Blinking Nightlight (by request)

Step 3 Blinking several LEDs

At this point, we've got our series resisitance, we can now blink several LEDs at once using our original timer circuit, simply by replacing the single LED and series resistor with our new series resistor and set of 4 parallel LEDs.

Below, you'll see a schematic of what we've got so far. It looks a little different than the circuit on the original link, but it's mostly just appearances. The only real difference between the circuit at http://www.satcure-focus.com/tutor/page11.htm and the one in this step are the resistance value for the current-limiting resistor, and the fact that we now have 4 LEDs in parallel, rather than just a single LED.

I didn't have a resistor of 127 ohms, so I used what I had. Normally we'd prefer to approximate upwards, selecting the next largest resistor value in order to ensure we don't let too much current through, but my next closest resistor was MUCH larger, so I chose a resistor slightly below our calculated value :(

We're making progress, but we still only have a bunch of blinking lights. On the next step, we'll make it turn off in daylight!

Comments

[stuuf]

You seem to have the LEDs in your schematic wrong. I was wondering how they would light up reverse biased like that; the circuit you linked to has the cathode connected to the discharge pin. How much current can the discharge pin sink? I know that the 555 usually can only handle a small amount of current on its output pin, so an external transistor is needed for driving a lot of LEDs, but how many can it drive in this configuration?

[inventorjack]

Thanks again for the correction. It's been fixed.

[skantu]

Mate Jack, I loved your circuit but , and i just have a small doubt, i think it will be easy far you to answer, im using what you done, but im trying to make it blink once a second (like turn lights on a car), you wrote.. “Basically, depending on what external components (resistors and capacitors) we use, we can change the rate of blinking.“ So could you please help me to decide which (capacitor or resistor) to change to make it blink faster? .. i will really appreciate it. Thanks mate.

[inventorjack]

Thanks for the catch on the LEDs. I've got to go to work today, but when I get back home, I'll fix the schematic. Not sure how I managed to screw up the orientation there :( Regarding the source/sink of the TLC555, the nominal parameters for this chip are 10mA Source and 100mA Sink, but it can operate up to 100mA Source. A better choice for the 555 timer would be the SA555, which can both source and sink up to 200mA, allowing a bit more flexibility in our design. I stuck with the TLC555 in the writeup, because that was the one originally used, and it provide enough current for the 4 LEDs, but not much more than that.