555 astable multivibrator

Hi,
I'm working on putting a multivibrator circuit together that will have varying output frequencies based on several DIP switches that select several resistors to be in series. I am using this circuit: **broken link removed** with C1=1µF and an LED from the output to ground. I am trying to get the off- and on-times to be equal, so my understanding is that R1 would be 0Ω and I just need to vary R2 (in my case, R2 varies from 100Ω to 26.5kΩ).

However, when I hooked everything up, the LED would be on for the on-time, but would then remain off. Each time I turned the circuit on this happened. I eventually tried placing a 360Ω resistor in for R1 and suddenly my LED would flash. I tried lower resistances but that produced the same results as before. Also, when I replaced my R2 with a 10MΩ resistor and used R1=360Ω, the LED just stayed on and never turned off.

Does anyone have any suggestions about what could be happening?
Thanks,
Reed

hi,
Connect the 555 for the astable mode #2, as shown in this image, this will give a 50/50 On/Off period.
You should have a resistor/transistor driver for your LED.

Also look at this link.
https://www.electro-tech-online.com/blogs/ericgibbs/125-simple-7555-cmos-timer-calculator-v1-2.html

Thanks for the suggestion. I ended up figuring it out this morning. I tried several resistor values for R1, and the lowest that I had on hand that would allow the circuit to function was a 220Ω. Using this resistor still gets me to a 51% duty cycle when my R2 is at 5.1k, so it's close enough for government work. I also found my error with the 10MΩ as I just had it hooked up incorrectly.
Thanks,
Reed

If you want to learn more about the formulas that was using for designing 555 circuits you may use 50-555 circuits. It gives detailed information and 50 designed circuits. You may found it only searching on google as '50-555 circuits' it is a free e-book.

Hi all,
I have a question.
I have a astable oscillator running. How do I Start and Stop the timer output. Just like an enable pin.
Any ideas? please....

You can use the reset pin to stop the output. However the first pulse after the reset will be a little longer than normal.

Thank you RONV,
U mean I have to pull the RESET pin low and set high when i need to enable?