ALeggeUp's Getting To Blinky Build Log


#1

I’ve watched the Getting To Blinky videos before, they helped convince me to give KiCad a try but I never actually made the Blinky board. In the spirit of contributing to the forum and some extra KiCad practice, I’m going to go through the videos again and follow along and log it here.


Design review groups
#2

I decided to try out a different 555 timer circuit for my Blinky, this one has two LEDs and alternates blinking between the two. It came from the same site that Chris uses in the videos:

http://www.555-timer-circuits.com/flashing-railroad-lights.html


#3

I decided I would like a switch on my Blinky so that I wouldn’t have to remove the battery to turn it off, otherwise I just captured the schematic in KiCad as is.


#4

I found a different battery holder that has a smaller footprint to save on board space, the suggested large pad in the middle made it a little more difficult than it needed to be, I’m sure I could have made it smaller but I ended up not needing to.

The 3D view in KiCad is pretty cool

Submitted to OSH Park, ended up with a size of 1.21x0.91 inches (30.63x23.01 mm) which means $5.45USD ($7.68CAD) for 3


#5

I played around with the values of the capacitor and resistors to get around 1 second blinks and get close to 20mA of current so that the LEDs will be as bright as possible.


#6

I selected all the parts for my blinky, I used the Digikey BOM manager (horrible interface) to collect all the parts, I was able to get everything I need except for the battery which I can grab anywhere.

As one off, the BOM cost is $4.61CDN before tax plus $8CDN to ship. I played around with the quantities and was able to get 100 of each resistor value, 25 LEDs and 10 of everything else for less than 5 times that. Total with shipping and tax ends up being $51.35CDN which will allow me to make 10 full blinkys (blinkies) and have some extra parts.


#7

Very cool, those prices aren’t bad at all. With that many sets of parts, perhaps you could set up a soldering tutorial at some point?


#8

Yes, that’s a good idea, I’ll have to look into how I’d do that while I wait for my boards to arrive. My 12yo daughter wants to give it a shot too, she’s already soldered a few kits with through-hole parts, but not SMD. I should probably order some more boards, I already spotted something I would change in Rev B.


#9

I was explaining to my daughter how everything should work on the board and when I was telling her how the switch would work with those 6 pads she pointed out that switching left for ON and right for OFF seemed wrong which I agreed with, so I made a Rev B that switches the pins used on the switch and while I was at it I tried out rounded corners.

On the plus side while making the new edge with the rounded corners I ended up with a slightly smaller board size 1.19x0.90 inches (30.12x22.86 mm) which saves me $0.15USD.


#10

Using children to do your error checking…I like it! :slight_smile:

On a serious note, explaining what you’re doing to others can be a great way to catch mistakes or generally help to figure things out. Board looks great!


#11

Good point, luckily only cost me $8 to learn that lesson.

Thanks! I’m pretty happy with the way it turned out, I hope it works.


#12

First batch of boards shipped, can’t wait to try them out!


#13

First batch of GTB’s arrived! Once the kids are in bed it’s solder time!


#14

Pretty happy with my solder job

Popped in the battery and…
… didn’t work.

Turns out I reversed the pins on the battery connector and made the big pad positive even though it clearly had a + on the top of the battery connected to the outside.

Luckily I can just flip the battery over and as long as the sides of the battery don’t touch the holder I can reverse the polarity.

And then…

Success-ish!

One of the LEDs seems brighter than the other and the timing doesn’t seem to be consistent, but I’ll have to think about those later.


#15

Congrats! Blinky achieved! Gotta take that as a win regardless :slight_smile:


#16

Ya, there should be a badge for it… Oh wait, it is it’s own badge! :stuck_out_tongue:


#17

Not all 555 timers are equal. While the datasheet might say:

Exact equivalent in most cases for SE/NE 555/556 or TLC555/556

This is an example that is not one of the “most cases”. Intersil’s CMOS 555 timer cannot be used in some circuits that were designed for the bipolar 555. This is one of those circuits. Intersil had to make some compromises in order to get the 7555 to work at a supply voltage of only 2V. The sink and source capacity is one of them.

The parametric table in the datasheet is not of much use here as you are using a supply voltage of 3V. You need to refer to the charts below and even there they only give curves for 2V, 5V and 15V. But the 2V curve is close enough for your purpose. Looking at the sink and source current curves for Vdd = 2V you can see that you never going to sink or source 10ma for your LEDs. And as you try to sink/source current from the output you can see that the output voltage will increase/decrease. As a result it is complicated to work out exactly how much current you are currently supplying to your LEDs but it is obvious that you are sinking much more current than you are sourcing, hence the difference in brightness.

As a result of this the current available to charge/discharge the capacitor also varies greatly between states which is why you don’t get the expected 50/50 duty cycle.

The correct way to use this IC in this application would be to use the discharge pin to control the charging/discharging of the capacitor for more consistent timing. The output pin should instead be driving transistors to provide the necessary drive current for your LEDs.

Note: The reason people sometimes design 555 circuits using the output pin to charge/discharge the capacitor is to get a 50/50 duty cycle at high frequencies. At low frequencies, such as in your case, the discharge pin can be used to get close enough to a 50/50 duty cycle that it becomes irrelevant.

Hope this helps.


NE555 Adjustable PWM Circuit
#18

That is a huge help, thank you. I was really confused, but that totally makes sense now that you point that out. I am excited to try a Rev C and see the difference.

Thanks @1.21Gigawatts!


#19

This is turning out to be much more complicated than I thought… I decided to give the falstad circuit simulator a shot which was a bit of a learning curve on its own. I have tried several times to get this right, but I’m stuck on getting the timing right, I’m not sure why I’m not getting 50/50 duty cycle in this circuit.


#20

I hinted at this earlier. Since the output pin can both sink and source current but the discharge pin can only sink current we need to add the pull up resistor, we’ll call it Ra, in order to charge the capacitor. You can see now that the capacitor will charge through Ra + Rb but discharge through Rb only. Therefore, unless Ra can be 0 we will never get a symmetric waveform. And since there isn’t an infinite amount of current available at the discharge pin Ra can never be 0. The trick is for Ra to be as small as possible relative to Rb without exceeding the discharge pin’s current capacity. In fact you want to stay well below the current capacity of the discharge pin. This way we can get very close to a symmetric waveform, especially at low frequencies. So in your case I would suggest making Ra about 470R while Rb should be 110K.

Also watch your transistor arrangement. As you have drawn it the NPN transistor will be on when the output is above ~0.7V while the PNP will be on when the output is below ~2.3V. As you can see there is a period of time when both are on which will briefly draw more current from your battery. But you are on the right track.