I have an assignment to create a circuit without a 555 timer or an op amp which flashes an LED at 2Hz for 5 seconds then stops it. Ive created the timer part using an RC circuit
however im not sure how to implement the flashing part. I have made other circuits which flash at 2Hz but dont stop and they all break down when I add the timing element
this is as close as i have got however it stops the simulation after 2 seconds. any help would be appreciated

 

When the switch closes the 12V supply is shorted out.

 

When the switch closes the 12V supply is shorted out.
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Does this help? Sorry I'm really lost on this one

 

This is what I have come up with however it doesnt work

 

You need to know what logic family you are using. 7400 series logic has different characteristics than CMOS logic such as 74HC00 or CD4000 series.

Logic switching circuits do not work well with 1 Ω and 10 Ω resistors. Use 200 Ω to 2000 Ω with 7400 series logic or as high as 20 kΩ with CMOS logic.

Use a Schmitt trigger NAND gate such as 74LS24 or CD4093.

Get the 2-Hz oscillator working first by itself.

 

For what you are doing you really need CMOS logic. CD40xx. With a 12V supply it is most likely CMOS.
For long delays R1, R2 should be 100k ohms or something large.
With Key=open, you have an unknown voltage on the inputs of U1.

 

This is what I have come up with however it doesnt work

Can be done using 2 inverters of a CD40106 chip or 2 gates of a CD4093.

 

Start with this basic CD4093 oscillator. Modify R1 and C1 for desired frequency.

 

I have an assignment to create a circuit without a 555 timer or an op amp

The how come you are doing it with digital gates?

 

The how come you are doing it with digital gates?

It was part of the brief

 

Start with this basic CD4093 oscillator. Modify R1 and C1 for desired frequency.

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I’m only allowed to use logic gates as active components

 

Start with this basic CD4093 oscillator. Modify R1 and C1 for desired frequency.

View attachment 337575
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Start with this basic CD4093 oscillator. Modify R1 and C1 for desired frequency.

View attachment 337575

im only allowed to use logic gates as active components

 

CD4093 is a quad 2-input NAND gate with Schmitt trigger characteristics. Last time I checked it was still considered a logic gate.

 

CD4093 is a quad 2-input NAND gate with Schmitt trigger characteristics. Last time I checked it was still considered a logic gate.

AH okay thank you! Will try it now

 

Start with this basic CD4093 oscillator. Modify R1 and C1 for desired frequency.

This is one good example. You could use something like a hex Schmitt trigger as well as the quad gate with Schmitt trigger. inputs.

Since the LED needs to be on for 4.5 seconds and only takes 0.5 seconds to switch off, the resistance or R1 between the gate output and C1 needs to be different for each phase of the cycle. Most people do that by placing a diode in series a second resistor in parallel with R1 so that C1 charges up faster when the output is one polarity or the other. A good starting place for the resistor and capacitor is where R1 x C1 = the desired delay , the actual output pulse width will be different because these CD4093's have a significant tolerance for their high and low input thresholds.

One other detail: You will need to place a resistor in series with the LED just to assure that the output is able to swing pretty close to V+. Otherwise, the voltage at the output will be clamped by the LED to a voltage that is lower than the lower threshold (that would prevent oscillation).

 

This is your basic NOT or NAND oscillator. It requires Schmitt Trigger gates to work well. For low frequency oscillation, keep R as large as possible and then select C for desired frequency.

 

We can buffer the output of the oscillator so that the load does not alter the frequency of oscillation.
After you have gotten the oscillator working, we will use one of the NAND gate inputs to stop it from oscillating.