A Real Newbie Q: LED Duty cycle?

arrow · Joined: 17 May 2005 Posts: 213

Hi

I have a real newbie question:
I would like to have an LED that shows that the circuit is on.
However, I would like to use as little current (power) as possible to do this e.g. by having a duty cycle (?). Can someone please suggest how an LED can show that the PIC is "on", while consuming as little current as possible?

If possible I would like to use the PIC itself to do this with no extra components. At the moment the PIC is executing a continous loop with INT_RB enabled.

Thank you in advance
arrow

rwyoung · Joined: 12 Nov 2003 Posts: 563 Location: Lawrence, KS USA

Use one of your timer interrupts to blink the LED. The blink rate needs to be high enough to seem constant on (persitance of vision). You can also fool around with the duty cycle to adjust the apparent brightness of the LED.

Or instead of setting up the period so the LED seems constant ON, you might consider blinking it. Think of it like a heartbeat. You can even alter the speed to indicate normal or error states.
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newguy · Joined: 24 Jun 2004 Posts: 1909

Just to expand on what was already said, if you blink the LED at a rate higher than about 100 Hz, it will look like it is continuously on. Just play with the duty cycle, starting at maybe 50%, and work down to about 1% or less. Stop where you feel comfortable with the brightness.

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia

Use one of the newish high efficiency LEDs - these require less current for the same effective light output.

Varying the duty cycle will not produce you a better result (lower average current) than limiting the current through the LED by increasing the value of the series resistor. The resistor is simple to implement - no code required :-)

You play with duty cycle when you want to be able to dim a led under program control.
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Humberto · Posted: Thu Jul 07, 2005 11:56 am

Another way is doing a short blink every 2 sec (aprox) using interrupts, it will be enough
to show that the circuit is running and is alive.

Humberto

John P · Joined: 17 Sep 2003 Posts: 331

What's the power supply voltage? If it's low, close to the LED's forward drop, then you're about as efficient as you can be already. If it's a 5V system, then you're losing power in the series resistor.

If that's the case and you really must minimize current flow, then you need a circuit involving an inductor--basically you'd be creating a switching voltage regulator; more like a current regulator. The idea would be output on, current builds up through inductor and LED. Output off, inductor keeps current flowing via a diode, Schottky type for best efficiency. You can improve on that if you replace the diode with a transistor controlled synchronously, but it ends up more complicated and needs an extra pin. Actually you might be able to do it with a transistor drive off a single pin, with the LED acting as the diode in the inductive circuit. No promises, but it would be fun to try.

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia

arrow · Joined: 17 May 2005 Posts: 213

Hi

Thank you all for your posts.
I like the flashing of the LED every 2 seconds idea.
However, I have a problem with interrupts.
I already have interrupts INT_RB- my sensors are on that interrupt, and I would not like to disturb things with another interrupt (unless it has no effect on the performance).
Can someone please tell me how I would implement a short flash with very little consequences?
In general will a delay_ms(100) effect my INT_RB?

Thank you and
Regards
arrow

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

It should have little impact. But there is nothing stopping you from polling the interrupt flag bits in the main loop. Since you are only concerned with 2 seconds, the accuracy doesn't have to be that great so just poll the flag bit and if set, then execute some function.

arrow · Joined: 17 May 2005 Posts: 213

Hi Mark

Thank you for your post.

Which has a higher priority: INT_RB or INT_Timer2.
I would like INT_RB to have a higher priority- is there any way of setting this?

I am assuming that INT_RB will interrupt a "delay_ms(100)" command- is this right?

Also, if an INT_RB occurs while I am in INT_TIMER2 subroutine, will the INT_RB start after INT_TIMER2 is complete? (or will the INT_RB not execute at all in this case?)

Thank you
arrow

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

I am assuming that INT_RB will interrupt a "delay_ms(100)" command- is this right?

Yes so long as you don't use a delay routine in an interrupt or a function called by an interrupt. The compiler will disable ints if you do.

Which has a higher priority: INT_RB or INT_Timer2.

They have the same on a PIC. You can use the priority statement to set the order for which the interrupt flags are checked.

Also, if an INT_RB occurs while I am in INT_TIMER2 subroutine, will the INT_RB start after INT_TIMER2 is complete?

Yes

prajeesh · Guest

hi,

am trying to communicate with a GSM modem using rs232. am facing some problem while reading the response from the modem. i used #int_rda for triggering the read operation. am not able to read the response from the modem

can anyone help me in this

thanks
prajeesh