low current flashing LED

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joecool85 said:
So, is it plausable to get a couple hours out of this off a 1Farad cap? If its not at all...then I will probably have to rethink my entire idea.
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To get a couple of hours I would suggest a total rethink!, bear in mind a 1F capacitor stores much less energy than a battery!.
 
Mabye something like a CMOS 7555 timer driving a low current LED would work better, the 7555 can use as little as 60uA.
 
Is that the same as the lm555 listed at Radioshack?

And I know that a 1Farad cap is way less storage than a battery. Isn't it something like 10,000Farads for the storage of one AA? But anyway, thats not the point. I want a LED to blink for 2-3 hours (ideally), with a short quick blink every 2 seconds or so. I could afford up to 5 of those 1Farad caps, run them in parallel, so I'd have 5Farads. But I'd like to do it on one if possible. I may have to try out one of those 555 chips.
 
joecool85 said:
Is that the same as the lm555 listed at Radioshack?
Click to expand...

No, it needs to be the CMOS version, NOT the standard 555.

What are you trying to do?, and why can't you use a battery?.
 
Yes, it must be the CMOS version to achieve the low current consumption. 2-3 hours may be achievable, but the LED will probably be quite dim, use a low current/high effeciency model. Make sure the resistors in the astable are large, the 7555 is happy to use over 1M, but don't go much over that or it may not function.
 
Where can I get one of these chips?

And the reason to use a cap as opposed to a battery is simply because I'm looking at doing something "cool" with a capacitor. I like playing with electronics.
 
joecool85 said:
Where can I get one of these chips?

And the reason to use a cap as opposed to a battery is simply because I'm looking at doing something "cool" with a capacitor. I like playing with electronics.
Click to expand...
yes u can make a ckt work from a high val cap. remeber as mentioned ca cap: stores far less energy than a batt. also have charge leakage .
to maximize the time u need a very low current consuming system .
blink the led with low duty cycle.
(it will be interesting to note that a 16f628a will only consume <0.2ma , if properly used with watchdog and sleep , u can build a very low current system )
 
They have the non RoHS one in stock though. That and I already have a bunch of electronics projects going right now, and spring break is almost over. So I may not get back to this for a couple months.
 
joecool85 said:
Is this the CMOS version?

https://www.radioshack.com/product/index.jsp?productId=2062595&cp

It says CMOS, but I don't know if its the same low power CMOS you guys were talking about.
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That's it. If you want constant brightness, you need a current source for the LED. The circuit below should work down to about 3V, at about 5ma current through the LED. You can change the duty cycle of the flash with the 1Meg pot. If you want to change the frequency, change the value of C2.
 

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An intersting off topic note:

The Q1, Q2 structure.. I use it alot. It likes to oscillate at high frequencies alot. Ever build this on the bench? I _almost_ always need about 100pf from base to base on this structure to calm it down.
 
Shhhhh! Mstechca will want to use it as a transmitter! :lol:
 
joecool85 said:
Cool, thanks Ron. By big cap, how big are we talking? 100uF or like, 10,000uF?
Click to expand...
That's the big cap you are playing with.

C=I*T/V

The current I = 5mA*dutycycle
So, if you flash the LED for 0.2 secs every second, or 2 secs out of 10 seconds (20% in either case), and you want it to last for T = 10 minutes (600 seconds):
If you charge it to 9V, and it works until it's down to 3V, then V=(9-3)=6.

C=(5*10^-3)*0.2*600/6,

C=0.1 Farad
If you already have a capacitor, just solve the equation for one of the other variables. You can change the duty cycle with the pot, as mentioned previously.

This is just an example, and assumes the TLC555 draws negligible current. In reality, the time may be about 10% less than calculated.
 
the duty cycle can be changed by adding a diode and, of course the flash rate can be adjusted by juggling the cap and/or resistor values
 
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