Alternatively blinking two LEDs with 555 Timer

Question

I tried to make a simple 555 timer circuit which turns on one LED when its output is HIGH and turns on another LED when its output is LOW. I tried connecting two LEDs with single 1k ohm resistor (connection 1). The top LED turns on and the bottom one turns off during LOW state of 555 timer. During HIGH state, top LED just reduces its brightness and bottom LED turns on. Moreover, it spoiled the IC. I initially thought there is some issue with IC and tried another IC. The results were same. Later I realised, both of them were spoiled because of this connection (when I used these ICs for some other project). I searched in the internet and found connection 2 in every search results.

I measured the voltage at pin 3. It was like 2 v lesser than VCC during its on time. My questions are

1. Why it spoiled the IC?
2. What difference it makes when there are two resistors which make the circuit to work as intended?

Note: R1 and R2 values were high enough to produce visual observation of LED. This image I just took from random site for this question

Answer 1

Connection 1 should not damage the IC, rather it would kill or damage one or both LEDs unless you had a much lower supply voltage than shown.

I can only assume that you had a connection error in the first case or something else happened to kill the ICs. Or perhaps the ICs are not dead but don't work because the other project was not well designed (and perhaps some other kind of 555 did). Nothing we will be able to figure out, I fear.

Bipolar 555 chips (eg. NE555) do not have a "rail to rail" output, even when loaded with just a few mA. At (say) 5mA sink/5mA source the output will typically go from about 0.1V to about 10.5V with a 12V supply. The problem is that 1.5V (12V - 10.5V) is enough to get some light out of many red LEDs, so it may not turn off completely with circuit 2.

CMOS 555s would typically work at a few mA but they tend to have weak drive capability and can suffer from the same problem if the LED current is even 5mA (worst-case).

Here is a way to use the NE555 that requires a couple more parts. R2 is made a bit lower than R4 to account for the typical 10.5 output high voltage (rather than 12V) with 5mA load and 12V supply. It wastes about 5mA though R1 when D1 is off, so average waste is 2.5mA. If you're concerned about 2.5mA you should not be using a bipolar 555 which wastes 5 or 10mA just sitting there.

^{simulate this circuit – Schematic created using CircuitLab}