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LED Lighting in Sequence Using Transistors

06/30/2009 9:10 AM

Hi there

I'd like to light up 8 LED in sequence using transistors such that by varying voltage from Vmin to Vmax, the LED will light up sequently and by varying voltage from Vmax to Vmin, the LED will switch off in the reverse sequence that they were lighting up (First On Last Off).

I have design a circuit using 8 transistors whereby their Bases are used as variable input to switch On and Off transistors accordind to the Voltage applied of course each base hase RB to limit IB and are all connected to a variable PS.

A resistor in serie with the collector to limit a voltage across the LED. the LED is connected between the Emetter and Ground and R in shunt with LED to limit a current flowing through the LED.

I'm struggling to light up 8 LED in sequence; I can only light up 2 LED in sequence.

Please any help will be appreciated.

Nganpet

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#1

Re: LED lighting in sequence using transistors

06/30/2009 9:16 AM

Add a simple digital counter chip and a 3:8 decoder chip. Drive the counter with a 555 timer IC. As the counter counts, the 3:8 decoder will give you 8 sequential signals that you can use to switch on each LED.

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#2

Re: LED Lighting in Sequence Using Transistors

06/30/2009 11:08 PM

LM3914 - search for it. It is designed as a LED driver with a adjustable comparator.

Otherwise, you should share a resistor across the grounds of the LEDs to limit current. I am thinking you need to use voltage dividers (VDD>R>BASE>R>VSS) to accomplish what you are attempting.

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#3
In reply to #2

Re: LED Lighting in Sequence Using Transistors

07/01/2009 1:31 AM

yes indeed, the LM3914 !s designed to to exactly what you want !

I wish you lots of succes !

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#4
In reply to #3

Re: LED Lighting in Sequence Using Transistors

07/01/2009 1:34 AM

or try this one :

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#5
In reply to #4

Re: LED Lighting in Sequence Using Transistors

07/01/2009 1:36 AM

This one might be a possible sollutioin for you ...

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#6
In reply to #5

Re: LED Lighting in Sequence Using Transistors

07/01/2009 1:43 AM

Or if you need more than 8 LEDS : (You might want to to modify the input circuit, this one is designed for measuring temperatures ...)

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#7

Re: LED Lighting in Sequence Using Transistors

07/01/2009 2:23 AM

As stated LM 3914 will do job- however many years ago I needed a simple LED voltage indicator for the charging system of my car- I used Zener diodes- I experimented till worked-the leds lit up in sequence- eg 9v6,11v,12v,13v,14v,15v. The leds all stayed on as v rose up & went off as v fell down in sequence- still have working in car- I used diff colours to represent low,normal, hi,over etc.

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#8

Re: LED Lighting in Sequence Using Transistors

07/01/2009 6:40 AM

assuming a 9V pp3 supply

The resistors in series with the LEDs are 360Ω

The resistors forming the pots driving the bases of the transistors are labeled RT1 to RT8 from bottom to top and RB1 to RB8 from left to right.

If you decrease the base current by say multiplying all resistors by 10 or 100 you will increase the fade in/fade out of each LED.

Does this make sense?

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#9
In reply to #8

Re: LED Lighting in Sequence Using Transistors

07/01/2009 4:11 PM

GA,

That is how I understood the OP wanted it done.

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#11
In reply to #8

Re: LED Lighting in Sequence Using Transistors

07/02/2009 6:11 AM

It looks as though I've missed out a stage in the calculation of the resistors.

In this spread sheet V is the voltage at the emitter of the first control NPN

The resistors are all in KΩ.

Vpot 1 (ie when V=1) is the voltage which would be present at the base of each driving NPN if the transistor was not there.

I've tried to keep the driving power (ie Rcomb) the same, and chosen E24 resistors to get the relevant voltage to 0.7V.

Of course you could do better if you used two resistors in each position or a series with a higher resolution. if you send me a private message with your EM address I will send you the spread sheet.

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#10

Re: LED Lighting in Sequence Using Transistors

07/01/2009 8:06 PM

Use some comparator chips with each comparator set to a different voltage with voltage dividers as references. Apply your Vmin to Vmax to the top of all the dividers. Keep a constant Vcc to the chips and LEDs.

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