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Battery powered toy design

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steaky1212:
Hi,

So it's nearly Christmas, and I figured I'd design a "toy" for my kids to play with (Ages 1,3, and 5) .
Currently its got a PIC18, 2x16 LCD display, 16 bi-colour LED's, 16 tactile switches, and a rotary encoder. Ive got some other bits and bobs I might throw in - but I was wondering how to power it.
I have constrained myself to using a dual gang back box as the enclosure with some clear acrylic on the front (with the PCB subflush). I want to introduce them to the idea that electronics isnt magic - but at the same time I dont want to cover ESD practices.
The LED's are driven from 595's with a Vdd 2-6V and the PIC is 1.8-5.5V. Its an old LCD so I doubt the LCD will stay powered at the lower voltages.
My idea is to power using 4 rechargable batteries. Rechargable batteries being 1.2V gives 4.8V which is under my max, and hopefully will stay at a decent level for a longish amount of time.

I've never designed anything battery powered so my question is, do I need to use a buckboost convertor or not bother?

IanB:
Rechargeable batteries are a nice plan--introducing kids to economical energy in a throwaway world is good.

With four batteries like eneloops you would have about 4 x 1.35 V when fresh and 4 x 1.2 V when empty.

Maybe you could just use a low dropout 5 V regulator for the PIC and power everything else that is not voltage critical directly from the batteries?

steaky1212:
Obviously with batteries I dont want to waste any energy - hence the 4x1.2V rechargable (standard NiMH) would give me a workable voltage.
I lot of the LDO regulators for 5V need over 6V input voltage, so unusable. The only option would be to use a 3.3V regulator, but then I'm wasting energy to regulate down.

Zero999:

--- Quote from: steaky1212 on December 10, 2011, 10:15:54 pm ---The LED's are driven from 595's with a Vdd 2-6V and the PIC is 1.8-5.5V. Its an old LCD so I doubt the LCD will stay powered at the lower voltages.
My idea is to power using 4 rechargable batteries. Rechargable batteries being 1.2V gives 4.8V which is under my max, and hopefully will stay at a decent level for a longish amount of time.
--- End quote ---
What's the 595? I've never come across that IC. What's the full part number?

If the components will all work over that voltage range why bother with a voltage regulator?

The only potential problem is NiMH cells have a higher voltage when hot off the charger, I've measured 1.5V or more in some cases. If probably won't cause a problem as most 5V devices can stand short term 7V surges but it's probably a good idea to stick a diode in series so it never exceeds the maximum rating.

steaky1212:
Sorry, 595 is a shift register - full part number is 74HC595PW.
I've used a different shift register to do the same before, but now I am having second thoughts about the design. The datasheet states Vcc or GND current as max 75mA. This works out as ~9mA per LED assuming they are all on at the same time.

I'm not using darlingtons to drive it - and I wanted to avoid it as I'm nearly running out of space in that area.

I'd normally be with you regarding the diode in series as it'd drop 0.7V - which is fine at 4*1.5V - but at 4*1.2V it drops me down to just over 4V.
Assuming my LCD drops off at 4.5V then it would stop when my batts run down to "usuable" levels.

Who'd have thought batteries would have  thrown so many curveballs.

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