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How do i make a 555 IC timer chip make two leds flash back and forth like a light bar? Answered

What i am trying to do is make my leds flash at different times. i have a 555 IC timer chip and have found how to make a single light flash using resistors and capacitors. How would i go about making two leds flash back and forth. Any information in regards to this would be greatly appreciated.



Well, according to this circuit by connecting LED1 between pin3 and + and LED2 between pin3 and -. I've done this at some point to make a push-pull curcuit.

The simplest way to do this is to hook up two transistors to the output of the 555 timer. One must be NPN, and the other must be PNP. This will provide two alternating outputs that you can hook LED - resistor combinations up to.

quick question - I have a bunch of NPN transistors but when I apply current to the base they only turn off very slightly and I am using 12 at 500ma and I am using a 2N3904. Is it toast or is more power needed to get the dam thing to turn off?

You've likely toasted your transistors. Keep in mind that transistors aren't exactly on-off switches (you're thinking about relays, aren't you?) Simplifying things greatly, they're current amplifiers: for example, a 1 ma base-emitter current will create a 100 ma collector-emitter current flow. If you don't have a resistor between your power source and the base of your transistor, you're likely using it wrong. Past a certain amount of current, the transistor will saturate, meaning you've sourced the base with as much current as it can handle. Being that most small signal transistors feature amplification in the range of 100x (also called "beta") you won't need much current to turn a transistor on. Based on your posts, it appears you're not familiar with the concept of current and may be confusing it with voltage. I suggest you study Ohm's law and Kirchhoff's circuit laws before worrying about transistors. Having a solid grasp of the fundamentals (current, voltage and resistance) will help you understand transistors much more easily. Specifically: just because a wall adapter says "12v 500ma", that doesn't mean the adapter is providing 500ma of current at all times. That's the limit it can provide. Ohm's law will tell you how much current flows through a circuit; it depends on the components in the circuit, not the power source.

You need two separate power supplies. One from the base to the emitter, and one from the collector to the emitter. You cannot use the same one. Draw some schematics to show me what you are doing.

Whoa, what are you talking about? Can't use the same power supply? Care to elaborate? I think you might be giving bad advice here...

thats what I have. A 12v 500ma adapter supplying power and I have tried everything up to an 18v 1.2 amp adapter on the transistors but they don't work.

You need to be more systematic. Try hooking up a 555 timer oscillator through a transistor and see if it works. Don't just plug the transistors into random power supplies to see what happens. Often times, if you plug a power supply into a transistor without some sort of resistive load, it will blow the transistor.

I know that. I do have a load attached, and sometimes I have most of my circuit built and working. I think I found the problem though, they seem to be working fine now. There seemed to be a dead spot on my protoboard and nothing was getting to the base of the transistor.

Also some of them turn on instead of off but I know they are all NPN. Whats wrong with them.

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