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The Wikipedia article on 3-way lamps says that the rotary or pull-chain switch mechanically goes through all the states, if I understand correctly. However, I noticed that my floor lamp still cycles through the 3 brightness levels when I flick a switch controlling the outlet instead of pulling the chain. If I toggle it within 13 seconds, that is. If I wait 14 seconds, it always comes on with the brightest setting. How might such a circuit be designed? Would it involve a capacitor that's responsible for the time effect I'm seeing?

mk12
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  • Probably similar to how a touch lamp does it. Don't know the method off the top of my head though but Technology Connections has a video about touch lamps. – DKNguyen Jul 23 '22 at 03:35
  • Most modern lamps have some sort of circuit that provides proper voltage and current to produce the desired light, especially if the lamp is a CFL or LED type. And nowadays many simple appliances such as lamps have microcontrollers and even Bluetooth, WiFi, and internet connectivity. So the circuit's time delay is most likely much more complex than a capacitor. – PStechPaul Jul 23 '22 at 03:48

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Probably an application-specific integrated circuit (ASIC) (but it could be a tiny microcontroller (MCU)) that has enough power supply capacitance that the circuit does not reset within your 13 seconds.

After the power supply drops to a certain point, the power-on reset is triggered at the next application of power (whether it's 14 seconds or 14 days) and the light comes on at the brightest setting. In this application probably there is little difference between 7 seconds and 30 seconds from the user's point of view, so the circuit can be pretty crude.

This would be quite easy to do in general (the difficulty might more lie in making it for pennies and in huge quantity to meet all safety and reliability requirements). There is similar functionality built into ASICs that control inexpensive LED lights- and bike lights. Just a few gates and flip-flops with a tiny bit of analog circuitry.

Many MCUs have a bit of EEPROM that is good for tens of thousands of write cycles so it would also be possible to remember the last setting regardless of the 'off' time if the EEPROM was used.

Spehro Pefhany
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  • Makes sense! I actually find the short time window to be a feature so I'm glad it doesn't use EEPROM. I was annoyed at first thinking I'd have to pull the chain 5 times every time since I rarely use the dimmer two levels. – mk12 Jul 23 '22 at 06:28