The max allowed output current of a 2N3904 transistor is listed on its datasheet as 200mA but it performs poorly above 100mA.
It is shown to turn on well when its base current is 1/10th its collector current.
So if its collector current is 100mA then its base current should be 10mA.
Your schematic does not show a supply voltage. If it is 9V then the output high from the 555 as shown on its datasheet is 7.6V and the base voltage of the transistor is 0.9V. then the base resistor has a voltage of 7.6V - 0.9V= 6.7V and its value should be 6.7V/10mA= 670 ohms which is not a standard value. Use 680 ohms.
You don't use trial and error when you see and calculate with the spec's of the parts on their datasheets.
Got the 1/10th of 'best performance' and that I should ignore the datasheet when it says max current 200mA.
Supply is 12V. Not having that degree in this stuff, datasheets are somewhat of an enigma with all those numbers. However, since your comment regarding output compared to Vcc I see that both 15V and 5V input lose 1.7V at the output, thus 12 - 1.7 = 10.3V output.
When you say the transistor base voltage, do you mean the base-emiiter voltage? If so I see on the datasheet that the saturation voltage (max) is 0.85V, saturation voltage (min) of 0.65V. So, going for the median, 10.3V - 0.75V = 9.55V. So following on your quote, 9.55V/10mA = 955R. 955 Ohms is not a standard value, use the next upwards, which is 1k. This I'm assuming is the V/I = R formula. As I've never seen it in action I had no idea how it fitted in with circuit design. Now I think I know a little more
I'm no spring chicken, but I do learn fast. However I can't learn from books as those books I have looked at tend to speak in a language that is directed towards those who already understand. I learnt ASP in 3 weeks building my first online shopping site with full checkout system etc. I learn quickly. But I had a friend I was able to ask what many would consider stupid questions, as I needed to know. I also had the internet which had various examples and explanations to draw on. It was then explained in words I could understand.
From your reply, although each part wasn't explained as to how you got each value, I was able to fill in the blanks. I feel I have learned a little more thanks to those who have taken the time to respond to this thread. Seeing the figures (current) in TINA has also helped in linking everything together. I didn't even know how a resistor effected a circuit, or why it was used before.
I wish books for learning electronics were written this basically. Maybe more kids would get back to building things, rather than buying ready made.
Again, thank you to all.
Angie