Electronic – Original 555 timer transistor output implementation

555outputtotem-poletransistors

I am a beginner casual electronics hobbyist trying to learn about how chips work internally. In order to accomplish, this I went out and got a 555 timer soldering kit and started experimenting with it…. things are not going so well.

I am working my way through the circuit as best as I can but I am currenlty stuck on the output stage section of the circuit, I am simply unable to understand how that works and I was hoping someone could lend me a hand.

For your reference, below is the original 555 timer schematics:

enter image description here

After looking at the diagram and reading throug some of the documentaion, I learned that the output stage constist of transistors Q20 throug Q24. With that in mind, in an effort to try to undetand how all this works, I tried to isolate the output stage from the circuit and recreate it using an online circuit simulator. This is what I did:

enter image description here

The problem is that the circuit does not work as expected (output does not sink and source current). Could someone please tell my what am I doing wrong? What do I need to do to my online symulator circuit to make it work? I would like to get it to work on the symulator so that once working I can mess aroudn with it and see if I am able to figure out how all these works.

click here to be taken to the online simulator

Thanks.

Best Answer

Here is something to try. Q20 wants to be a phase splitter. You can read about that here; enter link description here. Because you are tying Q20's base directly to 9V it is not working like a phase splitter. It is two forward biased diodes BC and BE. The means both the base of Q21 and the input to the 200 Ohm resistor are at 8.3V. So try this, put a resistor between the switch and Q20 base. Size it to get say 1V at the 200 Ohm resistor. When you do this, you will get about 1.5mA flowing thrugh Q20 emitter. This same current will flow into the collector trying to drop about 10V in the 6.8k resistor. Since it can't the transistor will saturate at maybe 1.1V. Now you will have Q24 on and Q22 off. Driving the base directly with 9V has them both on.

Related Topic