July/August 2018 43
checker shown at the top of Figure 1. It consists
of just a lithium-ion coin cell like a CR2032, a
blue LED from my junk box, and some jumper
wires. It’s much easier to use than a multimeter.
The schematic is shown in Figure 2. The
circuit can be built on a small piece of perfboard.
A strip board layout is shown in Figure 3.
A quick note on the coin-cell holder. I used a
vertical holder because I had one. However,
horizontal mounted holders may be
more common, although they take
up more space.
I hope you find this test jig
Connecting an LED
Directly to a Li-ion
At first glance, you may
wonder why the LED doesn’t burn
out when connected directly
across a 3V coin cell battery
without the usual series current-limiting resistor. The main reason
is that coin cell batteries like the
CR2032 already have a built-in
internal series resistance (IR). This
IR may range from 10 to 50 ohms
or higher as the cell discharges.
Also note that blue or white
LEDs are a good match to the
battery, with Vfs of approximately
2.9V at 10 mA. If you use a red,
green, or amber LED whose Vfs
are lower (around 1.8 to 2.0 volts),
it would be good practice to add
an external series resistor of 68
■ FIGURE 3.
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Solid, spring-action clip with screw. 5mm
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The little breakout boards are
available from suppliers such as
SparkFun. SparkFun part numbers
are as follows:
• Type A USB: BOB-12700
• Micro-B USB: BOB-12035
• Mini-B USB: BOB-09966
• Jumpers F/F: PRT-11710 or