555 astable multivibrator

[bnnnboy]

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

 

[ericgibbs]

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

 

[bnnnboy]

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

 

[karaapak]

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.

 

[ilias.cas]

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....

 

[ronv]

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

 

[ilias.cas]

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