Simple Battery Monitor Circuit Required

Discussion in 'Electronic Basics' started by Nigel Heather, Jan 19, 2005.

1. Nigel HeatherGuest

I'm trying to think up a very simple circuit which will monitor battery
level. Requirements are

Vbatt > Threshold --- LED is off
Vbatt < Threshold --- LED is on
Vbatt = Threshold --- Not fussed LED could be on or off
Accuracy of t=Threshold is not too important

Here's the catch. I'm looking for a very low component count - I'm think a
transitor, zener, LED and one or two resistors.

I can think of circuits using comparators, ICs, multi-stage transistors but
I feel there is a clever circuit that I'm missing.

Clever ideas would be most welcome.

Cheers

Nigel

2. Nigel HeatherGuest

Sorry didn't think it mattered that much and I didn't want to constrain the
creative juices - around the 6 - 6.4V mark. Nominal around 7.4V.
Undepleted O/C voltage around 8.2V. Can you guess what it is?

Cheers

Nigel

3. John PopelishGuest

The circuit depends very much on the specific threshold voltage you
have in mind but haven't specified.

4. John PopelishGuest

Not interested in guessing. But to put my original point into
perspective, how would the general concept you have in mind vary if
the battery was a 1.2 volt cell or a 300 volt UPS battery?

Now, the next question: How concerned are you with power consumption,
both when the LED is on and when it is off?

6. smpowellGuest

I suggest looking at a 1381 Voltage Trigger.

7. Clarence_AGuest

TOP POSTER PLONKED!

8. Robert MonsenGuest

Not so clever, but here is a circuit that may work:
(view in courier font, or other monospaced font)

Vin ---------o----------------.
| |
Q1 |------. Q2 |------|___|-----.
/| | /| 100k |
| | | |
| | | |
| '-------o |
LED V | |
- .-. 6.2V -
| 10k | | Zener ^
.------o | | |
| | '-' |
| .-. | |
| 1k | | | |
| | | | |
| '-' | |
| | | |
| |-+ 2N5460 | |
'---<| | |
|-+ P-JFET | |
| | |
GND ---------o----------------o------------------'

(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

If you omit the JFET, the LED will get brighter as the voltage increases
until it turns off. With the JFET, the brightness stays fairly constant
for a few volts until it turns off. The JFET forms a simple constant
current sink.

That's an N-JFET in the picture. You can also use a P-JFET; just put the
resistor below it, and connect the gate to below the resistor. You may
have to experiment with the value of the resistor, depending on the JFET.

If you wanted to get bold, you could use the beta of Q1 to limit the
current through the LED, and omit the JFET and collector resistor of Q1
altogether. Just increase the 10k resistor to 220k. Again, it'll
brighten as the voltage comes up, until it turns off.

For a battery monitor, however, you may want to consider a flashing
circuit, since this one will drain off 2mA at right before the LED turns
off, and thus decrease battery life. A little flashing 555 circuit,
along with a uPower comparator and voltage reference, might be a better
option; it would be more noticable, and probably consume far less power.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

9. jsmithGuest

Geese, what a lot of irrelevant questions!!!