Battery powered toy design - Page 1
Forum section: electronics / projects

author: steaky1212
author_rank: Regular Contributor
timestamp: December 10, 2011, 10:15:54 pm
content: 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?

author: IanB
author_rank: Super Contributor
timestamp: December 10, 2011, 10:54:13 pm
content: 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?

author: steaky1212
author_rank: Regular Contributor
timestamp: December 11, 2011, 11:09:13 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2011, 04:36:56 pm
content: Quote from: steaky1212 on December 10, 2011, 10:15:54 pmWhat'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.

author: steaky1212
author_rank: Regular Contributor
timestamp: December 11, 2011, 05:17:45 pm
content: 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.

author: Unknown
author_rank: Unknown
timestamp: December 11, 2011, 05:18:06 pm
content: Quote from: steaky1212 on December 11, 2011, 11:09:13 amThere are some LDO's with MOSFET pass elements that only have a few hundred millivolt of voltage drop, these tend to be limited to 5.5/6V max, though.Quote from: Hero999 on December 11, 2011, 04:36:56 pmProbably 74HC595 (or a '595 from some other logic family), a shift register.If the components will all work over that voltage range why bother with a voltage regulator?Quote from: Hero999 on December 11, 2011, 04:36:56 pmI agree, having the voltage slowly drop down to say 4V doesn't seem like a big deal here. An undervoltage lockout might be nice so you don't deep cycle the batteries. A diode in series will also dissipate power, especially at the lower output voltages. Would a shunt regulator (eg. 5.6V zener) be better? Leakage current should be fairly low once the voltage drops below the threshold voltage.

author: IanB
author_rank: Super Contributor
timestamp: December 11, 2011, 05:32:51 pm
content: NiMH may be 1.4 V fresh off the charger, and 4 x 1.4 = 5.6 V. Since it's possible that might be too much for a voltage sensitive device like a micro, I suggested an LDO just to protect it from accidental over voltage events (such as putting in fresh alkaline cells instead of NiMH.When fully discharged the voltage will drop to 4 x 1.2 = 4.8 V. At this point the 5 V regulator will drop out, but the micro should still see enough voltage to operate.Thinking that the regulator is "wasting power" is the wrong way to approach the subject. It is not wasting power, it is protecting the electronics. At the low power consumption of a micro the residual power dissipation is minute.

author: Unknown
author_rank: Unknown
timestamp: December 11, 2011, 05:45:03 pm
content: The LEDs may also need protection, since they're powered from a shift register which I believe is 5.5 V max. Not sure about the LCD. A linear regulator is wasting power to drop the voltage, that it also prevents the electronics from dying doesn't change this. An LDO with a bipolar pass element (eg. LT1086) might drop about 1 V, or about 20%. If the circuit will run down to 4 V, it will run down the cells to 1.2 V per cell, which sucks. An LDO with a MOSFET pass element (eg. MCP17/18xx) would do much better (at least the minimum cell voltage), but you do have to pay attention to the maximum input voltage, some are limited to 6 V, but I believe some go up to 10 V.

author: steaky1212
author_rank: Regular Contributor
timestamp: December 11, 2011, 06:17:39 pm
content: So how about I knock the AA cells on the head and power it from a 9V PP3 instead - and run a 5v regulator.Or use 4xAA's and use a 3.3V regulator. I've not found a cheap 5v regulator that will work for me.

author: Unknown
author_rank: Unknown
timestamp: December 11, 2011, 06:45:31 pm
content: MCP1802 is <$1 in onesies. 300 mA, ~100 mV dropout voltage, 10V max. input voltage. I'm sure there are plenty of others, your parametric search works as well as mine.I don't see the point of using a 9V battery, you might as well use only half of the capacity of the AA's if you don't care about battery life. Plus I'd definitely use a buck converter when powering a 5V circuit from 9V.

author: steaky1212
author_rank: Regular Contributor
timestamp: December 11, 2011, 07:05:25 pm
content: Quote from: alm on December 11, 2011, 06:45:31 pmIm in thet UK and was searching through RS - of which that part has MOQ of 5 - gives £2.00. Saying it outload makes me sound cheap.I picked RS as they've got a trade counter not too far (again cheap as didnt wanna pack postage). I guess I'l hit farnell (element14) instead

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2011, 10:19:56 pm
content: Quote from: steaky1212 on December 11, 2011, 05:17:45 pmHave you actually tested it?Do you have the datasheet? You'll probably find it works down to 4V and can tolerate 7V.

author: steaky1212
author_rank: Regular Contributor
timestamp: December 12, 2011, 10:12:05 am
content: LCD ishttp://www.powertipusa.com/pdf/pc1602d.pdfAssuming I use the MCP1802 what happens when the input voltage drops below the minimum Vin - obviously it doesnt regulate to 5v anymore, but does it just pass Vin through.

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2011, 05:16:06 pm
content: You could of course power the LCD directly from the battery.

author: steaky1212
author_rank: Regular Contributor
timestamp: December 13, 2011, 07:28:27 pm
content: Im expecting an MCP1802 to arrive sometime soon, and I an using the MAX16054 to control it. Basically it allows me to use a momentary switch to enable and disable the output of the MCP1802. Plus it has a "clear" pin that can be controlled via the PIC.I am sure you could prob use some flipflop derivative - but pff.