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I'm quite new to electronics, so hopefully I don't offend anyone by asking a question badly.

I'd like to make a circuit that uses a CR2032 (3v) button cell to pulse an LED (3mm blue clear super bright - forward voltage +-3.3v) for half a second, once per minute. The half-second part is quite important to me (1 second would be far too long), but the minute portion of the time scale is of less relevance (It could pulse once every 55 seconds for example, and that would be fine).

I'd like the final build to be as small as possible (in terms of form factor) and ideally to use analog components (best case) to keep the build cost-effective. At a stretch, I would also be happy to use something like a 555 timer, but as I understand it, its no simple task to have the LED spend more time off than it does on when using 555's, because of how they work.

Given the size, cost and voltage constraints, what direction should I be looking in to get started ?

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  • \$\begingroup\$ You'll need a boost converter to power the LED, and a 555 or a microcontroller to turn it on and off. There are small inductorless boost converters designed specifically for driving small LEDs like this. \$\endgroup\$
    – Nick Johnson
    Jul 17, 2015 at 9:50
  • \$\begingroup\$ Why do you expect using "analog components" to be cheaper, though? \$\endgroup\$
    – Nick Johnson
    Jul 17, 2015 at 9:52
  • \$\begingroup\$ I would look more in the direction of a 4060 (14-bit ripple counter + oscillator) and some gates to do the timing. And a couple of CMOS inverters in parallel with a diode and a capacitor to double the voltage, see creative-science.org.uk/multipliers.html since you only need the higher voltage for a fraction of a second, you can charge the capacitor in the 1 minute and discharge it through the LED when it needs to be on. \$\endgroup\$
    – Bimpelrekkie
    Jul 17, 2015 at 10:13
  • \$\begingroup\$ @IC_designer_Rimpelbekkie That's not going to have great quiescent power consumption, unfortunately. \$\endgroup\$
    – Nick Johnson
    Jul 17, 2015 at 10:36
  • \$\begingroup\$ You say half a second is important, because 1 second is too long. But is there also a too short? If it should just flash a bit quite brightly this can become a lot simpler. The timing can be easily done with just one 555, but I will await your response to my question before attempting an answer. \$\endgroup\$
    – Asmyldof
    Jul 17, 2015 at 10:50

1 Answer 1

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An absolutely minimalistic solution is to drive a microcontroller from the button cell, and use a simple bootstrap circuit to drive the LED:

schematic

simulate this circuit – Schematic created using CircuitLab

You can program the MCU - represented by the dotted rectangle in the above diagram - to idle in power-save mode 55.5 seconds of each minute, and to toggle its output pin using PWM for the remaining 0.5 seconds. The PWM waveform on the base of C1 provides pulses of boosted current through the LED, D2, lighting it up.

If the LED's forward voltage is sufficiently low it glows dimly even when off, you may want to drive the anode of D1 from another IO pin so you can shut it off entirely when not in use.

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  • \$\begingroup\$ That is indeed the LED circuit I was thinking of. \$\endgroup\$
    – Bimpelrekkie
    Jul 17, 2015 at 15:33
  • \$\begingroup\$ Thanks Nick - Looking a little further into it, something like an ATTINY13A and Dickson doubler strategy is going to be my next move. I was surprised to see those chips so cheap online, and have learned a bit in the process. \$\endgroup\$
    – Continuum
    Jul 18, 2015 at 16:05

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