Low voltage replacement for NE555 low frequency (0.2 Hz) astable timer circuit

[alfred1]

Hello,

I've built this simple circuit (http://www.555-timer-circuits.com/flashing-led.html) using a 555 IC to flash a LED at a certain frequency, determined by two resistors, R1 and R2.

I used an online calculator (http://www.ohmslawcalculator.com/555_astable.php) to calculate which resistors/capacitor I'd need to have the LED light up every 5s (thus having a frequency of 0.2 Hz).

Is there any way how I'd calculate how long does the LED stay lit up? I'd want to have it light up for 0.3s and then keep it shut off for 5s.

However, since this "toy firefly" is as bulky as a 9V battery gets, I'd prefer to operate it on something smaller, like two AAA batteries (3V).

I looked up the TLC555 however it looks like it can't get down to this low (0.2 Hz) frequency.


Thank you for your time :grin:

 

[FvM]

For a flasher, you'll connect the output in sink configuration. A CMOS 555 is perfect for this application because it can use large R1 values. e.g. 5 or 10 MOhm. Try R1=6.8 M, R2 = 430 k, C1=1 µF as a first guess.

 

[Audioguru]

The datasheet for the Cmos 555 ICM7555 from Intersil has graphs showing the typical output current.
It can source only 2mA (but its minimum current might be only 1mA) and sink 10mA (or a minimum of 5mA) when its supply is 3V from two brand new AAA alkaline cells then you will watch the LEDs dim even more as the battery voltage drops to 2V.
The Cmos 555 can drive the base of a transistor if it has a series base resistor then the transistor can drive the LEDs with as much current as you want.

 

[alfred1]

Thank you for your replies.

I need a flasher, but not with 50% duty cycle. Ideally, I'd want it to light that LED for 0.2s and then stay shut off for 5s. From what I've read up I'd need to modify somehow the circuit provided on the 555-website.

@Audioguru: which kind of transistor would I need to use to drive a 2V-somewhat LED?

Thanks again

 

[KlausST]

Hi,

What about using a small microcontroller.
There are some in SO-8 available.

Klaus

 

[dick_freebird]

The old LM3909 (if memory serves) was a 1-AA cell
capable red LED flasher IC. Meant for the job, able to
flash pretty slow. But the "on" time is pretty short,
just whatever the storage cap could hold.

 

[alfred1]

The old LM3909 (if memory serves) was a 1-AA cell
capable red LED flasher IC. Meant for the job, able to
flash pretty slow. But the "on" time is pretty short,
just whatever the storage cap could hold.

That was a good call. At this point I fear my best choice would be a discrete-components circuit to flash a LED for 0.1s every 5s.

Thanks

 

[FvM]

From what I've read up I'd need to modify somehow the circuit provided on the 555-website.

I did suggest a modification.

 

[alfred1]

I did suggest a modification.

You did, but wouldn't the 50% duty cycle (I'd need 0.1s of light every 5s, while a 555 either CMOS or old one, has the light time a minimum of 50% the dark time) issue persist with your configuration?

Thanks

 

[Audioguru]

The circuit will do exactly what you want.

When the timing capacitor of a 555 oscillator charges then the output goes high. The capacitor charges a long time through TWO timing resistors in series.
When the timing capacitor discharges then the output goes low. The capacitor discharges quickly through only one of the resistors.

If both resistors cause the output to go high for 5 seconds but one of them causes the output to go low for 0.1 seconds then a low power Cmos 555 can drive an NPN transistor through a series base resistor and its collector can have your LED with its series current-limiting resistor connected to the positive supply.

 

[FvM]

a low power Cmos 555 can drive an NPN transistor through a series base resistor and its collector can have your LED with its series current-limiting resistor connected to the positive supply.

The LED should be one during 555 output low, so you'll either drive the LED directly in sink configuration, or use a PNP transistor, I suppose.

 

[betwixt]

Incidentally, there is a discrete component equivalent of the LM3909 at this address:
http://www.talkingelectronics.com/projects/FlasherCircuits/Page83FlasherCircuitsP1.html together with a rather bizarre (although correct) explanation of how it doubles the battery voltage to drive the LED.

Brian.

 

[Audioguru]

The LED should be one during 555 output low, so you'll either drive the LED directly in sink configuration, or use a PNP transistor, I suppose.

I think I got the logic backwards. You are correct.
The time is short when the output of a 555 is low, then it turns off an NPN transistor and LED, but then the LED is supposed to be lighted. so a PNP transistor (and series base resistor) should be used with the LED and its series resistor and LED between its collector and 0V.