In this column, Tim answers questions about
all aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest to
the hobbyist. Feel free to participate with your
questions, comments, or suggestions. Send all
questions and comments to: Q&A@nutsvolts.com.
■ WITH TIM BROWN Q & A
Post comments on this article at
• Battery Monitor Circuit
• NiMHs or NiCADs
• Audio Synthesizer IC With
8 Ohm Speaker
Need Battery Monitor Circuit
QI'm trying to design circuitry that shows a low battery (once the voltage goes below a certain limit, e.g., 3.1V or 3.2V) by flashing an LED (0.1 sec every two to four sec). I'm using an Li-Ion
battery 14500 series and need to monitor the device
before the battery goes dead. I have to say there are many
designs for this purpose, but I'm seeking a design that has
the least impact on power consumption.
ALithium-Ion batteries (a.k.a., Li-Ion) use lithium- ions moving from the negative electrode (cathode) to the positive electrode (anode) to produce a flow of current in an electrical circuit.
To recharge the Li-Ion battery, you reverse the direction of
current flow. Li-Ion batteries do not need the
"conditioning" required by their Nickel-Cadmium (NiCads)
or Nickel-Metal Hydride (NiMH) cousins. However, Li-Ion
batteries are VERY sensitive to operating outside the range
of three to 4. 2 volts (overheating, fire, explosions).
Manufacturer specified chargers have the circuitry to avoid
overcharge, and battery packs have circuitry to avoid
operating below the three volt shutoff voltage. CAUTION:
Always follow manufacturer's recommendations for
charging and operating batteries — laptops, cell phones,
and even a factory have burned down due to Li-Ion
A simple monitor circuit could use an operational
amplifier (op-amp) to compare the Li-Ion battery's voltage
to a zener diode reference voltage to turn on an LED for
low battery voltage. Your requirement for a flashing LED,
however, dictates the use of a voltage detection IC such
as the MN13811-M which has a 3.2V voltage threshold.
Figure 1 shows a circuit schematic diagram for the low
voltage monitor adapted from circuitstoday.com. A
special thanks to Digi-Key for their free online schematic
drawing software, SchemeIt ( www.digikey.com/
NiMH Batteries Won't Replace NiCads
QI have an outdoor solar lantern. The original circuit died, and I have replaced it pretty much exactly as it was. I noted that candles don't actually flicker off and on, but the flame moves.
So, I have two LEDs controlled by a PIC. I have it run for
■ FIGURE 1.
14 December 2014