The Generation of Two Negative Pulses Separated by Time=t.

Hello,

I am attempting to create a circuit that generates two negative pulse signals separated by some amount of time. The trigger for the first pulse, however, is a switch, who's initial state is 'off', turning 'off' after being 'on' for some amount of time. I have attached an image to show what I mean.

The actual situation is that the switch is on a kettle and the two pulses need to be generated when the kettle has finished boiling, i.e. the switch has turned off. These pulses are going to a monostable multivibrator.

I can clarify if need be.

Any help would be welcome.

If time t is more than a few minutes, it is not practical to use analog RC delays. I would use a 4017 CMOS counter and oscillator made out of gates.

How long is t, and how long should the pulses be?

Time t is of the order of a few seconds and the pulses need to be about 1-5ms in duration.

So just to be clear, you want the first down-going 5ms pulse right after the down-going edge of the input, and the second down-going 5ms pulse an adjustable delay (1 to 10sec) later?

What voltage do you want to power it on?

What is the amplitude of the input pulse?

What do the two output pulses drive?

So just to be clear, you want the first down-going 5ms pulse right after the down-going edge of the input, and the second down-going 5ms pulse an adjustable delay (1 to 10sec) later?

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That is correct, however it must not generate a pulse when the input goes from 'low' to 'high' , only 'high' to 'low'.

What voltage do you want to power it on?

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5V

What is the amplitude of the input pulse?

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5V

What do the two output pulses drive?

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They trip a bistable multivibrator.

BLUE_CHIP said:

...They trip a bistable multivibrator.

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You mean they set and reset a flip-flop? What is the input impedance of these inputs? TTL? CMOS?

You mean they set and reset a flip-flop? What is the input impedance of these inputs? TTL? CMOS?

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Sorry, I made a mistake. What I meant to say was the pulses go directly to pin 2 of a monostable 555.

What I am trying to do is control a flip flop circuit which in turn controls the length of pulse on a astable 555. The astable and flip flop are shown in the attachments.

What I had thought to do was control the flip flop with a monostable 555 which receives two pulses separated by time t ≈ 8s

The context of the whole system is to control a servo. i.e. servo is in initial state. The switch on the pulse generator is closed and then opened again which generates a pulse and then shortly after, another pulse. The first pulse switches the flip flop which causes the astable to generate a longer pulse length, changing the position of the servo. Shortly afterwards the second pulse trips the flop flop again causing the astable pulse length to shorten back to its initial length and so the servo returns to its initial position.



I thought to use a monostable 555 as a pulse generator with the circuit I am asking for help with providing the input to pin 2.

If you can recommend a better solution it would be most welcome.

Maybe this is what you want: Look at the light blue trace V(da), which is really the voltage at the out pin of the 555, node A in the circuit. Is that what would control your PWM stage?

Otherwise, the two pulses you requested are V(db) and V(dd), nodes B and D in the circuit, respectively.

They are all really 5V signals; it is the DView gadget that compresses the traces and spreads them out so they dont cover each other up.

Thank you for your help. What is the name of the software you used to create that circuit simulation?

BLUE_CHIP said:

Thank you for your help. What is the name of the software you used to create that circuit simulation?

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LTSpice. See this thread.