[CHP] Blinking LED Resistor/capacitor question

OK, so I am going to head to Debco at lunch to see if I can pick up these items locally. Here’s my shopping list so far per set of eyes:

I already own or have ordered:
555 Timer
2 LEDs
multipurpose PC board to cut into squares and hold everything together
9v battery connector
ping pong balls for the eyes

I need to get:
10uF cap
150K resistor
440K resistor

I also need to get a resistor between the the 9v battery and two LEDs. The white LEDs that I already have are from bestshop2008hk and can be found here:


While I sat in on James’ electronics class and can remember BBROYGBVGW, I failed at remembering the formula to compute resistance. (thought I would just find an android app to assist, but failed at that!) What type of resistor should I get to put in front of two LEDs in serial to prevent frying them from a 9v source?

Thanks again to everyone for your help!



To plug DebCo again, they sell 100-packs of LEDs for maybe $5 a pack. I think the choice is between green or red. Also - that eBay shop says ‘Estimated delivery in 13-22 business days’… it would be near well November before they arrived! (Or have you already received them?)

I had a new thought about the LED resistor - and that is the voltage drop on the LEDs (~0.7V each, or ~1.4V for two).

The basic formula for calculating the LED resistor is Voltage = Current * Resistance. Solving for R, it’s Resistance = Voltage / Current. Typically, about 20mA is the most current you can safely push through an LED. With a 9V, and taking into account the 0.7V drop on each LED: 7.6V / 20mA = 380 ohm. Also, you’ll want at least 15mA to be satisfied with the brightness ( 7.6V / 15mA = 506 ohm). So, you will want a resistor between 380 and 506 ohm, but the closer to 380, the brighter. I suggested 470 before because it is (perhaps surprisingly) a fairly common resistor value to find. A 470 resistor would give you about 16mA of current which should light two series LED’s decently. I think 390 might also be a commonly found value.

If you need me to, I could stop by DebCo on Monday… they should have the caps and resistors you need in stock for cheap.


OK, I picked up several items from DebCo today - enough to build 4 prototypes. I’ll keep everyone posted!


Have fun! :slight_smile:

A few things.... Ed, I know you already ordered your 555 timers,
however, I may point out another alternative, just so it is possibly
an option in the future. Rather than the 555 timer, you could use a
small microcontroller. The one that comes to mind is the PIC10F200. In
100pcs it is about $.38 from Mouser (less if you want SMD). it has a
built in timer module, and you could also use large software delays.
With this, you would have to run it on 2 AA batteries. Microchip does
sell a microcontroller that will run directly at up to 12v but it
costs over $1.00.

From a battery life standpoint, operating at 9V, the 555 timer uses

approximately 6mA. The added circuitry would possibly add a few mA to
the overall power consumption. The PIC10F200 uses 500uA at 3V. That's .
5mA. Considerably less. If we wanted to spend some time tweaking and
putting the microcontroller to sleep in between instructions, the
current consumption would be a little over .2uA or .0002mA. Given this
would only be while the LED is off. The LED would add its current to
the overall consumption.

The circuit design would be even simpler, involving the
microcontroller, LED and current limiting resistor. I agree that
initially the 555 timer is cheaper, but I believe the caps and added
resistors to the 555 timer circuit may drive your cost up. I know we
have a PICKIT2 programmer at the Hive, and I have a PIC programmer if
it is needed. I could help you program one if needed. I may also point
you to http://www.oksolar.com/led/led_color_chart.htm . This site
gives you approximate LED voltage drops based upon their color. A red
LED has approximately 1.8v drop across the LED. I have a lot of red
superbrights, if you are interested. If you would like to see an
example of the PIC microcontroller, I could dig up an 8 pin PIC and
throw up a prototype. I also have a lot of 555 timers, I may bring
them to the hive because I don't really use them.

Either way you go, have fun. The 555 timer is a great device. I have
just gotten away from using them because a small 8pin micro is only
slightly more expensive, and a lot more capable. I'm not sure about
the Android, but I have a 555 timer calculator app for my iPhone. It
covers most basic circuits and calculates the values needed.

Have fun!
See you Tuesday!


Thanks for the suggestion. I have wanted to do some PIC programming for quite a while and purchased the hardware to program them several months ago, just never found a project to do some development with.

One reason I like using the 555 is because it doesn’t require any programming or downloading of the code - just basic soldering. That being said, once I tackle this basic project with the 555, I’ll look at a version 2 that utilizes a PIC chip.

I won’t be at this Tuesday’s meeting since I will be out of town AGAIN. I can’t wait until things settle down and I can regularly make it to the meetings. I saw so much at Maker Faire and haven’t been able to build anything myself for over a month.



A quick look at cost of my proposed idea:

PIC10F200 - .46 (Quantities of 10)
Resistor - .04 (Debco 25 for $1.00)
LED - .50 (Debco Superbrights 2 for $1.00 Free from me depending on
Batteries - .66 (Walmart.com Rayovac 30 for $10.00)
If you choose to make your own free arrangement for the battery holder
Total cost = $1.66
I figured with as little components there are, we could solder
directly onto the IC
With the battery holder @ mouser
add $.69 to total cost.


You’re my hero. I love reading your posts :smiley:


The only thing to not then with the 555 would be your current limiting
resistor should be based on 1.8v drop on tue led.
Enjoy :slight_smile:

OK, I just had an interesting encounter with TSA on my way to a conference in San Antonio, Texas. Since I have a lot of down time, I brought the components to build a few of these flashing LED kits.

Any “tool” longer than 7" is not allowed on a flight, so they took my soldering iron. The thing that was the red flag that made them scrutinize my bag was the 9v batteries, which are not common in electronics these days but used for other “bad stuff.”

They were very friendly during the whole process and I would rather them be more strict than less… Luckily I was able to keep the electronics and the Ada fruit kits. I left my wire cutters at home knowing they probably wouldn’t make it - never thought of the other items.

Moral of the story is if you’re traveling with electronics, make sure you have them in a bag you’re checking in…


Also, if you are building a bomb, use AA batteries instead of 9v. That screening guideline is utterly arbitrary and stupid.

Ah, yes, the TSA… Good times, good times…