Driving Schmitt Trigger to 4017 counter to MOSFET

Hello,

The o/p of a Schmitt trigger will be the clock of a 4017 Decade counter.
But I'm worried that this may affect my calculations of R (in the Schmitt trigger) and how to keep the High/Low voltage compatible! Does a simple voltage divider work, a buffer, ..?

What should I look for exactly in the datasheet? I was recommended the LM393 but I'm not exactly sure what are the criteria, why not LM324!
The Schmitt trigger o/p will be 40kHz but the i/p pass the threshold and goes out in a very short time (duty cycle is very low), although I don't think I'd care much if the o/p of the Schmitt trigger does the same, as long as it gives a 40kHz signal!

I'll take an o/p of the 4017 counter and it will act as the input of a MOSFET (probably MC14007) for sample/hold purposes. Do I connect the o/p of the 4017 to the gate MOSFET directly or I need a series resistance!

Thanks!

The o/p of a Schmitt trigger will be the clock of a 4017 Decade counter.
But I'm worried that this may affect my calculations of R (in the Schmitt trigger) and how to keep the High/Low voltage compatible! Does a simple voltage divider work, a buffer, ..?

Click to expand...

Don’t worry, opamp's output is like a voltage source and connecting load such as the input of 4017 won’t effect it .. (next to nothing - bugger all)

What should I look for exactly in the datasheet? I was recommended the LM393 but I'm not exactly sure what are the criteria, why not LM324!

Click to expand...

The difference between LM393 (comparator) and LM324 (opamp) is mainly in the rising and falling edges of the output pulse, but if you are not after nano-seconds and are satisfied with a micro-second then at 40kHz it doesn’t make much difference ..

I'll take an o/p of the 4017 counter and it will act as the input of a MOSFET (probably MC14007) for sample/hold purposes. Do I connect the o/p of the 4017 to the gate MOSFET directly or I need a series resistance!

Click to expand...

You can connect output directly to the Gate ..

IanP

wsalem said:

Hello,

The o/p of a Schmitt trigger will be the clock of a 4017 Decade counter.
But I'm worried that this may affect my calculations of R (in the Schmitt trigger) and how to keep the High/Low voltage compatible! Does a simple voltage divider work, a buffer, ..?

What should I look for exactly in the datasheet? I was recommended the LM393 but I'm not exactly sure what are the criteria, why not LM324!
The Schmitt trigger o/p will be 40kHz but the i/p pass the threshold and goes out in a very short time (duty cycle is very low), although I don't think I'd care much if the o/p of the Schmitt trigger does the same, as long as it gives a 40kHz signal!

I'll take an o/p of the 4017 counter and it will act as the input of a MOSFET (probably MC14007) for sample/hold purposes. Do I connect the o/p of the 4017 to the gate MOSFET directly or I need a series resistance!

Thanks!

Click to expand...

Hi

To use MOSFET at the output of Cd4017 you can use a resistor of 47 OHM if supply is under 9 volts to CD4017 or 100
OHM if supply is over 9 volts and under 15 volts.
If you do found any noise at the output of your MOSFET then use a resistor of 1k from Gate to ground 0v of CD4017.

Rajguru

I didn't check all the other brands but CD4017B ICs made by Texas Instruments already have a Schmitt-trigger clock input.

I'm using the Schmitt trigger because I need a device that gives a clock whenever the signal passes a certain threshold!
The signal I'll be handling is from a Time division multiplexer, it changes between 5 channels (info is contained between 0.5 and 2.5v) and whenever it changes channels it goes very briefly to 3v!
The receiver has no clock but will obtain it by noticing when the signal reaches ~3v! That's why I'll use a "custom" Schmitt trigger instead of relying on a built it Schmitt trigger (or a dedicated IC such as 40106 and 74HC14) has a very wide hysteresis rendering them useless for this application!

But if there's a simpler solution (especially that I'm worried from my calculations of the Schmitt Trigger resistances/V+/V- using single supply op amp!) would be glad to hear!

I managed to get HCF4017 (STMicroelectronics) for the decade counter, LM393 for the Schmitt Trigger, and Analog Switch TC4066 for Sample/Hold instead of the MOSFET!

Which brings me to another question, for protecting the ICs from the input signal going above 5v or below 0, what should I do?
As I recall I may put a 5v Zener diode to ground, and a silicon diode in series? But Wouldn't this alter the levels (0-0.5 for alignment signal, 2.5-3 for clock) of which the comparators are assigned? How do I solve this?

Thanks a lot for your help!

The ordinary Cmos ICs work from a supply that is from 3V to 18V. They do not need a 5V supply like old TTL ICs.

High Speed Cmos (74HCxxxx) work from a supply from 2V to 6V.

Indeed, but even if I choose a higher supply voltage I'm not in control of the i/p signal and I could receive a higher/lower voltage and I'll still need protection!!

Cmos logic ICs use input levels that are the same as the supply voltage. The inputs have protection diodes to the supply voltages that will be damaged if a input voltage is higher than the supply voltage and has a fairly high current. The protection diodes limit the voltage. Add a series resistor to limit the current.

A "classic Cmos oscillator" (two inverters) has a voltage swing at the input to one inverter above the supply voltage and below ground. It uses a series resistor to limit the current.

If an input voltage is too low then it needs to be amplified.