4047 monoshot(1ms pulse width)

[vinodquilon]

I have designed oneshot for 1ms Pulse width using 4047.

R5- makes output current >= 10mA to ensure load regulation.
C4- prevents bouncing of trigger switch.
1N4002- prevents discharging of c1 through LM317T.

1. Is it possible to see the 1ms 'short time' switching between RED & GREEN LEDs at output of 4047 ?

2. Can I use C4 in parallel with C1 as both have different roles in the circuit ?

3. 10K or 100K which one is better to pull down trigger switch to GND ?

 

[BioniC187]

1. Don't really understand this question.
2. Yes you can, they do have different roles.
3. Use a 10k

 

[vinodquilon]

1. Don't really understand this question.

The persistence of vision at least is 1/16 Sec=62.5mS. But LED at 10th pin of 4047 turns on for 1ms only.

The output source current of HCF4047 at 5V is 1mA. Is it enough to drive output LEDs ?

 

[alec_t]

I don't think a 1ms pulse would be enough to make the LED flash visibly.
1mA (if persistent) would certainly give a visible indication, but very dim unless an ultra-bright type sensitive LED is used.

 

[vinodquilon]

Here is another arrangement of de-bouncing...

 

[vinodquilon]

ouput drive with debouncer

Here is another arrangement with output drive and debouncer.

 

[alec_t]

1) Your debounce circuit has a slow rise time and fast fall time, so would best be used for triggering the -T rather than the +T pin of the mono.
2) R5 is unnecessary as LED3 passes more than the minimum load current of the regulator.
3) The LM324 is unnecessary. You could drive the base of T1 directly by the mono.
4) I'm guessing that even with the increased LED1 current you still won't get a visible 1ms flash; but I could be wrong

 

[vinodquilon]

1) Your debounce circuit has a slow rise time and fast fall time, so would best be used for triggering the -T rather than the +T pin of the mono.

As per your opinion I simply change the trigger line from 8th pin to 6th pin (no changes in other parts).

OR

Can I put two schmitt NOT gates in series between trigger line and debounce output so that sharp edges trigger signals can generate ?. How to take care Lower and Upper threshold of schmitt trigger NOT during the design of debouncer ?

 

[audioguru]

Texas Instruments datasheets for Cmos logic ICs show detailed minimum and typical output currents.
With a 5V supply, the typical output current to a 2V red LED WITHOUT USING A CURRENT-LIMITING RESISTOR is only 1.8mA.
With a 10V supply, the typical output current is 9mA.

 

[alec_t]

OR...

Can I put two schmitt NOT gates in series between trigger line and debounce output so that sharp edges trigger signals can generate ?

You could, but it seems unnecessarily complicated to do that.

How to take care Lower and Upper threshold of schmitt trigger NOT during the design of debouncer ?

They take care of themselves.

 

[vinodquilon]

1) Your debounce circuit has a slow rise time and fast fall time, so would best be used for triggering the -T rather than the +T pin of the mono.

If I change the trigger line from 8th pin to 6th pin (no changes in other parts) pulse will generate only when the trigger is released.

To compensate the anomaly I just reverse the connections on the switch as shown below.
It has an added advantage that the minimum time required for keeping the switch pressed is also get reduced.

I'm afraid that the very low discharge time(0.01ms) will lose the function of debounce(as bounces lasts for some time after releasing the switch- from NO to NC).
Can I solve the problem by swapping 100 Ohm & 100K ?

But it will increases the minimum time required for keeping the switch pressed.

 

[vinodquilon]

Most of the bounce occurs when the switch makes contact at the NO contact. At this point the -trigger input is grounded and any bounce is suppresses by C5 as it tries to charge up through R8. Same thing happens when the switch is released. Any bounce at the NC contact doesn't matter.

Hence the below circuit would be fool proof design