capacitorpulseresistors
How could I make a circuit where whenever a push button is pressed, the output would be a single pulse ? The pulses duration has to always be the same regardless of how long the button is held down. What would be the simplest way of doing this (ideally only using discrete components, so no 555 timers) ?
Thanks,
Liam
If you run the CMOS from the same supply voltage there should be no problem in directly connecting the 555 to a CMOS input.
The TTL chips are pretty much limited to a supply of around 5V +/- 0.5V. This may be an issue. If the 555 is running from a 5V supply they can be directly connected as the output of the 555 is both source and sink (sink being required for TTL input)
I've done this on multiple products to give a short pulse on the reset line once, no matter how long the button is pressed. In my case VDDS is a 3.3V rail, and nRESET is the reset line to the MCU.
Start with power on, button not pressed (open).
a. nRESET is pulled high through R1.
b. C26 has 0V across it.
When button is pressed, the node of R4 and C26 is grounded.
a. Since the voltage across a capacitor can't change instantaneously, C26 will pull nRESET low as well.
b. C26 will begin to charge through R1. The time it takes C26 to charge will be dependent on the value of R1 and C26. This RC time constant as well as the IC's VIL threshold will determine the low pulse time. The voltage across C26 will eventually charge to VDDS as long as the button is held.
When the button is released, the node of R4 and C26 will be pulled up to VDDS through R4.
a. This will cause nRESET to try to jump up to VDDS + the charge across C26 (which is VDDS). This would be 2X VDDS, which may exceed the voltage rating of the IC. This is the reason for D1 to clamp the voltage at nRESET to a safe level.
C3 is present to provide high frequency decoupling of the nRESET line, and may or may not be needed, depending on the application.
As mentioned above, tuning of the timing of the reset pulse can be accomplished through changing the values of R1 and C26.
Best Answer
This is the simplest way without the 555 timer. You have to play with the RC values and the gate's input should be schmitt triggered.
simulate this circuit – Schematic created using CircuitLab