Hello all,
I am trying to create a flashing 3 LED circuit with a 9v battery. I want a specific flash time of 20ms and a duty cycle of 1.3 seconds. My question is what size should R1, R2, and C1 be in order to achieve that timing. I found the equations for figuring it out, I was hoping maybe someone would rather take the challenge of giving me the answer instead of me going back to school to relearn algebra. Any help is very much appreciated.
Thanks.
Hi Rob,
I don't know if you keep on wanting to try it with 555.
If yes, try this sch.
See some rude calculations:
But I am not sure if it is posible for the human eye seeing so short pulse, try it.
Also take into account that timing may vary depending on the resistors and capacitor tolerance, if you are looking for accuracy just use microcontrollers as our colleagues suggest.
Hope that helps,
Miguel
Hi Gihu,
For the fun I bread boarded your circuit and it seems to work just as you predicted. I couldn't quite get to the 18 meg value of Ra so it was flashing a little more often than 1.3 seconds. I ran the Vcc at 9 volts and used an NTE 29 equivalent for the output transistor. The duration of the pulse is not a problem for the human eye which can detect even one photon under proper circumstances. The problem is as soon as the pulse gets shorter than about 70 mS the eye continues to interpret the pulse as being 70 mS. This is because of the residual image effect of the photoreceptors in the eye. Here is a photo of my circuit.
Rob should be able to use your design to do what he wants and the values that you have calculated are very close. I did not put the scope on them but from what I gather he may not be that critical for his application.
John
Hi Gihu,
For the fun I bread boarded your circuit and it seems to work just as you predicted. I couldn't quite get to the 18 meg value of Ra so it was flashing a little more often than 1.3 seconds. I ran the Vcc at 9 volts and used an NTE 29 equivalent for the output transistor. The duration of the pulse is not a problem for the human eye which can detect even one photon under proper circumstances. The problem is as soon as the pulse gets shorter than about 70 mS the eye continues to interpret the pulse as being 70 mS. This is because of the residual image effect of the photoreceptors in the eye. Here is a photo of my circuit.
Rob should be able to use your design to do what he wants and the values that you have calculated are very close. I did not put the scope on them but from what I gather he may not be that critical for his application.
John
Cool!!
John, thanks for checking it!! 
