Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 35)

-35-
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. The part of the circuit to the left of
the dashed line in the schematic is the voltmeter, the two
resistors on the right produce a voltage that you will
measure. Notice that the variable resistor (VR) will
always act as a 50K
Ω across the battery but by turning its
knob you adjust the voltage at the base of NPN1. By
turning this knob you can make one LED brighter than
the other, indicating that the voltages at the bases of
NPN1 and NPN2 are not equal. Adjust the VR so that the
two LEDs are equally bright. The transistor base
voltages are now equal. To determine what voltage you
have measured, simply subtract the percentage shown
on your VR dial from 100 and multiply by 0.09.
If you like you can calculate what voltage you should
have measured. Your measurement may differ from this
due to the tolerances in the resistors and the VR dial, but
you should be close. The resistors on the right are a
voltage adjuster, just like the VR is, and the voltage you
measured (at the base of NPN2) is:
V
Calculated
= x V
Battery
= x 9V = 6.9V
This circuit is a form of the Differential Pair transistor
configuration, which is widely used in integrated circuits.
If the transistor base voltages are equal then the currents
through the LEDs and collectors will also be equal. If one
base voltage is higher than the other then that transistor
will have more current flowing through it’s collector and
associated LED.
You can now replace the two resistors on the right with a
different combination and make a new voltage
measurement. The table below lists different
combinations of your Electronic Playground resistors that
you can measure, but you don’t have to measure them
all. In some combinations resistors are placed in series
or parallel to create new values.
Remember to disconnect the battery wire when you’re
not using the circuit to avoid draining the battery.
EXPERIMENT #21: The Voltmeter
R
Lower
R
Upper
+ R
Lower
33K Ω
10K Ω + 33KΩ
Wiring Checklist:
Schematic
Upper
Resistor
Lower
Resistor
Measured
Voltage
Calculated
Voltage
10K Ω 33K Ω 6.9V
33K Ω 10K Ω 2.1V
33K Ω 100K Ω 6.8V
100K Ω 33K Ω 2.2V
3.3K Ω 10K Ω 6.8V
10K Ω 3.3K Ω 2.2V
1K Ω 3.3K Ω 6.9V
3.3K Ω 1K Ω 2.1V
10K Ω
parallel
33K Ω, 100KΩ
6.4V
parallel
33K Ω, 100KΩ
10K Ω 2.6V
series
10K Ω, 33KΩ
100K Ω 6.3V
100K Ω
series
10K Ω, 33KΩ
2.7V
1K Ω
parallel
3.3K Ω, 10KΩ
6.4V
parallel
3.3K Ω, 10KΩ
1K Ω 2.6V
series 1K Ω,
3.3K Ω
10K Ω 6.3V
10K Ω
series 1K Ω,
3.3K Ω
2.7V
o 1-to-3-to-27-to-48-to-45
o 26-to-50-to-39-to-47
o 15-to-49
o 2-to-16
o 4-to-19
o 38-to-17-to-20
o 18-to-44-to-46