Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 36)

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Make sure you have a strong 9V battery for this
experiment. Connect the wires according to the Wiring
Checklist, connecting the wire to the battery last since
this will turn on the circuit. And be sure to disconnect this
battery wire when you’re not using the circuit to avoid
draining the battery. This circuit is a variation of the
differential pair configuration used in Experiment 21, you
will use it to test your 1.5V batteries. Take any 1.5V
battery you have (AAA, AA, A, B, C, or D cells) and
connect it between the base of NPN2 and ground. The
easiest way to do this is to stand your battery (negative
side down) on battery spring 26, connect a wire to spring
18 (the base of NPN2), and hold the wire to the positive
side of your battery.
If LED2 is bright and LED1 is off then your 1.5V battery
is good, otherwise your 1.5V battery is weak and should
be replaced soon. Don’t throw any weak batteries away
without making sure some measure good with this test
because all batteries could fail if your circuit is wired
incorrectly, or if your 9V battery is weak.
This circuit uses two diodes (NPN3 is being used as a
diode) to create a voltage reference. The turn-on voltage
drops for the diodes are combined to produce a constant
voltage of about 1.1V at the base of transistor NPN1.
(We said earlier that a diode turn-on voltage is 0.7V, but
it varies slightly depending on the current. In this
application the drops will be about 0.55V for each). This
is compared to the 1.5V battery voltage at the base of
NPN2, in the same manner as Experiment 21. A strong
1.5V battery will easily exceed 1.1V and only NPN2 and
LED2 will be on, while NPN1 will be shut off. But if the
1.5V battery is weak then the base voltages will be nearly
equal and NPN1 and LED1 will also be on. Diodes are
often used to make voltage references like this in
electronic circuits.
Remember to disconnect the battery wire when you’re
not using the circuit to avoid draining the 9V battery.
EXPERIMENT #22: 1.5 Volt Battery Tester
Wiring Checklist:
o 1-to-3-to-27-to-47
o 26-to-39-to-14
o 10-to-15-to-46
o 11-to-12-to-13
o 2-to-16
o 4-to-19
o 38-to-17-to-20
Schematic