2
\$\begingroup\$

I bought a Christmas LED light yesterday but it is static. I don't know much about electronic circuits but I learned something in college on RC circuits. I am wondering if we can build a flasher with one capacitor and resistor and put them in series with the AC socket. I Googled the idea of flashing Christmas lights but all schematics shown online were too complicated for me.

\$\endgroup\$
  • 9
    \$\begingroup\$ If you are not familiar with electronic circuits, I wouldn't start with line-voltage circuits. For your safety. \$\endgroup\$ – Brian Carlton Dec 18 '12 at 23:09
  • \$\begingroup\$ I've seen a variety of devices in places like home-improvement stores which will switch AC120 in response to a safety-isolated "low-voltage" signal. Some turn on the load when current is supplied on the low-voltage side. Some turn on the load when when current is supplied in one direction, and turn it off when current is supplied in reverse. Some output a low AC voltage, turn on the load if it's "shorted" in one direction, and turn off the load if it's "shorted" in the other (designed for control via center-off momentary switch with two diodes). Pre-made isolation is probably a good idea. \$\endgroup\$ – supercat Jan 18 '13 at 17:27
  • \$\begingroup\$ If you are new to all this, heed the warnings, mains voltage is not the thing to learn with. There are a few versions of what you want, from basic LED flasher to a mains-voltage version, in the excellent "Getting Started in Electronics" by Forrest M. Mims III (crazy name, crazy guy!) \$\endgroup\$ – John U Jun 18 '13 at 8:01
1
\$\begingroup\$

Unfortunately, without knowing exactly what type of product you bought, and having a reliable schematic for it, there's not much advice that can be offered.

As @Brian says, since you're just getting started, you should start with something with less energy than mains.

I'd recommend getting a breadboard, LED, battery, and some components according to any number of online searches for "simple LED blink circuit."

Some things I recommend you research:

  • powering LED's, forward voltage, and how much current to supply according to the datasheet
  • current limiting resistors
  • Ohm's law
  • resistor-capacitor timing circuits
  • the venerable 555 timer integrated circuit

If you've gotten to the point where you can blink an LED on a breadboard, you'll be in a better position to ask about making a commercial product modification, but I'd still not recommend doing anything on the mains (AC) side.

Adding or modifying components on the DC side is more approachable and safer, but please seek some help from someone with more experience before tackling anything that uses mains.

\$\endgroup\$
  • \$\begingroup\$ Thanks all. Well, from all comments, seems that I should not try that by myself before I have more knowledge on the circuit. But anyway, here is what I bought biglots.com/p/c/lights/35l-led-dome-lights \$\endgroup\$ – user1285419 Dec 19 '12 at 17:10
1
\$\begingroup\$

As @JYelton states:

Unfortunately, without knowing exactly what type of product you bought, and having a reliable schematic for it, there's not much advice that can be offered.

As @Brian says, since you're just getting started, you should start with something with less energy than mains.

But depending on power rating and the kind of circuitry, this is how I made a flashing lamp when I was a little boy. Put a Fluorescent tube lamp starter (a classic one, not an electronic one) in series with the mains power supply. This works with reasonably low power strings (say < 25W, usually the max. power is printed on the starter) and with a classic string of incandescent light bulbs. I have no idea if it works with these modern LED strings, hence the repeat of JYelton's words of warning.

\$\endgroup\$
  • \$\begingroup\$ +1 because I learnt something new from that, and it's a rather elegant solution. \$\endgroup\$ – John U Jun 18 '13 at 7:57
0
\$\begingroup\$

Trying to harness or control mains power isn't overly difficult to do safely, but the potential consequences of mistakes are sufficiently nasty (death and destruction--literally) that I'd suggest splitting your project into two parts: finding a pre-built device which will use a safety-isolated low-voltage signal to control mains-powered devices, and then building a device to generate a low-voltage signal that can be used to operate it. If you use this approach, and also use a pre-fab power supply to operate your low-voltage device, then even if you mess everything else up, your device would not endanger anyone. You might start a fire if you carelessly leave a soldering iron unattended near flammable materials, but your device wouldn't.

I don't know of any particular "standard" for devices to interface things as I described. There are a number of styles you may encounter.

  1. Device turns on when current flows through a pin, and turns off when it stops.
  2. Device turns on when externally-applied current is driven one way through a pair of pins, and turns off when it it is driven the other way.
  3. Device turns on when current is allowed to flow one way between a pair of pins, and turns off when it it's allowed to flow through the other way.

In some cases, the device will supply a voltage, such that it can be operated directly via simple contact closure (such as a "doorbell" button). In other cases, the device will require that an external voltage be supplied to drive the current (such that the circuitry that drives it need not worry about voltage being fed back through it). Some devices may have enough terminals to permit wiring in either configuration (offering a voltage terminal that can be used if one wants to operate the device with a contact closure, or ignored if one wants to operate it with an external voltage). I'd suggest that you plan on using a small relay or two to switch the low-voltage side--such an approach should allow you to work with any style of power controller.

Once you have figured out what sort of power controller you can use, then the issue becomes one of designing a blinker circuit that can pulse a small relay. Look around and you should have no trouble finding one. I'd expect that the simplest customizable circuits would either have three transistors, two caps, and a handful of resistors, or a one transistor, a 555 timer chip, one cap, and three resistors.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.