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19 August 2010
Author: Giorgos Lazaridis
LED Off Delay with dimming effect

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.

Bill Of Materials
 Resistors R1 Resistor 47 Ohm 1/4 Watt 5% Carbon Film R2 Resistor 2.2 KOhm 1/4 Watt 5% Carbon Film R3 500 KOhm Potentiometer Capacitors C1 100uF 25V electrolytic capacitor Transistors T1 2N2222 Switching NPN Transistor