low current flashing LED

[joecool85]

I'd like to make a low current flashing LED circuit. Ideally 5-6mA. One red LED.

Here is a schematic I found for two LEDs:

**broken link removed**

Any idea how to get it to work with only one LED?

 

[akg]

I'd like to make a low current flashing LED circuit. Ideally 5-6mA. One red LED.
Any idea how to get it to work with only one LED?

remove one LED..

 

[joecool85]

I'm assuming I would have to remove the entire left or right side of the circuit? Will it still flash?

 

[Nigel Goodwin]

I'm assuming I would have to remove the entire left or right side of the circuit? Will it still flash?

No, just remove one LED and replace it by a piece of wire - even better, increase the value of the series resistor to compensate for the LED no longer being there.

The circuit is a simple slow speed monostable multivibrator, a VERY standard old circuit.

 

[k7elp60]

I agree with Nigel, I calculate the collector current to be about 15Ma so replace the 470 ohm resistor and the straight wire with a 620 resistor. In other words put a 620 ohm resistor from the collector of one transistor to the 9V battery connection point.

 

[akg]

No, just remove one LED and replace it by a piece of wire - even better, increase the value of the series resistor to compensate for the LED no longer being there.

The circuit is a simple slow speed monostable multivibrator, a VERY standard old circuit.

i think u meant astable multivibrator

 

[joecool85]

I agree with Nigel, I calculate the collector current to be about 15Ma so replace the 470 ohm resistor and the straight wire with a 620 resistor. In other words put a 620 ohm resistor from the collector of one transistor to the 9V battery connection point.

Would I replace both 470s with 620s or just the one on the side I'm pulling the LED from?

 

[joecool85]

Also, I don't have two 10uF caps in my parts bin, I do have a 22uF, a 10uF and a 4.7uF...would it work with a 10 and a 4.7?

 

[akg]

Also, I don't have two 10uF caps in my parts bin, I do have a 22uF, a 10uF and a 4.7uF...would it work with a 10 and a 4.7?

yup..it's a general purpose ckt..no values are very much critical here.

 

[joecool85]

Cool, I'll be trying this out later. Now...how do I calculate how many mA its using?

 

[Nigel Goodwin]

No, just remove one LED and replace it by a piece of wire - even better, increase the value of the series resistor to compensate for the LED no longer being there.

The circuit is a simple slow speed monostable multivibrator, a VERY standard old circuit.

i think u meant astable multivibrator

Oops! - so I did!

May as well mention bistable as well, just to them all mentioned in the thread! :lol:

 

[Nigel Goodwin]

Also, I don't have two 10uF caps in my parts bin, I do have a 22uF, a 10uF and a 4.7uF...would it work with a 10 and a 4.7?

As suggested, it's a very non-critical circuit, changing the capacitors will affect the flash speed, one capacitor setting the ON time, the other the OFF time. The smaller the caps, the faster the flashing.

 

[Roff]

Just a few comments:
1. The minimum beta of 2N3904 is 100. This circuit requires a beta of at least 180 to keep the transistors in saturation. I would change the base resistors to 47k. If the same timing is required, raise the caps to 20uF.
Of course, it might work fine as a one-off with the values in the schematic.

2. The collector resistor on the side without the LED can be as large as 10k, saving power and making the battery last almost twice as long.

3. Base-emitter breakdown is probable with a 9V supply. I would switch to 6V (with appropriate resistor and capacitor changes), or put a 1N4148 diode in series with each emitter.

EDIT:
I just noticed Mr. Cool wants 5-6ma. In that case (using the emitter diodes as I suggested), the LED-side collector resistor can be 1.2k, and the base resistors and capacitors can retain their original values.

 

[joecool85]

Is it possible to get this to work on 2-3 AAs? So 3 - 4.5 volts? 5mA was a starting point, if I can get it lower, that'd be great! So what would be the best values for this to run of a few AAs for as long as possible? I want it to blink about once every second. Once every two seconds at the slowest.

 

[joecool85]

Well I think I'm going to be build it as per the original schematic for now, if you guys have ideas as to how to run it off 3-4 volts for as long as possible, feel free to post 'em. Thanks.

 

[Dr.EM]

This is the schematic following Rons comments

 

[joecool85]

Ok. Here is the whole plan.

I want to run this circuit off from a capacitor. Right now I'm using a 10,000uF as my test cap. If I can get things going well I want to grab a 1Farad 5volt "memory backup" style cap for it. The longest I can get it to run off the 10kuF cap is 10 seconds or so...This is using the original schematic, and charging the cap with a 9volt battery.

**edit**
Would it help save charge if I could get it to have a longer period in between flashing, or do the transistors eat up a lot of power while waiting?

Right now it is flashing a shade faster than once a second. Also, what would I change to make the flashes themselves shorter?

 

[Nigel Goodwin]

This is a astable circuit, which means it has two semi-stable states, with either one or the other transistor been turned ON, and the other being turned OFF. When the LED is lit, then the current is basically that off the current through the LED (set by it's series resistor), when it's OFF, then the other transistor is turned ON, and the current is set by it's collector resistor. So to reduce the current you can increase the value of that collector resistor - but it will have some effect on the timing, and if increased too far will stop working all together.

You can alter the ON/OFF ration by changing the capacitor values, or the base resistor values - probably simplest for you to alter the capacitor, unequal values will give an unequal flashing ratio.

Try different values and see what happens! - it's a good learning experience!.

 

[joecool85]

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.

 

[Dr.EM]

I presume a 1F cap would ideally give 100 times the life of the 10,000uf cap. If that was 10 seconds, you might expect 1000 seconds, around 16 minutes. But then you'd have to use a lower voltage, so it might well be less than that.