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How to run higher current through a 555 timer using transistors? Answered

Hi, I'm trying to run 2 1W LEDs (each one is 3.5V and 350mA) through a 555 'one-shot' timer circuit.  555 timer has a current limit of 200mA but the LEDs have a rated current of 350 mA.  

How do I spec the proper transistor to use to boost the current?  I'm using a 12V power supply.  

I'm also not 100% sure about how to add the transistor to my circuit, though I'm pretty sure that the LED/LED resistor/(+) connection go on the collector side, (-) on the emitter side, and pin 3/base resistor on the base side of the transistor.  

Any insights will be greatly appreciated! 



Jack A Lopez

6 years ago

For LEDs that size, the usual trick is to find, or build, a driver circuit which can supply them with constant current, and that current is set by the designer to a level corresponding to the desired level of brightness.  That's usually what you do if you want your LEDs to be just on, continuously, at some level of brightness.

I'm not sure what you're doing with this 555-based one-shot circuit. Guessing that you want the LEDs to turn on briefly, and then turn off.

One way to do this would be to find, or build, an LED driver circuit that has an "enable" input; i.e. a logic level input that basically tells the driver to turn on or turn off.  Then drive that enable line with the output from your one-shot.

Here I am assuming that the amount of time, the duration of the one-shot, is long enough that the usual rules and reasons for wanting a constant current driver for the LEDs apply.

However, if you are doing something else, like turning the LEDs on for such a brief amount of time, or such a low duty cycle,  that they don't even get warm... well then maybe different rules apply.

I guess all I'm saying here is that the usual methods for LEDs that size, approximately 1 W or greater,  call for a constant current driver of some kind.


6 years ago

As you using the transistor as a switch pick one with a sufficiently high Collector current to comfortably drive the LEDs.

The data sheet will give you the gain from which you can calculate the appropriate base current / voltage to drive the transistor fully on - Make sure you do as driving it partially on will a) leave your LEDs dim and b) cause the transistor to dissipate heat.

Lots of solutions here



6 years ago

You have to use a transistor to switch and carry the extra load.