3v LED timer

Please excuse what may be a very basic question. I’m wanting to make a 20-30 second timer LED that runs on 3v. I’ve found timers using 555 or 547 but they all seem to run on more than 3 volts. When using 3v, they end up very dim and if using high voltage, they heat up and burn out. I’ve found the perfect LED’s that run on 2-1.5v button batteries and I was hoping to use them and 2 AA batteries.

by ShalinG
August 08, 2020

Could you draw your schematic in CircuitLab and add it to your question? How much LED current are you seeking?

The LM555 datasheet suggests a minimum supply voltage of 4.5V and max of 16V, suggesting a range of 5-15V.

If it's working down at 3V, it's certainly out of the "guaranteed" range and it still might work but with issues. One of these issues might be that it can't turn on the output transistors strongly enough. It's a bit hacky to base your design around running the 555 so far out of spec, but this can probably be solved by adding an external output transistor to drive the LED.

by mrobbins
August 08, 2020

I’m a tinkerer/inventor without much EE/circuit experience. I really wish I’d gotten an Engineering degree in the 80’s instead of education/math. So, as you can tell I’m older but educated. I was trying to learn circuitry for this one product and realized, since it’s just one thing, asking here might be easier. I’ve purchased a circuit kit and a few other things trying to figure it out, using YouTube videos. I’ve tried 2 designs, one using a 555 and the other using a 547 but neither did what I wanted. I’ll see if I can use your last idea. I’ll also work on making a schematic here later.

by ShalinG
August 09, 2020

Have you tried to use a Nand Schmitt Gate? TTL levels are compatible with your 3V. Note that you have to use a 74x13, not a 74x00 (the 7400 is NOT Schmitt triggered).

Tie one of the input of the nand date to 3.3V. Tie the output to the led in series with a resistor. Between the output and the led, add a bypass which will be connected to the second input, but passing through a resistor in serial to a capacitor. That's all. Chose the resistor value, R, and the capacitor value, C, in such a way that RC will be proportional to the flashing delay. If that product is too small, the led could flash, but not in a perceptive manner to the human eye. You can increase the delay by increasing either R either C. The "duty cycle" will be approximatively 50% (50% of time on, 50% of time off).

Why it works: If the output of the gate is 1, the capacitor will "slowly" charge up, until 2.0 volt are present to the second input of the nand, The nand gate will thus see its two inputs as being high, so the output will become low, or 0 volt (in fact, not more than 0.4 volt). When the output of the nand is 0 ( below 0.4 volt), the capacitor will discharge until it reaches 0.8 volt for which the input at the nand will be perceived as a logic low value. With one input high and one input low, the nand output becomes high.

A 74x00 won't work since as soon as the second input reaches 2.0 volt, the output becomes 0 and the second input starts to get under 2.0 volt, which bring the input into an invalid voltage (neither logic high, neither logic low).and the 74x00 output ends up by being always up, either always down.

A 74x13 has 4 nand gates, that "oscillator" uses only one of them. The oscillator is not very precise and is highly temperature defendant, but the whole setting requires less hardware than a 555 (and is more economical in energy consumption).

Haven't found a 74x13 model, so I could not supply a working circuit, sorry.

by vanderghast
August 10, 2020

It is even easier with a Not Schmitt Gate (74LS14, as example) since one of the input of the Nand Schmitt was always set to logic high, in my previous reply, we can eliminate it by using a Not instead of a Nand. Less wiring.

See https://www.allaboutcircuits.com/technical-articles/exactly-how-schmitt-trigger-oscillators-work/

by vanderghast
August 10, 2020

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