Okay this is my first post here and I am not looking for help with homework (I read a lot before registering and posting) and I am trying to build a simple dual circuit with a 556 (I read that it is a dual timer chip) and I want circuit one to be on for ~2 seconds and off for ~5 seconds, which I want to use as the trigger for the second circuit which will operate at a 50% duty cycle at ~.5Hz ,

I have read and understand the need for the diode on r2 to acheive a longer off time than on time and understand the second circuit and should not have a problem with the values for that one.


I am hoping that someone can help me with the R1, R2 and c1 values to acheive the desired results so I can trigger the second timer circuit.

Hope I explained what I am looking for and hapoefully someone can help.

Nabs1964

 

You can find a lot of helpful information in Bill Marsden's 555 timer blog at this link.

hgmjr

 

thanks for pointing me towards more information, everything I have read is showing me stuff in hertz, I need to figure out a way to trigger in seconds per cycle and not in cycles per second. I did a lot of reading and have found the formulas for making leds flash, but there is not much out there once you go above a second per cycle it becomes difficult.

 

Keep in mind that:

\(\frac{SECONDS}{CYCLE}=\frac{1}{\frac{CYCLES}{SECOND}}\)

hgmjr

 

yes I am conversant with the formulas, trying to get 5 seconds per cycle or .20
cycles
second

anyway, I figure I can just go with 2 pots for the resistors, adjust them until I get the desired result, measure the resistance on the pots and use an equal resistor.

Kind of backward, but it will work.

 

 

Okay let me put up my work (paper) that I have done so far and see what, if anything, I can do to drop the duty cycle to about 30 percent with a total time of 7 seconds.

r1=3.3Ω r2=1kΩ C=3.3μf

This gives me the total time I want, but I change values and I can never get the duty cycle where I want/need it without totally screwing up the time. The duty cycle using the above values gives me a 66% duty cycle, can I use this to hold a transistor (switch) off while high and let it pass while low and use the output of the transistor for the trigger of the second circuit, acheiving a time on for the second circuit of ~2.3Seconds and a time off of 4.7 seconds?

The above is a question and is not stated as fact, light bulb went on and maybe I need to use circuit one as what I would describe as a normally closed circuit that would turn off circuit 2 when circuit 1 was in a powered state?

 

Or would I be better off using the output of a 555 to power a normally closed relay to open the relay while the output is high and run the working voltage to the motor through the relay with an automotive flasher to acheive the desired result?

 

Are you having trouble with the inverted signal of a 555 for some duty cycles? This can be tweaked with diodes in the right places, or simply inverting the signal with a second 555 wired as an inverter.

 

this circuit is to turn a 12 volt 1.3A motor on for ~2seconds and off for ~4, during the 2 seconds on, the power can be flashed using an automotive blinker/flasher.

I understand the need for diodes to stop feedback, I have not yet peg boarded this circuit up yet, I went to school for electronics in 1990 and worked in the field until 95, worked construction for the last 15 years and with the way the housing market in california, well lets just say I am trying to brush up on some basics to get back into the field and was using this circuit/project to both brush up and use for a small money maker for an Idea that I have that I think people would want/buy.

 

Sorry about the delay getting back with you. I had to think this one through.

I think you have the right idea. The basic schematic will work as shown, but your values are a bit off.

Going through the math you need

C1 = 100µF
R1 = 27KΩ
R2 = 68KΩ

Your original choice of cap would raise the resistances too high a value.

You will need a transistor to let the 555 drive this. A MOSFET will work at 12 nicely, so going with your schematic I would do something like this...

You can get a MOSFET from your local Radio Shack, a IRF510.

 

Is this right: motor runs on & off at 50% duty for 2 sec & then off for 5 sec. Motor runs at .5 CPS or 2 sec per cycle = 1 sec on, 1 sec off for one cycle or alltogether motor runs 1 sec & off 6 sec.??

 

I'm not judging his application, just providing information.

He also said he was using a turn signal flasher with this circuit.

 

Thanks for the math work, the diode on the right connecting 0V to Vs (right side near motor and flasher) is necessary for?

I planned on coming out of a NC relay (in place of the transistor) to the 0V with a diode inline, a transistor would probably be better, and a 3 amp fuse inline to the Vs. That connection there back to Vs has me befuddled.

 

Motors, like coils, can generate spikes. The diode absorbs said spikes, keeping them from blowing the MOSFET. It is accepted practice pretty much everywhere. They are also needed for relays too.

 

Bill, I was just trying to get the timing figured out, but did not get an answer. If it is as I saw it there is no need for a flasher unless a faster flashing rate than .5 PPS is desired.