Archive for April, 2011

Improving battery lifetime of Le Dominoux

April 26, 2011

Some time ago I saw on Hack a Day this post about Le Dominoux, an electronic based version of dominoes. At first it didn’t seem much fun, but watching the video I quickly changed my opinion.

In the end, the author also measures the power consumption of the device when the led is not lit, showing it’s around 100uA, and it’s from there that I thought this little device might be improved a bit.

Now, since this device was designed for a 555 contest, the author had to use a 555 IC, and given this constraint I think he has done a good job in optimizing it for low power, using a CMOS variant of the 555 and high brightness leds.

But set aside that constraint, it would be easy to optimize it event further. First of all let’s check the battery lifetime of the original device. Since the power cosumption when the led is lit (probably 5 to 10mA) differs greatly from the standby value of 100uA, battery lifetime largely depends on how much you play with it, but an upper bound is easy to calculate, and it’s 200mAh (CR2032 battery capacity) / 100uA = 2000h / 24 = 83 days. This isn’t very good since even if you almost never play with it, after ~80 days the battery is dead.

One possibility to improve it is to use standard 40xx CMOS logic gates to design the one shot timer, like this:

The fun of CMOS gates is that other than being very versatile, they draw next to zero current when they’re not switching. I built this circuit to measure the power consumption, ant that’s the result:

As you can see, it is drawing just 4uA. The actual standby current depends on how much light is shining on the phototransistor. For example in the dark it draws just 1uA, while if a bright light is pointed at the phototransistor it draws up to 300uA, which is 3V(battery voltage) /10K(R3). This happens also in the original author’s schematic, since the input stage with the phototransistor and the 10K resistor is the same, so that’s no worse than the original circuit. Anyway, in normal ambient light conditions very few light reaches the phototransistor, and the current is very low, around 5uA.

This gives an upper bound to the battery lifetime of 200mAh / 5uA = 40000h / 24 / 365 = 4.5 years!

Last notes about this circuit:

  • it uses the same number of components as the original one, just replaces the 555 with a 4011.
  • you have to connect pin 7 of the 4011 to ground, and pin 14 to Vcc, even if in the schematic these connections are omitted.

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