse is a perfect trigger for U2 without using the differentiator.

Here are five consecutive simulations, with R2 = 2K, 4K, 6K, 8K and 10K. Note at V(t1) the Frequency of oscillation of U1 isn't effected (because R1*C1 are not changing). Frequency=3534 Hz (Period = 283us) for all five simulation runs.

However, as R2 varies, so does the PulseWidth. The PulseWidth is proportional to R2, but trouble happens if it gets too big. This varies the duty-cycle of V(out2). Note the five traces V(t2) and V(out2), which show duty cycles from ~30% to ~85%.

This circuit as depicted is a useful pulse generator, whose freq can be adjusted with one pot and whose duty-cycle (via the pulse-width) can be adjusted with another pot.

Double555a.png
 
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