Battery level indication : a circuit

[kinabaloo]

Here's a circuit to indicate battery level. Haven't made it but so simple that it can be designed well just on paper.

I thought of using a quad op-amp that comes in 1 x 14 pin chip (get a socket too and solder that) to give a four LED level indication; one could use green, amber and red LEDs.

An op-amp is a comparator circuit. It will compare actual voltage to a known value(s). To get a fixed reference we use a zener diode. This needs to be less than the lowest the battery will go (about 3v - to 3.5v) so let's say a 2.7v zener.

Now, if a 3.7v lithium is freshly charged it will be abut 4.2v. So the first LED could indicate a value of say 4v or greater. How to do that if our reference is only 2.7v? By dividing the voltage with 2 series resistors (or more than two to get a sequence of fractions; we do the same with the reference voltage). Put an LED on each op-amp output and connect the power lines to the op-amp chip.

A simpler single op-amp circuit could be made instead that will detect when the voltage is nearing needing a recharge.

To have the LED come on when power drops below rather than the reverse, swap the + and - on the op-amp.

Very simple! The trick is to realise tha one can work on fractions of the supply voltage.

BTW, resistors these days are all 1% as standard.

[kinabaloo]

ps: Across the four black resistors next to op-amps is 2.7v. Use these to give a suitable scaling, such as 95%, 90%, 85% and 80%; The bottom resistor will be the highest; Use values in the 1k to 47k range.

Similarly the two blue resistors will divide the actual battery voltage so that the ratio is 4/2.7; i.e. the voltage at the divide is 2.7v when the battery actual voltage is 4v; this will light all four LEDs (just).

I'll let you do the math

If worried about power use, you could raise the 82 resistors to 220 or 330 and choose high brightness LEDs; or put the whole thing on a push-to-make button.

pps: I've only drawn one of the 4 outputs; do same on the outputs of the other opamps.

[Laredo7mm]

Here is the one I use:

[kinabaloo]

Laredo - interesting - can get a chip for this now!

But it's basically just an opamp and a zener diode in one

But same principle.

A bit mean with the LED current though !

[kinabaloo]

I'm thinking that a NicoStick box has enough space for an MP3 chip, amplifier and small speaker so am wondering about some suitable sounds effects to accompany each puff ... something the sound of a blowtorch perhaps ...

[Laredo7mm]

Yeah, it is a nice little chip. It came in soic8 format so I bought it. For some reason I really like that chip size. In my PV Pak, I had it turn on at 3.1V with a 3.1V forward voltage LED, so I don't even run a current limiting resitor.

[kinabaloo]

Yeah, it is a nice little chip. It came in soic8 format so I bought it. For some reason I really like that chip size. In my PV Pak, I had it turn on at 3.1V with a 3.1V forward voltage LED, so I don't even run a current limiting resitor.

The main problem with ready made chips is not only need to get hold of them but also that one might not be able to achieve quite what one wants sometimes. So i like to build things from the simple building blocks - trannies, diodes, opamps; perhaps splash out on a timer or logic gates

[kinabaloo]

For example: thinking that a recharging circuit could be much the same as the level circuit - suitable current feeds on each opamp o/p to slow recharge as it fills. Could even keep the LEDs to indicate progress.

[solonguniverse]

I'm thinking that a NicoStick box has enough space for an MP3 chip, amplifier and small speaker so am wondering about some suitable sounds effects to accompany each puff ... something the sound of a blowtorch perhaps ...

Lightsaber sound effects!

[ichaya]

OK, for those of us that are NOT electrical engineers!


WHAT???

I would be interested in something that showed my charge progress, and also when it was time to re-charge.

However the previous conversation has me lost.