555 switch

[chopperdog]

I have learned a little bit about what I am trying to create. Hopefully I can learn this stuff.

On a 12v circuit, my desire is to have 250us sending out 12v, then 750us with no voltage.
I want this to continuously cycle, as long as the power is applied to the circuit.
The output voltage will close a relay, then let the relay open when the voltage is stopped.

Is it possible to have the no output time longer than the output time?

I have looked at the 555 calculators and have not been able to achieve this.

Any help is greeeaaatly appreciated.

Thanks,

Chopperdog

 

[k7elp60]

The attached circuit may do the job. The calculations for the 555 timer for the output to be high for a time shorter than the low time don't work. For the value shown the output is low for 250uS and high for 750uS. The times are really too short to operate a relay. A mechanical relay typically takes about 15mS to operate. By using a P channel Mosfet the load will be connected for 250uS when the 555's output is low, and the load will be disconnected for 750uS when the output is high. As long as power is applied to the 555 the sequence will continue.

 

[PCBWING]

A mechanical relay may find it difficult this cycle, especially in a long time.

 

[chopperdog]

Thank you very much.
I'll give it a try.
I'm hoping I can buy the components at the local radio shack.
Thanks again.

 

[chopperdog]

Oh, I am quite the beginner.
I actually meant milli seconds, not micro seconds.
So, 250 milli seconds, which is .25 seconds, correct?
What I am after is actually .25 seconds power on, .75 seconds power off.

Man, I feel a little embarrased, but a little more educated as well.

Thanks.

The attached circuit may do the job. The calculations for the 555 timer for the output to be high for a time shorter than the low time don't work. For the value shown the output is low for 250uS and high for 750uS. The times are really too short to operate a relay. A mechanical relay typically takes about 15mS to operate. By using a P channel Mosfet the load will be connected for 250uS when the 555's output is low, and the load will be disconnected for 750uS when the output is high. As long as power is applied to the 555 the sequence will continue.

 

[k7elp60]

I can still help you with the design. Several things I need to know are 1. How accurate is the timing required, and 2. The load current that you will be switching. The timing accuracy depends on the components connected to pins 2,6,and 7. We can replace the power mosfet with a PNP transistor to turn on the relay when the output of the 555 is still low for the 250mS period, if necessary