# How can I make astable multivibrator using nand gate for getting 50% duty cycle?

Thread Starter

#### Apurva Watekar

Joined Sep 22, 2016
3
I want to make Astable multivibrator using Nand gate which will give 10kHz frequency with 50% duty cycle. I have tried several ways but felt to get 50 % duty cycle please suggest me some solution with mathematical explanation.

Thank you in advance
Apurva

#### jpanhalt

Joined Jan 18, 2008
11,087
This sounds like homework, but just in case it isn't, if you connect the inputs together, you get an inverter. Making an oscillator from inverter(s) is common. Here is a link to an old Fairchild app note on that: http://www.fairchildsemi.com/an/AN/AN-118.pdf

Forgot about the 50% part. If you can't get close enough to 50% by selection of the resistors and/or steering diodes, make the oscillator at 2X desired frequency and use a divide by 2 flip-flop.

John

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Thread Starter

#### Apurva Watekar

Joined Sep 22, 2016
3
Thank you for your reply.

#### Tonyr1084

Joined Sep 24, 2015
7,967
The dual D type flip flop - that's what I was thinking. But now you have to add another chip to your project. That always bugs me.

I was building a Chase Light using a CD4017 Decade counter. Had to use a second chip for the clock pulse. Used a 555 timer. Then I wanted to chase the lights further by adding a second decade counter. OK, that meant another two chips. In order to toggle between the two counters I had to use a flip flop. So I grabbed a dual D type FF. (Flip Flop) But the dual D has TWO FF's on it. Using the otherwise unused FF I used the set and reset to build an oscillator. I'll look for the diagram and post it. It's an easy way to use the extra component of a chip the tis otherwise unused.

#### AnalogKid

Joined Aug 1, 2013
11,160
I was building a Chase Light using a CD4017 Decade counter. Had to use a second chip for the clock pulse. Used a 555 timer. Then I wanted to chase the lights further by adding a second decade counter. OK, that meant another two chips. In order to toggle between the two counters I had to use a flip flop.
You can cascade 4017s with one 2-input gate, but you get only 9 active stages per part.

ak

#### Tonyr1084

Joined Sep 24, 2015
7,967
You can get up to 100 lights to sequence using just two 4017's. I'm still searching through my stuff to find it. But when I do I'll post it along with details on how to use the set and reset of a FF to produce a clock pulse. In fact, I think it produces a rather good square wave, close to if not 50% duty cycle. It might be on the other computer, I'll have to go down stairs to my lab and search for it. But I DO have it.

#### Tonyr1084

Joined Sep 24, 2015
7,967
Using the first 4017 wired to LED's, output 0 goes LED's 1, 11, 21, 31, 41, 51, 61, 71, 81 and 91. Output 1 goes to LED's 2, 12, 22 - - - and so on up to 92. Finally, output 9 goes to LED's 10, 20, 30 - - - 100. The first 4017 chases between 0 & 9. Those LED's are grounded through an NPN transistor. (resistors ignored for the sake of the argument). When the first 4017 goes beyond output #9, the "Carry Out" acts like a clock input to the second 4017 moving from output 0 to output 1. The next transistor provides ground for the next set of LED's, 11, 12, 13 - - - 20. With the next Carry Out, the second 4017 clocks to output #2 - and so on and so on.

OK, back to just chasing 20 LED's using a single 4017 and a single 4013 (FF): Using one half of the 4013 you toggle between two transistors. Q and NOT Q reverse and only one transistor provides ground. As the 4017 clocks through the outputs, both LED's (for instance LED 1 & LED 11) are powered. However, depending on the state of the FF, only one of them is grounded and thus, only one of them will light.

So using a 4017 and one half of a 4013 you have the makings for a chase light. Only thing missing is the clock pulse. You can ADD another chip OR you can use the SET and RESET functions of the second FF (4013 has two FF's on it) to produce the clock pulse. Please refer to the attached drawing:

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Thread Starter

#### Apurva Watekar

Joined Sep 22, 2016
3
I have used this circuit diagram as a reference
with using formula T =2.2RC. Is it possible to get 50% duty cycle with only adding up FF in toggling mode or i can get with adding up some other component, please suggest.

#### Bordodynov

Joined May 20, 2015
3,182
See

#### jpanhalt

Joined Jan 18, 2008
11,087
Divide by 2 is a fairly standard circuit. This is from the Paisley 555 site:

Source: http://home.cogeco.ca/~rpaisley4/LM555.html#23

Replace the 555 with the output from your oscillator. Chose either the D flip-flop or the JK flip-flop. Doing that will maintain the crtiterion of using NAND gates.

John

#### eetech00

Joined Jun 8, 2013
4,014
Here is another option.

Single CD4093 Schmitt NAND oscillator outputs 10Khz at 50% duty cycle.
The simulation could be better as the output produced by a physical circuit is much more square than the sim shows.
Calculation shown is approximate. Tune to 10khz by adjusting the pot.

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