Making a 5v digital signal

[ProDistraction]

All right. I've been trying to figure this out most of the morning and decided to go to a forum to see if the internet can help me any more than it already has.

I've been trying to figure out how I can build a circuit that takes a 12 volt source voltage and changes it into a 5 volt digital signal switching at 813.8 Hz. I know it's easy to step down 12 volts to 5, but I'm having a hard time coming up with an answer to changing that 5 volts into a digital signal. Any help at all would be appreciated.

~ProDistraction

 

[Laplace]

What do you mean by 'digital' signal? A 5V CMOS digital signal is somewhat different than a 5V TTL digital signal. Is it the 12 V that is switching at 1.6 Hz or do you need a circuit to generate the 1.6 Hz? Maybe all you need is a 5V regulator and 555 timer.

 

[ProDistraction]

The 12 volts is just from a regular car battery. It doesn't switch. The Digital Signal I need is just a simple on/off square wave that's on for 614.4 microseconds at 5 volts and off for another 614.4 microseconds. this is replacing the signal coming from a ECM in a car going to a Ford EDIS-4 to control the spark timing. It reads the signal in micro seconds. If you need any more information, just say so.

 

[john monks]

Can you restate the problem?
614.4uS on and 614.4uS of gives a frequency of 813.8Hz.

 

[ProDistraction]

Hmm. So it does. That'll show me to trust someone else's calculations.

Restated: I need to turn a 12 volt source voltage into a 5 volt digital signal at 813.8 Hz

 

[jackorocko]

If you need exactly 50% duty cycle then an IC or uC with PWM output pins would probably be the route to go.

If you can do with a little error on the duty cycle a 555 timer with pin 6 connected to pin 3 via a resistor will give you a very close approximation to a 50% duty cycle

 

[ProDistraction]

This is BUS communications though... I'm not looking for duty cycle. At least I don't think so. If I'm wrong, then please correct me. But I don't think just having a 50% duty cycle will satisfy needing a frequency of 813.8 Hz...

 

[jackorocko]

You can't have a frequency without a duty of <100% and >0%

if you need the same ON time as OFF time then you need a 50% duty cycle of a 5V square wave

The formula for the frequency is in the image I posted. You know the frequency, so use your algebra skills and work backwards. Best to pick a sane capacitor value and do the math for the resistor.

 

[ProDistraction]

Then can I have a duty cycle of 50% at 813.8 hz?

 

[jackorocko]

Yes, read my edited post above and look at the image, the formula is right there.

 

[ProDistraction]

Okay. Got it. So all I need to do is figure out R and C I'm good to go with that setup?

 

[jackorocko]

It may work and it may not for your particular problem. Cheap enough to try at least. I don't pretend to understand vehicle timing circuits in the least bit.

 

[ProDistraction]

Fair enough. I'm looking through IC's with PWM outputs as well. I'm not an electrical engineer, but I'm not completely lost. Yet. I'll try the 555 first. If that doesn't work, I'll head to the more complicated IC. Hope you don't mind if I come to you with any questions I end up with?

 

[jackorocko]

Post back, someone will help you out.