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General Category => General Discussion => Topic started by: The enD on April 22, 2014, 15:47:59 PM

Title: How do I make ON delay circuit?
Post by: The enD on April 22, 2014, 15:47:59 PM
I want to wire a RGB LED strip in my side view mirrors and use it both as white DRL (daylight running lights) and orange turn indicators.
Want thing I want to do is delay the white light before it turns on when given power, like 10 seconds delay.

The plan is this. When I start the car both mirrors light up in white. When I use my turn signal a 5 pin relay switches and the LED strip lights up in orange, I will figure out a way to wire resistors to achieve orange, or maybe just go with red. Then when the turn signal is off I don't want the DRL to coma back instantly, this is where the delay ON circuit comes.

5 pin relay which when not triggered will allow the power from the circuit to flow and white to shine.
When the turn signal is applied, it switches the relay and opens a constant 12+ which is wired to light up in red.

This are my mirrors now. 5mm LED, not strips.
Title: Re: How do I make ON delay circuit?
Post by: The enD on April 24, 2014, 14:37:07 PM
Nothing? :(
If my previous comment is a little confusing, ignore it.

How do I make ON delay circuit? Apply 12 volt to it, after a short period of time the circuit closes and the 12 volts reach a LED strip. I would prefer, if I could control after how much time it closes. That's my goal.
Title: Re: How do I make ON delay circuit?
Post by: kam on April 25, 2014, 10:36:12 AM
Hehehe that's more easy to understand :D

The simplest way to make such an analog (i presume) circuit is with a charging capacitor. You let current through a resistor to charge a big capacitor. The capacitor slowly charges. You have a sort of comparator with a predetermined voltage to compare. When the voltage of the cap is above or equal to the predetermined one, the comparator output goes HIGH.

So here is how to do it - remember this is the very basic circuit, no fancy operations whatsoever.
First get a comparator. An op-amp (like ye old good 741) will also work since you do not want a rail-to-rail. Make sure that you have provide power to the power pins of the chip. Then, get a potentiometer, a 10K or 20K pot is ok. Connect the two external leads of the pot to the + and - of your power supply and the middle lead to the negative (or inverting) input of the comparator. This sets the voltage threshold, in other words the delay.

Now for your RC delay network. Here you need some educative guess. A n RC network charges at a specific level within RC time. So, if your capacitor is say 47uF and your resistor 2200 ohm, it will charge in:
t=2200 x 47x10^-6 = 0.1sec or 100mSec

You want to have 10Sec delay, so the RC should be about 12Sec. You do not want a very big RC because the setting will be difficult. So, lets refine:

t=33000 x 470 10^-6 = 15,51 sec

The RC network goes like this. The 33K resistor comes from the circuit input (from where the "Start" signal comes) and connects to the + input (or non-inverting) of the comparator. The capacitor connects from the + input of the comparator to the ground.

When you give power to the input (the one edge of the resistor), the capacitor will slowly start to charge and voltage will build across its leads. This voltage is also the non-inverting input of the comparator. When this voltage is higher than the voltage of the inverting input (as set by the potentiometer) the comparator output goes high.


Title: Re: How do I make ON delay circuit?
Post by: The enD on April 25, 2014, 23:15:58 PM
Thank you very much.

I'll post, if I meet some complications. Or with a video of the finished product.
Title: Re: How do I make ON delay circuit?
Post by: kam on April 26, 2014, 00:09:25 AM
looking forward to see the video