[SOLVED] 7400 issue as a clock generator

[rajaram04]

Hello sir

i ve a common circuit diagram for a simple clock generator circuit



but this one is not working anyway . . please check

 

[FvM]

It will definitely work - with a schmitt-trigger NAND gate (or inverter) and reasonable component values. 7400 however is no schmitt trigger. A respective component would be 74132 or CD4093. Only CD4093 or CD40106 will allow a supply voltage > 5V.

 

[rajaram04]

It will definitely work - with a schmitt-trigger NAND gate (or inverter) and reasonable component values. 7400 however is no schmitt trigger. A respective component would be 74132 or CD4093. Only CD4093 or CD40106 will allow a supply voltage > 5V.

please check this link . . why they described with 7400 ?

**broken link removed**

i followed all that & for now the circuit is on board before me but not working as per explanations

 

[FvM]

Ignorants copied a correct circuit diagram (with schmitt trigger NAND symbol!) and added misleading and mostly erroneous text.

P.S.: The circuit notes printed below the circuit are also copied from the original circuit:
Place a CD4093 and the circuit works with the specified component values.

Please also notice https://forum.allaboutcircuits.com/showthread.php?t=56684

 

[bigdogguru]

i followed all that & for now the circuit is on board before me but not working as per explanations

Well I'm shocked!

Do you suppose that was the first time someone posted, incomplete, inaccurate, misleading or incorrect information on the Internet?

BigDog

 

[Audioguru]

The symbol for a Schmitt trigger inputs gate and the 5V to 15V supply voltage was a "giveaway" that a TTL 7400 gate WILL NOT WORK.
I wonder how much smoke a 7400 will make with a 15 supply?

Almost all projects on that circuit-diagram website are wrong.

 

[rajaram04]

well well well ya previously it was observed that many of the circuits on net world contain wrong info.
thus we are all thankfull to god to have these forums

i applied CD4093 & ya its working but what i am observing here is the ON & OFF time are not same for the LED i connected at output pin 3 . .
No matter whats the value of capacitor & resistor but it always happen with output

 

[Audioguru]

i applied CD4093 & ya its working but what i am observing here is the ON & OFF time are not same for the LED i connected at output pin 3 . .
No matter whats the value of capacitor & resistor but it always happen with output

When you look at the datasheet for a Cmos logic IC like the CD4093 then you will see that its output current is low. Then its output current is used to light your LED instead of charging the timing capacitor.
Simply use another gate in the CD4093 as a buffer that is driven by the oscillator then the buffer drives the LED.

The Schmitt trigger threshold voltages are not exactly symmetrical so the output of the oscillator will not be perfectly symmetrical but it will be close.

 

[rajaram04]

When you look at the datasheet for a Cmos logic IC like the CD4093 then you will see that its output current is low. Then its output current is used to light your LED instead of charging the timing capacitor.
Simply use another gate in the CD4093 as a buffer that is driven by the oscillator then the buffer drives the LED.

The Schmitt trigger threshold voltages are not exactly symmetrical so the output of the oscillator will not be perfectly symmetrical but it will be close.

ya ya i was reffering that infact & found the same issue . . thanks

well in the datasheet a gated oscillator diagram is also there
but when i left pin 1 open i found no change in oscillations
so how to biased it properly so as to make control over ? ? ?

 

[Audioguru]

You NEVER leave a Cmos logic input open. It must be made high or low.
The CD4093 is a NAND gate (why didn't you look it up?) so if its second input is high then the oscillator runs but if the second input is low then the oscillator stops with its output high.

 

[rajaram04]

You NEVER leave a Cmos logic input open. It must be made high or low.
The CD4093 is a NAND gate (why didn't you look it up?) so if its second input is high then the oscillator runs but if the second input is low then the oscillator stops with its output high.

ya i did so many times with CMOS logics but in this forum i didn't remember but some one adviced about no need to connect it to ground or high so i got confused again , well now thats all ok sir . .

so i am connecting the terminal to ground with a 100k resistor . .
is that ok ?

 

[Audioguru]

To turn the CD4093 oscillator off then make its second input a logic low which can be ground, the logic low output of another logic gate or inverter, the collector of a turned on NPN transistor or a resistor to ground.

The input of a Cmos Logic gate or inverter draws no current so the resistor to ground can have any value.

 

[rajaram04]

To turn the CD4093 oscillator off then make its second input a logic low which can be ground, the logic low output of another logic gate or inverter, the collector of a turned on NPN transistor or a resistor to ground.

The input of a Cmos Logic gate or inverter draws no current so the resistor to ground can have any value.

i see . . . ya this structure is working very smoothly now . .

is it possible to design a schmitt trigger with 4093 ?

how to do that ?

 

[betwixt]

The 4093 has schmitt inputs already!

Brian.

 

[godfreyl]

You're already using it as a Schmitt trigger. With one input held high, the other input and the output act as an inverting Shmitt trigger.

 

[rajaram04]

You're already using it as a Schmitt trigger. With one input held high, the other input and the output act as an inverting Shmitt trigger.

ya but thats an automatic type , i mean in astable mode . .
i want actual biasing configuration so as to apply external input & all that

 

[Audioguru]

ya but thats an automatic type , i mean in astable mode . .

Of course it is automatic. It is already an astable oscillator.

i want actual biasing configuration so as to apply external input & all that

It does not need biasing. The second input is used to turn the oscillator on and off.

 

[rajaram04]

Of course it is automatic. It is already an astable oscillator.



It does not need biasing. The second input is used to turn the oscillator on and off.

ya definitly , but as we do in schmitt trigger case ,
like an input,threshhold voltage setting etc & all that

 

[betwixt]

The upper and lower thresholds are designed into the device, they can't be changed from outside. All that astable circuit needs is a feedback resistor and capacitor to introduce delay between the output and input pin(s).

Brian.

 

[Audioguru]

ya definitly , but as we do in schmitt trigger case ,
like an input, threshhold voltage setting etc & all that

What are you talking about? An SN7400 TTL gate package that will not work in this circuit? It does not have Schmitt trigger inputs.
The CD4093 has Schmitt trigger inputs and the threshold voltages are fixed and are listed in its datasheet.