Electronics Forum

by **tangolpe** » Sun Dec 04, 2011 7:39 am

G'day everyone. I am brand new in this neighbourhood
I need to switch a 300 mA DC - LED's light circuit on-off via any SCR, fine no problem, easy to find in the market one BUT, my problem is that need to work in reverse, meaning that when the power at the SCR gate is "on" there should be no power in the LED's, meaning the power should be "off", and vice-versa, when the power is "off" at the gate, the LED's should be "on". At the moment I am powering a mechanical relay with the LED's load in the NO terminals, so when the power is "off" the relay closes the circuit and the LED's goes on

by **Thomas W** » Sun Dec 04, 2011 9:38 am

Since you're using an SCR instead of a transistor I assume that you need the holding function of an SCR, and your input signal is a pulse?
And instead of a positive pulse you have a negative pulse?
Or is your input signal constant?

by **tangolpe** » Sun Dec 04, 2011 12:27 pm

I choose an SCR because I have very basic knowlegde of electronics and i saw some articles in google. If a transistor can do the job, fine, as polarity can be chosen in either way. Yes, it is a constant pulse as I instaled the circuit in series in a PIR sensor that has a NC contactis (with no polarity), so when the PIR goes active, it becomes NO for whatever period of time is a active

by **tangolpe** » Sun Dec 04, 2011 12:32 pm

I forgot to say thanks Thomas for your reply

by **Thomas W** » Mon Dec 05, 2011 12:27 pm

You're welcome, Tangolpe

The BCX19 transistor is able to switch 500 mA.
What I mean by a pulse is for instance a signal that has been 0 V for a long time / minutes, and then changes to 5 V for a couple of seconds, followed by 0 V for a long time again. What does your PIR sensor do?
I assume that NC means Not Connected, but what does NO stand for? What is the output from the PIR sensor? A contact / relay, or perhaps a voltage?
Do you want your LED to do the exact opposite of the PIR sensor, i.e. when the sensor is ON the LED is OFF, and vice versa, no matter what?

by **tangolpe** » Mon Dec 05, 2011 12:49 pm

The PIR sensor is a "pasive infrared sensor' that detect any IR variation in a pre-set determided area (i.e. people)
Unit itself has four terminals: two for the 12VDC power and two for the NC (normally closed) contacts.
I opened the unit and does not show any mechanical relays, so i asume that it is an elrctronic switch. I am actually powering (12VDC) the mechanical relay through this NC contacts, so when the PIR detects movement ( me going to pee at 6:00 AM at the batroom) it switch the power off to the rellay so the active NO (normally open) contact at the relay becomes closed, therefore the LED's at the skirting board iluminate nice dimmed blue...

by **Thomas W** » Mon Dec 05, 2011 8:47 pm

Take a look a this circuit:
[img]http://diagramtips.com/images/2011/pir_sensor_leds.jpg[/img]
Activity in front of the PIR sensor -> sensor contact open -> current flows into pin 1 ('input') of T1 instead of through J2 to ground -> T1 leads current from pin 3 to pin 2 (closed switch) -> LED's light up.
R2 protects the 'input' of T1 against too much current. R1 controls the current in the LED's (may not be needed). T1 = 2N2222 is just an example.
Thoughts?

by **tangolpe** » Wed Dec 07, 2011 10:23 am

Thanks Thomas, it looks really promising,
Tomorrow I will go to Jaycar and buy the transistor.
I already dropped the LED's current with a trimpot is series, so they are dimmed
What ohms value should I use for the protection resistor R1?
I let you know the outcomes
regards from down under

by **tangolpe** » Wed Dec 07, 2011 10:24 am

sorry i made a mistake, I mean resistor R2

by **Thomas W** » Wed Dec 07, 2011 4:37 pm

What is the resistance in the trimpot? Is 300 mA the maximum current you want in the LED's?
R2 is opening up the transistor more or less, so you could put in the trimpot in series with R2 instead. For instance, R2 limits the maximum current in the LED's and the trimpot would dim the light further if you choose to.
What voltage does the LED's require? R1 could be removed if it was 12 V.
What is the voltage across the PIR output when the contact is closed? If the voltage is not low enough (above 0.5 V) the transistor would not turn off properly, and the LED's would be ON.

by **tangolpe** » Sun Dec 11, 2011 2:46 am

Hi Thomas
I measured the output voltage at the LED's and it is 8.5 v after the trimpot set to 1.5 K
The trimpot is a 10K[attachment=0]

: modified; Circuit 2.JPG (21.98 KiB) Viewed 29912 times

[/attachment]

by **tangolpe** » Sun Dec 11, 2011 3:25 am

Hi Thomas,

Here are the goods news; the circuit is working perfectly. After experimenting with few capacitors, I added a 4000mF electrolitic cap at the LED's terminals so they fade slowly when the power goes off.

Thanks you and the forum administrator for this oportunity to receive help in this little project.

I am sure that I will be bodering you in the future

Regards, happy Christmas and prosperous New year from Melbourne

Daniel

by **Thomas W** » Sun Dec 11, 2011 2:54 pm

You're welcome, Daniel

Happy Christmas to you too!

by **tangolpe** » Tue Dec 13, 2011 12:54 pm

Hi Thomas,
yes it is me........again
I forgot to mention that after i connected the cap, the LED's are turning on in three seconds, meaning the full 8.5 V.
The cap discharge so slowly that it takes one minute to disappear any light
I do understand the discharging concept of the cap but do not undesratnd why in the way up is not instant

by **admin** » Tue Dec 13, 2011 2:50 pm

There are some mistakes in the circuit. Do the circuit as attached and it should light up immediately now. I'm assuming that your LED is ok with 12V across it. Otherwise you may need to add a resistor of small value R (say 10ohm) in the circuit as shown. If the LED arrangement can take 12V, then R is not needed and can be replaced with a short.