hello everyone..
I'm planning to build a VU meter based on LM3915N with LED strips as the display. I have already made such a circuit and it works fine. Now I'm trying to build a much more advanced version of this where a group of led strips within the LED display are separately controlled to form different light pattern. I'm planning to do it by using switching transistors. But the problem is that I don't know which transistor to choose from. What I'm looking for is that a 12v input must trigger the transistor to provide a 12v supply as its output to light up the LED strips. the output load current wont be greater than 500mA at 12v. so could anyone give me some suggestions??

 

You could use Tip42 pnp series transistors, with a 4k7 base resistor and put your leds on the positive supply and Collector of the transistor.

Or for smaller current upto 600mA, P2N2222.

 

You could use Tip42 pnp series transistors, with a 4k7 base resistor and put your leds on the positive supply and Collector of the transistor.

Or for smaller current upto 600mA, P2N2222.

View attachment 130116

Thank you Dodgydave for you help. Actually I'm using a 2N3906 transistor to drive the LED strip, (you could see in the schematics which I have attached). What I'm trying to do is that the 12 v output from The 2N3906 should again go to another transistor which would be operated by a separate 12v source for its switching. Actually I find it very difficult to fully convey my Idea about switching the LEDs and I'm sorry for that.

 

Your leds are drawn backwards, and you shouldn't need the 1k B /E resistor, also the leds have no series resistors, do they come with them?

 

Your leds are drawn backwards, and you shouldn't need the 1k B /E resistor, also the leds have no series resistors, do they come with them?

Actually I got the circuit from the internet and it was explained that the resistors were to protect the IC from the reverse current which will eliminate the problem of heating up of the IC. Actually I didn't examined the circuit properly and I admit it. YES the LEds are drawn backwards, but I have done correctly in my practical circuit. The LEDs which I'm talking about are LED strips which comes with the resistors. So you mentioned about the resistors, did you mean that there is no need of the resistor or perhaps a lower value resistor will do.??

Any suggestion about the transistors??

 

I got the circuit from the internet and it was explained that the resistors were to protect the IC from the reverse current which will eliminate the problem of heating up of the IC.

Not everything on the internet has been peer reviewed...

When that chip was designed, it would have been uncommon for a comparator output to not be open collector, so a pull-up/current limit resistor isn't required for your application.

Limiting current forced into an input/output is more relevant for CMOS where all outputs have parasitic diodes and most inputs incorporate clamp diodes for ESD protection.

 

Not everything on the internet has been peer reviewed...

When that chip was designed, it would have been uncommon for a comparator output to not be open collector, so a pull-up/current limit resistor isn't required for your application.

Limiting current forced into an input/output is more relevant for CMOS where all outputs have parasitic diodes and most inputs incorporate clamp diodes for ESD protection.

would there be any negative effect due to the resistor which may effect the lighting up of the LED strips like its response time or something like that??

 

The pull up resistor in parallel with the B/E terminals is obsolete with this chip, as it uses constant current output.

 

The pull up resistor in parallel with the B/E terminals is obsolete with this chip, as it uses constant current output.

OK thank you for your reply

 

Could anyone give any info on which transistors to choose. I have changed my idea a little bit. Now I'm looking for a transistor which should switched on when a 5v (its from the output of a 7805 IC which is fed from a 12V,1A dc adapter)is applied to the base emitter junction which will conduct a 12v through the collector and the load current wont be greater than 500mA.