The circuit on a breadboard controlling 6 high brightness 10mm LEDs
Many times have i been told that i design difficult circuits that are not particularly practical. Well, here is a very simple and very practical circuit. How simple do you think that an LED Off-Delay circuit is? It is enough to tell you that the main off-delay timer is composed by a transistor and 4 more simple components (resistors-capacitors)! And yet, it is an extremely practical circuit. It can be used as a car dome off-delay light, or in your tent when you go camping, or even as a security light. It can be powered with any voltage from 4 to 15 volts, and it can control up to 78 high brightness LEDs @ 12V power supply (or 104 LEDs @ 15V).
Moreover, instead of shutting down the LEDs when the delay time is passed, this circuit will dim the LEDs slowly until they are completely turned off, making this way a nice effect.
The Circuit
To make it more easy, i will break the circuit in two parts. The first part will be the controller, which is exactly the same no matter what power supply is used. The other part is the LEDs. I will show you some different configurations for different voltages and LED numbers. So, here is the controller:
The R1 protects the capacitor from over-current shock, when the pushbutton is pressed. R2 and R3 will determine the time that the transistor will supply current to the LEDs, as they actually determine the discharge of the capacitor. You should be very careful with the connector "LEDS", because the transistor is NOT protected from over-current. If this connector is grounded for any reason without a limiting resistor, then the transistor will wave bye bye immediately. The LEDs are connected to this connector. According to the power supply and the LEDs that you want to light, you should choose the connection from the bellow suggestions. Note that these schematics applies to LEDs with operation current 30 mA and voltage drop 3.6 Volts! In case you have different LEDs, you need to calculate the protective resistor yourself (go to the LED Resistor Calculator)
To adjust the off-delay time, play with the R3 potentiometer. Higher resistance means more time to turn the LEDs off. You can further increase the off-delay time, by changing the C1 capacitor. You can use for example a 220uF or a 470 uF, or bigger. The more the capacitance, the more the off-delay. Just keep in mind that if you plan to use the circuit at 15Volts power supply, do not use a 16V capacitor. Use a 25Volts or bigger.
Moreover, if you plan to push the circuit to the limits, I suggest that you measure one row of LEDs to see how much current flows, and check if this is the same (or very close) value as i have measure in my circuit. This is because there may be big differences due to the different LEDs that you may use.
5 Volts power supply
To save space, in every schematic that will follow, i will mark the first row with the letter RP for the protective resistor and LED (LED1, LED2...) for the LEDs. This row can be multiplied a number of times, according to the current that is drawn. I will mark the 2nd row with RPN for the protective resistor and LEDN (LEDN1, LEDN2...) for the LEDs, but this line can be multiplied more times.
When powering with 5 volts, only one LED can exist in each row, as each LED drops 3.6 volts. The protective resistor for each row must be 47 Ohms. Each row draws about 24mA. The 2N2222 can handle up to 800 mA IC. So, with 5V power supply you can control up to 33 rows of LEDs, which means that you can control up to 33 LEDs.
9 Volts power supply
With 9V power supply, each row can have 2 LEDs. The limiting resistor for each row is 68 Ohms, and a current of 24 mA is drawn. This means that the transistor can supply 33 lines of LEDs. So with 9V power supply, the circuit can control up to 66 LEDs.
12 Volts power supply
With 12V power supply (or 13.4 from the car), each row can have 3 LEDs. The limiting resistor for each row is 47 Ohms, and a current of 30 mA is drawn. This means that the transistor can supply 26 lines of LEDs. So with 12V power supply, the circuit can control 78 LEDs.
15 Volts power supply
With 15V power supply, each row can have 4 LEDs. The limiting resistor for each row is 20 Ohms, and a current of 30 mA is drawn. This means that the transistor can supply 26 lines of LEDs. So with 15V power supply, the circuit can control 104 LEDs.
@sharaz you don't need an image, let me explain: the pot has 3 pins: we will call them LEFT MID and RIGHT pins. The LEFT pin is connected to the R2 and the transistor. The MID and RIGHT pins are connected together to the 0 volts. If you need more explanation please open a thread in the forum. Here we cannot upload images.
At 18 December 2011, 11:20:50 user sharaz wrote: [reply @ sharaz]
hello sir,
might be my question is very basic as i am not familiar with most of the electronics. can you please explain the potentiometer a bit more how you have connected it with the circut. as i am getting problem when i attach potentiometer the led light get dimm and the capistor also discharge very soon. i am using the circut without potentiometer, i need the result as you have shown in youtube video.
can you please upload the picture of your potentiometer and how you have connected it, please,
There seems to be a write-up error with the last 2 samples (12v & 15v).
"So with 9V power supply, the circuit can control 104 LEDs."
"So with 9V power supply, the circuit can control 78 LEDs."
Shouldn't this be 12v & 15 respectively?
Nevermind the answer. I see it is not critical but, relative the values. I used what I had a 100 Ohm R1 and a Pot of 100Kfor R3. The circuit works. Brilliant. Now, I just do the math and play with the t=RC values to vary the time. If r3 is 0 ohms then the capacitor discharges through the light too fast to see. As I raise the resistance through R3 the light stays on longer. If higher capacitance the light also stays on longer. FUN. Thank You!
@Rick i'm very sorry but due to lack of time, i cannot do that. I try to administer the whole site by myself and myself only.
At 3 October 2011, 3:38:20 user Rick wrote: [reply @ Rick]
Thanks, I read that after I posted the comment. I see you have one on another page about the dome light. I'm still curious to know how to read the circuit. I'm having trouble understanding how to wire everything together. I'm not reall familiar with electronics. I was wondering if you could do a how to on the done light? From start to end showing all the solder points. If you could that would be greatly appreciated.
@Rick this circuit is actually not proper to use for car dome light because usually, the car's on-off signal comes from earth and not from positive. It is for educational reasons mainly.
At 28 September 2011, 19:47:35 user Rick wrote: [reply @ Rick]
I came across your video on you tube and would like to make this for the dome light in my car. I have never messed with electronics like this before. I kinda understand what needs to be done but I'm having trouble reading the diagram. I don't know what the different symbols and stuff mean. I am asking if you could be explain and maybe make a vedio of you assembling this. I plan on using a project box from radio shack and a curcuit board to soder everything to. Thanks.
@Rollie you do not solder on the breadboard. if you want to solder use perforated prototyping PCBs
At 21 September 2011, 8:53:10 user Rollie wrote: [reply @ Rollie]
Your Video was very interesting specially to those who are new to the electronic world like me. could you please show to us how did you connect each materials on the breadboard by soldering. I\'m planning to put also into my car. thanks
@RAIN i do not recommend to use this circuit for dome light, because dome lights usually use negative to trigger. If your car use positive, then connect the positive to the input of R1
At 20 August 2011, 4:48:05 user RAIN wrote: [reply @ RAIN]
if i will use it in a car dome light hows the connection? im really interested to this proj. i hope you could help me especially in wiring to my existing dome light, thanks
hi!!!
i have been seeing your videos and u explain them very well. It is giving me a great amount of practical knowledge. Basically i am an electronics engineer.
thank you
@Mark It has to do with the current that your strip will draw. Normally, it should work, but if the current gets high, then the transistor will not be able to deliver enough current. You then may consider using a power darlington pair transistor instead as a second stage.
Regarding the button, no, you can only use an on-off switch.
At 28 July 2011, 9:39:58 user Mark wrote: [reply @ Mark]
Just wondering if this would work with a row of strip LEDS like these: http://www.ebay.com.au/itm/12V-5M-300-LED-3528-SMD-White-Flexible-Light-Strip-car-/220745713088?pt=AU_Car_Parts_Accessories&hash=item33657845c0
if so, would i need to make any modifications, and could i use a momentary switch, or will i need an on off type??
But there shouldn't really be any risk of this happening since there's no reason the emitter would be connected directly to ground, am I right? Or do you mean the ground from the leds?
Perhaps what you're saying is that there needs to be a resistance to the collector if I use a power supply with more Volts than the collector can handle? If so, it would make sense to me.
oh, i mean the connector for the LEDs, which is the emitter of the transistor. If there is no limiting resistor to the collector or the emitter, and the emitter (LEDS connector) is grounded directly, then the transistor will die immediately.
Yes Devinz, you can use a transistor to invert your signal.
At 15 February 2011, 6:17:11 user Devinz wrote: [reply @ Devinz]
I'm thinking can I still keep your circuit but I just convert my negative signal to positive by a transistor.
Is this make sense?
With a transistor, my original negative signal go to the transistor 'base' and the positive for collector and the emitter as the 'S1'.
* The EBC terminals might be wrong from above as I\'m just an amateur..
Devinz, you need to make the symmetric circuit with a PNP transistor. For example, replace 2n2222 with 2n2907, C1 and R3 should be hooked to positive, emitter of T1 goes to positive and you get output of the collector. Roughly this is the idea, but i have not test it. It may not work at once and you may need to run some extra experiments. If you test it and it works, please be kind and tell us the results.
At 14 February 2011, 22:03:35 user Devinz wrote: [reply @ Devinz]
If I can only use the negative signal to turn on the circuit, how can I convert my negative signal to positive signal for this circuit (for the S1).
I know how to do it by a relay but how can I do it with a transistor?
Thx.
Hello Sebastien,
I am not sure, but you can try this by adding a big potentiometer (like 500K) in between R1 and the pushbutton (S1). Do try it and tell us the results.
Is it possible to add an ON Delay with dimming effect too on the same circuit ? I mean, when the pushbutton is pressed, it will have an ON Delay with dimming effect and when you release the button, the OFF Delay with dimming effect will be.
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