2 Circuit Questions: 555 astable flipflop & 9V blinkers

[Steelwolf]

Would really like some help from the electronics geniuses in our midsts.

First one is about the ubiquitous 555 timer chip. Apparantly, you can run it as a stable or astable flipflop. The stable flipflop is the most common circuit I can find as it is used commonly as a timer. Setting it up with certain values of resistances and capacitances will cause the chip output to switch states at a predictable rate.

It is the astable mode which I am not sure about. Apparantly, the output will switch states when the input reaches a certain condition. Can anyone explain this more clearly?

The 555 chip is used in a lot of PWM applications as the output can be used to provide the pulsing and changing a resistance value will change the pulsing rate.

But can anyone devise a method to use the astable configuration in such a way that it monitors the condition of a charging device, say an inductor, and then when the inductor reaches a saturated state, to then pulse the output channel, so that a IRF20 MOSFET or something similar can deliver the current to an LED array? Essentially, to use the 555 chip (together with other required components) in such a way that the 555 becomes the regulator, not just the pulse provider in a PWM or step-up circuit.


The other question is about using semi-depleted 9V cells in blinking LED or continous LED operation. There is already a commercial product for this, but I was wondering about a simple circuit for home hobbyists to build. Perhaps something like a capacitor charging from the 9V cell, then when it reaches saturation, a switch activates automatically to discharge it through the LED array. Similar requirement to the above idea, but a little more specific.

Thanks in advance to all.

 

[Evan]

There was a circuit diagram in "Nuts & Volts" magazine a few months back that used a 555 as a charge controller. If you think about it, the 555 is perfect; it has 2 voltage comparitors connected to a flip-flop; The circuit was close the basic astable circuit, with the battery as the capacitor and voltage dividers to set the limits so when the battery was discharged, the charger kicked in, and when the upper limit was reached, the charger was stopped.

Here is a link to Tony van Roon's 555 Tutorial that might help.

 

[**DONOTDELETE**]

Hey
The chip you might want to check out is a National LM2621. It is a step up regulator, up to 30 volts from as little as .6 v in.
It can make 5 v at 500mA. (Luxeon star driver dee-luxe). It can also operate with an external switching transistor for even more power. The have a nice eval board for 20 bucks. Check out their site.

For your flasher, try the National LM 3909 (I think that's the number) it is an application specific LED flasher, which uses a cap in a charge pump circuit. Runs just about 4-ever.

 

[Evan]

I've still got a few 3909s around here. I was wondering if they were up to driving the new blue-green-white LEDs, but lost track of my National book.

The circuit would run more than a month on an AAAA out of a Duracell 9 volt. I once sent a letter to my brother with an "Urgent" blinking red light visible thru a window in the envelope. Guess if I tried that today I'd get a knock on my door. Still, I am not aware of any postal regulation that says your letter may not glow or blink.

 

[dat2zip]

I believe the LM3909 is obsolete. Last time I checked National web site I could no longer find it. I think you can still get this part out in surplus.

I could be wrong...

The LM3909 is actually very cute and is specfically designed to use a few capacitors to blink an LED. The duty cycle is very low but it works great.

I had a design to blink a DC/DC converter using the LM3909. Since the LM3909 will run down to 0.8 Volts or so, the output could be used to drive the enable/disable pin on some DC/DC converters making a high efficiency emergency type light.

 

[MrAl]

Hi there Evan,

Im not sure what you want to do with the
LM3909, but if you need specs or something
you can go to the National site and do
a search for the part number.
That will lead you to a data sheet at
some point.
I think all or most of National's
parts are online now.

Good luck with it,
Al

 

[Steelwolf]

Thanks for all the suggestions. I know about the LM3909, but I didn't know about the LM2621 except for the little bit that I glanced at from that extra long thread. I will certainly investigate them further, especially the LM2621 as I want to make an LS mod soon.

Why do I still want to try and set up a circuit using the 555? Partially because the 555 is such an easily available chip. I have had difficulty finding the LM2621 and LM3909 chips before, but never the 555. Also, the essence of all these step up/step down DC-DC converters is an astable switch which switches the charging circuit between the battery and the load. All the better if it can respond to the input voltage so as to drive the charging circuit harder or slower (regulation).

Since, in monostable mode, the 555 is able to respond to the condition of the input to determine when to switch, I thought it might be possible to device a simple step up/step down circuit around this chip which is able to self-regulate as well. (Step up and step downs without regulation are common, e.g. the PWM circuit we all know).

 

[**DONOTDELETE**]

Hi
I ve got data fresh from the lab . Steelwolf, the 2621 is available on-line from NS, and they have a cool eval board for $20. All ya have to do is replace one resistor (RF2 on the board)with a 20 turn 100k pot, and yer set. I ran a LS at350 mA for 2:15, on two Ray O Vac NiMH. At that point, the bottom fell out of the batts, and in another 5 min the LS was only gettin' 15mA. The 2621 is not easily configurable as a constant current driver, but with precise output adjustment, AND proper heatsinking, you can set and maintain any current you want. Heatsinking is important - i removed the sink at 350 mA, and I was at 400mA in less than 60 seconds. The V-I curve is temperature dependant.......
Peace

 

[vicbin]

Hi Steelwolf,

Still trying to make boost converter using 555 ? I think you should look at the Zetex ZSCT1555 IC datasheet.

Zetex Product Datasheets - ZSCT1555

Yes, its the popular 555 timer IC but this one can be driven down to 0.9 V operating voltage.

Just browse to the last page (page 10), it has a example circuit for single cell boost converter to 5 Volt with 555 and discrete components. IMHO even with SMD the PCB size can be quite big.

With the plain bipolar 555 the higher minimum voltage requirement , I don't know if this schema can work without further modification.

Hope this is close to what you're looking.

Vic