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I switched some electronic candles to be powered by a power adapter instead of batteries but when I power them up I noticed they all now "flicker" (which is a feature) at the same rate probably because they all use the same circuit. With batteries this isn't a problem because you'd turn them on/off at different times but now they all turn on at the exact same time when I plug it in. Is there a cheap way to delay these at different rates, possible with simple passive components?

enter image description here enter image description here

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  • \$\begingroup\$ These candles have a look up table that it cycles through to simulate a random flicker. \$\endgroup\$
    – Makoto
    Commented Apr 10, 2018 at 12:04
  • \$\begingroup\$ Yes you can, given the right circuit. \$\endgroup\$
    – user16324
    Commented Apr 10, 2018 at 12:06
  • \$\begingroup\$ If the flickering is started based on when you power on the circuit, why not just install a switch for each candle instead of having a single power on point? \$\endgroup\$
    – SolveEtCoagula07
    Commented Apr 10, 2018 at 12:16
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    \$\begingroup\$ I have 15 candles that I want to power on a timer so manual switching is not an option. \$\endgroup\$
    – Ryan Detzel
    Commented Apr 10, 2018 at 12:49
  • \$\begingroup\$ Have you tried to change a bit the capacitors ? I see two of them, maybe one is related with the system clock? Edited , the ceramic smd is not , it's on the supply voltage line. \$\endgroup\$
    – Dorian
    Commented Apr 10, 2018 at 15:41

4 Answers 4

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This is impossible to do well with only passive components. It would require that your candles switch on/off at a precise voltage threshold, plus an RC circuit which is able to source any sensible amount of current with a delay of several seconds will have ridiculous component values.

Instead, you could implement one of the NE555 timer delay circuits, which can be easily found online. Or you could design a circuit yourself. You will need:

  • an RC divider to create ramping up voltage
  • a comparator to transform that voltage into a digital signal
  • a voltage divider to generate the threshold value for the comparator
  • an active switch (BJT, MOSFET) which would switch the candle on based on the digital signal
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  • \$\begingroup\$ I'd have to source some TTL 555 right? I think I only have cmos as they're common. \$\endgroup\$
    – Ryan Detzel
    Commented Apr 10, 2018 at 15:01
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Maybe something like that is good? Put higher C values for higher delays. You will have ~1s delay for each 50uF added. Take a ~1V voltage drop on Q into account. Use the link "simulate this circuit" then in "Run" menu "Time domain simulation" to see the results. Use lower resistors and higher capacitors if your candles ar drawing more than 10mA.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Using a darlington pair would work even better, since you'd be able to increase R (and reduce C) for the same delay and load current values. \$\endgroup\$
    – Dmitry Grigoryev
    Commented Apr 11, 2018 at 13:43
  • \$\begingroup\$ Yes if the voltage drop is not an issue. He can also make combinations of R and C values for various time constants not being limited by the Q1 base current. \$\endgroup\$
    – Dorian
    Commented Apr 11, 2018 at 14:01
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The easiest way me be directly to change the capacitor value on the candle, which will change the behaviour of each candle. If not, a RLC circuit may solve your issue!

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    \$\begingroup\$ You have the schematic for these candles, do you? \$\endgroup\$
    – Finbarr
    Commented Apr 10, 2018 at 12:19
  • \$\begingroup\$ "The capacitor". Which are you referring to? \$\endgroup\$
    – Bort
    Commented Apr 10, 2018 at 12:21
  • \$\begingroup\$ Of course it depends of the candle and of the schematic, but I've a similar device (a snowman with a light for xmax) with a small IC and only 4 other components (2 resistors and 2 capacitors). Since in my case this is a crap pcb , it's very easy to put another capacitor in parallel to observe behaviour. \$\endgroup\$
    – Golgot
    Commented Apr 10, 2018 at 12:32
  • \$\begingroup\$ @golgot These candles don't typically have an accessible circuit. Usually just a blob circuit. Or more recently the circuit is embedded inside the LED \$\endgroup\$
    – Makoto
    Commented Apr 10, 2018 at 12:50
  • \$\begingroup\$ I posted pictures, so there are some smd caps on that back of the board. \$\endgroup\$
    – Ryan Detzel
    Commented Apr 10, 2018 at 12:55
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You can use a simple RC timing circuit for each candle and you can change the value of each capacitor to get the desired delay

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  • \$\begingroup\$ How big of a delay would this give though? \$\endgroup\$
    – Ryan Detzel
    Commented Apr 10, 2018 at 12:49
  • \$\begingroup\$ You can use this equation T = 5*RC T:Is the time required for the capacitor to charge to the supply voltage(more precisely about 99.99% of the supply voltage) \$\endgroup\$
    – Mohammed Hisham
    Commented Apr 10, 2018 at 13:42

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