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- Joined
- Nov 28, 2011
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With a 3V supply your options are a bit limited.
There was a device called the LM3909 but it only flashes a single LED.
A monostable multivibrator using two transistors will work at 3V. Put an LED in series with a current limiting resistor across each collector resistor.
Personally I would probably use a CMOS hex Schmitt inverter IC (CD40106, CD4584, or 74C14) with one gate wired as an oscillator and the rest "paralleled" as buffers, driving LEDs in the same way as the 555 circuit provided by markm in this thread.
Please feel free to respond if you want further details.
There was a device called the LM3909 but it only flashes a single LED.
A monostable multivibrator using two transistors will work at 3V. Put an LED in series with a current limiting resistor across each collector resistor.
Personally I would probably use a CMOS hex Schmitt inverter IC (CD40106, CD4584, or 74C14) with one gate wired as an oscillator and the rest "paralleled" as buffers, driving LEDs in the same way as the 555 circuit provided by markm in this thread.
Please feel free to respond if you want further details.
The LM3909 allows you to flash LEDs with supply voltages below the LED voltage.
It was designed back in the days that you could get any colour LED as long as it was red, so circuits typically show it running from a single 1.5V cell, and continuing to operate down to 0.8 volts or thereabouts (from memory).
Using a LM3909, you could conceivably flash a white LED from 3V -- although I've not tried (I doubt I have an LM3909 to try it).
For LEDs with a forward voltage drop well under 3V, it is trivial to flash them using CMOS logic. There are several families which can run from low voltages (often they have LV in the part number) and are thus fully specced to operate from low voltage ranges. However even standard 4000 series CMOS is specced down to 3V and will probably operate in an acceptable manner for something like this at significantly lower voltages.
There are circuits using a single CMOS Schmitt trigger that will flash LEDs, and these are probably the simplest.
The available output drive from logic falls with supply voltage, and this may be an issue. There are several "standard" solutions to this problem. KrisBlueNZ gave you one (and in this case probably the one I would pick).
If you want a discrete solution, the astable multivibrator (google that) is the obvious choice. I think that is what Kris meant. I would replace the collector resistor with a LED and a resistor, but there are also reasons for doing it Kris' way with a parallel resistor.
It was designed back in the days that you could get any colour LED as long as it was red, so circuits typically show it running from a single 1.5V cell, and continuing to operate down to 0.8 volts or thereabouts (from memory).
Using a LM3909, you could conceivably flash a white LED from 3V -- although I've not tried (I doubt I have an LM3909 to try it).
For LEDs with a forward voltage drop well under 3V, it is trivial to flash them using CMOS logic. There are several families which can run from low voltages (often they have LV in the part number) and are thus fully specced to operate from low voltage ranges. However even standard 4000 series CMOS is specced down to 3V and will probably operate in an acceptable manner for something like this at significantly lower voltages.
There are circuits using a single CMOS Schmitt trigger that will flash LEDs, and these are probably the simplest.
The available output drive from logic falls with supply voltage, and this may be an issue. There are several "standard" solutions to this problem. KrisBlueNZ gave you one (and in this case probably the one I would pick).
If you want a discrete solution, the astable multivibrator (google that) is the obvious choice. I think that is what Kris meant. I would replace the collector resistor with a LED and a resistor, but there are also reasons for doing it Kris' way with a parallel resistor.
- Joined
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D'oh! Yes I meant astable.
The reason I suggested leaving the collector resistors in, and adding LEDs with resistors in series with them across the collector resistors, is that without a resistor directly from collector to VCC the collectors will not pull fully high when the transistor turns off, because of the LED forward voltage. This will be more of a problem at lower supply voltages.
To the original poster, a few more questions:
1. What colour are the LEDs?
2. Do you want them to flash together or alternately? We have assumed alternately.
3. Do you want them to remain steadily illuminated for their whole ON-time or do you want a bright blink at the start of each ON-time?
4. What constraints? Cost? Part availability? Current consumption? What kind of battery are you using? How are you going to build it? Stripboard? What quantity?
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I want the circuit of the 3V 2 LED flash circuit
At 3Vcc your really pushing the limits because of the voltage drop of LEDs. This is the best I can give you but you must use common low power red LEDs. I've included pertinent notes on the schematic.
