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Tiny low power LED clock

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Oh no, not another clock...

My first standalone MSP430 project and also my first project to
be programmed "in circuit", and a soldering exercise for the TSSOP housing

A tiny LED clock. It is minimalistic by all means:
-a TSSOP G2452, no other IC
-only one button to set the time
-a dumb 4 digit white LED 7 segment display, https://www.sparkfun.com/products/11409
-a PCB size of 25mm x 25mm
-extremely low power

Wanted to find out whether I can use the TSSOP with my usual home made setup,
that is Eagle, Deskjet printer, photosensitive base material, UV-lamp, develop,
the ugly FeCl3 etching, and then hand-solder.
The PCB is single sided of course.

I use the lauchpad for programming.

The white LEDs of the display are pretty bright at their nominal current.
I wanted something low power and just enough brightness to see the time in the
middle of the night with darkness-adapted eyes.
So at 4:30 in the morning, the display is much more useful than an LCD.

The whole thing draws around 0.3- 0.5mA, LEDs included.
CPU runs at 1Mhz, but is in LPM3 most of the time, even though the poor thing has
a restless sleep: it has to wake up 512 times a second, to do the multiplexing.

At this low current there are quite a few options for a power supply, I considered

- 2 AA cells; would run around half a year
- something solar, with supercap
- I ended up with direct AC supply via a capacitive dropper, 4700pF mains rated film
cap. Where I live, plugs are polarized, so I know which one if life and which one
is neutral. Still I think that this would not be street-legal without double insulation

Overall I calculated 2milliwatt total power consumption, probably the lowest of
any mains powered clock.

If anybody is interested, I might bring schematic and software into
human-readable form.



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Neat!  I've vaguely read about AC capacitive coupled power sources but never tried it myself.  What's the math you use to figure out the capacitance for that?  Any other details on the power circuit?

(I could totally dig making some small embedded trinkets for around the house if I had a reliable AC capacitive coupled circuit put together...)

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one good read is from microchip:



my circuit is slightly different; i use full bridge rectifier, so I get double the current.


R10 limits inrush current, I chose 4700 Ohm

C3  4700pF, 1000V,

rectifier: can be a low voltage type, 4x 1N4148 do the job here

C4,C5 10uF, 16V ceramic

R11  1k

D2   3.9V Zener (don't wanna fry the MSP430)


As long as R10 is on the "hot" side, right side is low voltage.


Current is defined by C3, line voltage, and frequency: the cap gets charged

from - peak voltage to + peak and again discharged from + to -  at mains

frequency, therefore:


Idc = f * C3 * 2 U mains peak 


for Europe and 4700pF I get


50Hz* 2 * 4700pF * 325V * 2 = 0.3mA


This is the linear current, the effective is slightly higher:


I eff  = U * 2 * Pi * f * C


230V * 2 * Pi * 50Hz * 4700pF= 0.34mA.





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Heh actually TI just had some blog post including two "reference designs" based on that concept- http://e2e.ti.com/blogs_/b/powerhouse/archive/2014/06/11/powerlab-notes-powering-the-iot-in-our-homes.aspx?DCMP=powerlab&HQS=pwr-powerlab-fb-pwrhouse-20140611-en&hootPostID=48a1b82e4945e738c76d9c40733f2173


The last two "PMP9310" and "PMP9311" cover the general concept.  I'll take a look at the stuff you posted though!

edit: The designs you posted look simpler, probably cheaper too.

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Neat! I love small clocks :D


Very unique power option. Do you keep the mains stuff away from the clock? I'd imagine if it was also tucked behind then changing the time would be a dangerous afair.


Does it only display the time?


I have some small LED displays like this, but I've left them because I wasn't sure how to power them.

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@greeeg: R10 and C3 have built into the mains plug, they cannot be touched. Our plugs are polarized, so I know (and double checked)
where phase (life) is.

So I can only touch neutral.This does not hurt;-)  However, IEC-wise this is still illegal, so to make this a product, the whole

thing would need proper insulation, including the button.

And I use a capacitor I can trust, a shorted cap would be a baaad idea. I use one of those, 1000V version:


And last but not least, we do have an RCBO.


Yes, it only shows the time as of now: but.. mmmhhh, don't I have a  temperature sensor in the processor?

Let me change it to a temperature- time display.  And seven segment displays can easily show

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  • 4 weeks later...

I just love clocks (especially cold-cathode and similar...)... Nice project...


And I use a capacitor I can trust, a shorted cap would be a baaad idea. I use one of those, 1000V version:


And last but not least, we do have an RCBO.


That's a good capacitor but not rated for this application - you should really be using an X1 or X2-rated cap for safety... not just for shock, but fire hazard etc. "X" capacitors are flame retardant and mandatory in Europe - don't know about US but both the Microchip App note and TI blog above also specify an X2-rated cap (as they should too!).


A good one here would be VISHAY - MKP3382 X2, 4n7 @ 630VAC, p/n BFC233820472, Farnell p/n 121-5463


Also, you should probably have a MOV across the mains on the hot side of the cap - the cap will do its work with a nice 50Hz (and 60Hz) sinusoidal waveform, but a mains-borne transient from, say, a motor (fridge/washingmachine/nearby lightning etc.), has far higher dV/dT and will cause a DC spike, quite possibly in the 100+V region. A MOV won't be 100% effective in all cases, but its a good move.


Have you measured or 'scoped the final DC voltage - is the zener doing the clipping or is the voltage below 3.9V? The two smoothing caps will also sink current - typically between 50 and 100uA each - with such a low delivery current to the uP & LEDs, these parasitics become very significant and can drop the final DC value significantly - maybe consider upping the 4n7 to 5n6 or 6n8?


You can also lose one diode (D2) by replacing the lower two diodes in the bridge by 4.2V zeners and changing R11 to about 470R. With C3 at 22n you'll get a nice steady 3.6V with about 20mV ripple...

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you are 100% correct with the cap. I would need an X (or even Y, can't remember which one is which), if I would even dream

about making it a "product" and try to certify it. The same applies for the MOV.


During normal working, the Zener does not conduct, it is only there to to limit the voltage, otherwise you could kill the poor

controller by software: turn the outputs off, set it to sleep and the voltage would go up until the poor thing (probably) dies...

The smoothing caps are MLCCs, they have all kinds of strange effects (capacitance depends on voltage...), but I guess

leakage is lower than with electrolytics.

Doing a rectifier bridge with zeners is also cool. However, I used a tiny full bridge (4x1N4148) in a four pin sot23ish housing, I do not

think they sell full bridge zeners at digikey.




mmh, might think about this, later. The solar cell is not the problem, the problem is the supercap.

One cell is not enough voltage-wise. Two cells need balancing resistors... hurts !

For one hour, one volt, half a milliamp I need  3600sec x 0.5mA =1.8F, so around 25F, 5V would work for one night. So two cells 50F each.

Not easy, but not impossible either.

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you are 100% correct with the cap. I would need an X (or even Y, can't remember which one is which), if I would even dream

about making it a "product" and try to certify it. The same applies for the MOV.


To be honest, I wasn't thinking so much about productisation, more about your safety. Even for personal use, I'd never use anything other than an X2 (you don't need X1 or Y-class) in this situation - you have a genuine fire hazard otherwise  :smile:


An X2 cap and a MOV are just a few cents, and could save you a world of pain !


Even better, use a Fairchild FSAR001B http://www.fairchildsemi.com/ds/FS/FSAR001B.pdf - cheap, small, no worries about capacitor types and does the job properly!

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