Elenco Basic Electronic Experiments - Manual (Page 36)

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EXPERIMENT #23: 1.5 VOLT BATTERY TESTER
Make sure you have a strong 9V battery for this experiment. Connect the circuit, and connect the battery last since this
will turn on the circuit. And be sure to disconnect the battery 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 22, 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 hold it between the loose wires (the base of
the right transistor and ground, be sure to connect to the (+) and (-) battery terminals as shown).
If the right LED is bright and the center LED 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 (the left transistor 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 the center transistor. (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 the right transistor, in the same
manner as Experiment 21. A strong 1.5V battery will easily exceed 1.1V and only the right transistor and LED will be on,
while the center transistor and left LED will be shut off. But if the 1.5V battery is weak then the base voltages will be nearly
equal and NPN-center and LED-left will also be on. Diodes are often used to make voltage references like this in electronic
circuits.
Remember to disconnect the battery when you’re not using the circuit to avoid draining the battery.
(left unconnected for
future experiments)
+9V
470
Ω
33k Ω
to (+) side of battery
to (-) side of battery