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555 timer problem

Can anyone lend advice on this? I am needing a circuit that takes an input of 12 - 15VDC and gives an out put that switches between 0V/.7V to 0V/.9V at about 3.3 sec intervals. I don't know enough to design it, so I found the attached diagram. I can't get it to work. D1 LED should come on solid with power and D2 should blink at the high output pulses (about every 3.3 sec). Can anyone take a look and see if maybe the schematic is wrong? I can't even get the 1st LED to power....I have built it twice and quadruple checked solder points and paths...pulling out my hair!

Thanks in advance for any help!

555_diagram.pdf13.91 KB

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great page on the 555 chip


use the Time formula half down the page to solve for the resistor and capacitor values

**this one

f=Frequency (Hz) 1Hz=1 second period 

f = 1/(.693 x C x (R1 + 2 x R2))

What exactly do you mean by "switches between 0V/.7V to 0V/.9V at about 3.3 sec intervals"?

It would seem to me that lowering the voltage from 12 to less than 1 Volt could be done without the timing by the 555. Leaves you with the pulses every 3.3 seconds. You never told how long these pulses need to be.

Pulsing sounds like an astable timer circuit to me. Did you use our search engine? Did you find the 555-howto?

That is a range of .7V to .9V for the pulse. I think the pulses need to be around .5 to 1 second in duration.


I am checking the 555 howto now...and thanks for your quick response!

Pin 4 should go to power, pin 5 should have a small 10 nF capacitor to ground. Otherwise your circuit resembles the 555 tester circuit about a quarter of the way down on that page. Oh, R3 and R4 look way too big, should be 200 to 300 ohm each, not in the k range at all. But you are trying low voltage, may have to adjust them some.

So power to PIN4 and the cap on PIN5 are in addition to the shown diagram?


Thanks again!

I see....if the rest circuit is not used then it is recommended to apply V+ to PIN4. It looks like the cap on PIN5 was just ommitted from the diagram.