Driving an LED through RC circuit(RC time constant)

[hary536]

Hi, i am trying to design a circuit, where i am trying to light up an LED long enough based on my pulse signal.
Here is what i am trying to do.

a 10us pulse(every second) --->74HCT04 buffer---> (RC circuit)----> LED.

since this is a 10us pulse, i want to make sure to light up the led for long enough time, so that one can see it. So i am trying to use RC time constant to add delay so that the LED remains ON for a longer time. I am not able to figure out the exact circuit for that and how i should design the RC circuit and how and where to place it with/before/after the LED.
Also i am not able to figure out how to calculate R and C.

It would be great, if someone can help me out.
Any kind of input is appreciated.
Thanks,
Hary.

 

[berkeman]

Traditionally, you'd buffer the pulse, drive through a diode into a cap (with parallel bleed resistor to give the RC time constant), and follow that with a Schmidtt trigger gate to drive the LED.

Quiz Question -- why does the second gate need to have a Schmidtt trigger input?

 

[oswaler]

Hello Hary,

It's great that you are experimenting with designing circuits to achieve your desired outcome. Using an RC circuit to control the timing of an LED is a common technique that can be very useful in various applications.

First, let's discuss the basics of an RC circuit. An RC circuit is composed of a resistor (R) and a capacitor (C) connected in series. When a voltage is applied to the circuit, the capacitor will charge up to the same voltage as the source. However, the capacitor will take some time to charge up, and this charging time is determined by the values of R and C. This is known as the RC time constant, and it is calculated by multiplying the resistance (R) by the capacitance (C).

In your case, you want to use the RC circuit to create a delay in turning off the LED after the pulse signal has ended. To do this, you can place the RC circuit in parallel with the LED, with the positive terminal of the LED connected to the positive terminal of the capacitor. This will allow the capacitor to discharge through the LED, keeping it lit for a longer period of time.

To calculate the appropriate values for R and C, you will need to know the desired delay time and the voltage of your source. The formula for calculating the RC time constant is τ = RC, where τ is the time constant, R is the resistance, and C is the capacitance. Rearranging this formula, we can calculate the values of R and C as R = τ/C and C = τ/R.

For example, if you want a delay of 1 ms and your source voltage is 5V, you can use a 1 kΩ resistor and a 1 μF capacitor. This will give you a time constant of 1 ms, and the LED will remain lit for approximately 1 ms after the pulse signal has ended.

I hope this helps you in designing your circuit. Don't be afraid to experiment with different values of R and C to achieve your desired delay time. Good luck!