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I have a lot of LED flashlights and lanterns from China that use a simple IC as their driver to get these different light modes. For instance I have this small keyfob lamp that lights in the following modes when the momentary tactile button is pressed - (High mode -> Low mode -> Blinking Mode -> Off), and yes I have to cycle through all modes to turn it off. Additionally long pressing the button will make it go into an SOS flasher mode.

I opened up the lamp and took a close look and found there only a total of 4 components, the LED, a momentary switch, a 3R0 smd resistor and finally a SOT23-6 type IC with “2819” written on it. I googled for that IC and found it’s FM2819 by “Shenzhen Fuman Elec” and found it’s datasheet too. The circuit used in the lamp is exactly the same as the one shown in the datasheet.

What I want to know is that is there any way of modifying the circuit/wiring to get rid of all these light modes? since all I want is just plain ‘ON’ and ‘OFF’.

  • SW in the circuit is for switch.

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Your S1 is almost certainly not an closing switch,

schematic

simulate this circuit – Schematic created using CircuitLab

but a momentary pushbutton,

schematic

simulate this circuit

considering it's used to cycle through modes.

This sounds very much like

Throw away existing circuit, replace S1 by toggling button, be done.

Alternatively, you could also

Throw away existing circuit, replace with circuit that toggles output based on push button.

But that'll require a couple more components (two transistors for toggling, plus a bit of capacitance for de-ringing the pushbutton) or a small microcontroller (e.g. an Attiny8).

Modification of the existing circuit is not really an option – it's almost certain that the IC is single-purpose; I can't read the datasheet in Chinese, but seeing that it's only 2 pages in length, it's certainly not a programmable controller.

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  • \$\begingroup\$ The circuit board is really small, so is the enclosure. If I have to use a toggle/latching switch then it must the same size of that momentary button, which is 5mm*5mm. I don’t think latching switches come in that size. \$\endgroup\$ – Kokachi Mar 15 at 19:34
  • \$\begingroup\$ You are right about the button diagram. Will 2 additional transistors and a capacitor replace this circuit? \$\endgroup\$ – Kokachi Mar 15 at 19:40
  • \$\begingroup\$ ... together with at least two resistors, yes, you can make a latch from two transistors. probably larger than an Attiny8. \$\endgroup\$ – Marcus Müller Mar 15 at 19:43
  • \$\begingroup\$ The IC provides more than just the light mode, from an translation it does current limiting, pwm, and other functions, based on R. Replacing it with a single switch would likely poof the led. \$\endgroup\$ – Passerby Mar 15 at 19:58
  • \$\begingroup\$ @Passerby won’t the smd resistor do the trick for current limiting? \$\endgroup\$ – Kokachi Mar 15 at 20:04

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