how to generate only one rectangular wave???

datasheet 74hct121

hi,

i need a circuit that give only 1 rectangular wave after i push my push button.
and everytime i push the button,it will give me 1 constant frequency/period for that rectangular wave.

does anyone has any idea to do that???
please help me.

i has design that circuit using,resistor capacitor and schmitt trigger inverter,
but so far i dont know this circuit can be trusted or not and i still dont know how
to make 1 constant frequency everytime i push the button.

thanx

I think what you want is a one shot (o/s) triggered by your switch.

An easy way to get this is via a logic monostable multivibrator, such as the 74hct121/221.

As I remember it, you can trigger it via a rising or falling edge (make sure it isn't retriggerable by the way (that's the 123 I think), unless you also debounce your switch), and the pulse width is t=RCln(2) -- just a battery, an R and C, a pullup or down resistor, the IC, and your switch to either supply.

You could also do it with a 555, but then you'll need to differentiate to develop your trigger.

Added after 4 minutes:

However, you're confusing me with "one rectangular wave."

Do you mean a pulse? (If so take my above suggestion.) Or do you mean a periodic waveform? (If so, what's the point of pressing the switch more than once?)

I gathered that you meant a pulse, not a wave, in my response above.

emmm...

i m also confuse with my question ;-)
sorry about my lame english,
but it is 1 shot button switch and i want to use it to change my counter state,
usually we use clock with periodic rectangular waveform right?
but this time i want to use only "one rectangular wave" so that i can see my counter change it state when i m pressing the push button

If you want your counter to advance by one state each time you push the button, all you need is a switch debouncer (Google). Counters don't need a complete clock cycle. They only need a transition in the proper direction (e.g., from 0 to 1).

Ok, now I'm understanding.

Yeah, all you need is a switch debounce.

When you press a switch or flip a switch, although you may not realize it, the switch actually momentarily opens and closes, or closes and opens, several times before coming to its final state. You need to have a circuit that takes the initial change and ignores the rest. That's a switch debouncer.

Currently you have an R, a C, and a Schmitt trigger inverter. Assuming your counter is a CMOS device, it likely has a clock input that responds to the rising (leading) edge of the clock signal. Hence, you'll need your clock to go from low to high when you push your switch/button. Therefore, you'll need your input to your inverter to be normally high and then go low with the button.

For a normally high input, you'll want to pull up the input through your R to Vdd (your supply). Then you'll want your C to discharge quickly through your switch/button to Vss (Ground) and then charge again through your pull up R back to a high upon release. The charge time is determined by τ=RC.

Hence, at power up, your input will go high keeping your output low, and it willl charge your C according to τ. Then, when you press your normally open switch, which is across C, the C's charge gets quickly shorted to Vss and pulls your input low which, in turn, sends your output abruptly high and triggering your counter, via the leading edge of the output pulse (your rectangle). BUT...

As your C was charged, for that brief instant when the switch is closed, your input isn't going to go to Vss, 0v, but, rather, -Vdd, for it will look like a battery that's been reversed. This may cause a problem. (I'm not sure what a Schmitt gate input looks like, but good practice is not to use the input clip diodes onboard.) To limit the negative spike, you should add a diode from the input to Vss, with anode to Vss. This way the -Vdd spike will cause the diode to conduct (anode is at Vss=0v), bypassing the input of the inverter with the low impedance of an on diode.

However, you may have a normally closed switch. (You didn't say.) If this is the case, then you still need that normally high input (as an inverter is stilll an inverter), but you'll need the opening of your switch to pull the input low. This seems to me to be a little more tricky. I'll have to think on it, if that's what you need.

you can use TLC555 timer cdircuit as a one shot.

it's easy and powerful.

best regards

inifinitrix said:

hi,

i need a circuit that give only 1 rectangular wave after i push my push button.
and everytime i push the button,it will give me 1 constant frequency/period for that rectangular wave.

does anyone has any idea to do that???
please help me.

i has design that circuit using,resistor capacitor and schmitt trigger inverter,
but so far i dont know this circuit can be trusted or not and i still dont know how
to make 1 constant frequency everytime i push the button.

thanx

Click to expand...

hey there,
i think that a 555 can solve your problem easily.
Take a look at the datasheet (CA555) and the typical applications, especially this:
*Reset Timer (Monostable Operation)

Hi
If I understood your question ucan use 2 of NE555 1 as monostable and 1 as astable and AND their outputs and you will have the requuired waveform each time ypu push the trigger input of the monostable.
Regards

samy alex said:

Hi
If I understood your question ucan use 2 of NE555 1 as monostable and 1 as astable and AND their outputs and you will have the requuired waveform each time ypu push the trigger input of the monostable.
Regards

Click to expand...

As I read his second post, our OP only wants to change the state of a counter every time he pushes a buttion. If this is the case, all he needs is a debouncer. As I said above,

all you need is a switch debouncer (Google). Counters don't need a complete clock cycle. They only need a transition in the proper direction (e.g., from 0 to 1)

Click to expand...

.