Instructables

Technology; LED flasher

I would like to make a simple single LED flasher powered by a 9 volt battery.  I would like the LED to flash approximately once per minute.  So far with my limited knowledge of electronics the best I have come up with is one that is off for one minute and then on for one minute.  The one I made that flashed once every minute using a large resistor had a very short battery life.  Any help will be appreciated..............................................phil

samaddon1 year ago
use a 2n22a transistor and sorry to say but with little knowledge you are not able to make its flash per minute!
AndyGadget3 years ago
 
Yep, a 555 is the way to go but you'll need to use the CMOS version (7555) because of the long time delay involved.

Look HERE and scroll down to where it says "extended duty cycle astable".  This is the circuit you need.  Using values of 100uF for C,  10K for R1 and 820K for R2 will give you a flash of just under a second every 58 seconds or so. (Reduce R1 to shorten the flash.)  This will be approximate because of component tolerances and temperature drift, so tweak the value of R2 if you need to.

Don't forget the resistor in series with your LED to limit the current.

This isn't entirely true, the TTL 555 can be set up to provide accurate time delays, even at times as long as 15 minutes. It may be unnecessary, I can buy 555s for 18p each and 7555s are about £1.20 each, I have only ever bought 2 for use in high-power model rocketry where weight is critical, so the whole thing needed to be powered by a coin cell. Apart from that, I didn't really notice any differences in accuracy. Assuming you set up the circuit as a standard 555 astable, suggested values are 330Ω for R1, 10kΩ for R2, and 250µF for C1 (don't bother connecting pin 5. I think it is just for noise cancellation or something).

As long as you are only using is occasionally with a switch, it should be fine, however, if it is going to be hard to access, you may wish to use a 7555. Even so, the power usage is still marginal, and average LED draws about 25mA, which still isn't much. I had an LED run off a 3v coin cell for several weeks constantly.
 
I had it in my mind from ages ago (we're talking decades here) that the trigger and threshold and currents became significant when you're timing delays of over 30 seconds or so.  I checked after I'd posted the original answer and found combined they'd be in the order of a microamp or so.  With a minute's delay you'd be looking at a charging current of 10uA ish, so it would have some effect.  However, the datasheet I'm looking at is dated 2006 and I'm wondering whether the chip fabrication has changed over time to reduce this.  Of course, the downside to the 7555 (but not significant in this case) is the 60mA output current as opposed to 200mA for the 555.

 
Also, the 7555 takes under a tenth of a milliamp as opposed to 5mA for the 555, so battery life will be much longer.
Use a good quality non-rechargable to power this if it's in a 'fit it and forget it' situation. Rechargable batteries will 'self discharge' and last nowhere near as long.
verence3 years ago
Use _two_ 7555! With the first, make an astable multivibrator with a frequency of 1 minute. This would give you a 30 seconds off period followed by 30 seconds on.

Now don't use the output of the first 7555 to power the LED directly, but trigger a monostable stage (built with the second 7555) with it. This second stage can power the LED for (let's say) 1 second, or maybe even just for a short flash.
ARJOON3 years ago
use a 555 astable.