Manual

-147-
T
his experiment is a pulse-tone oscillator with an
adjustable frequency that can obtain a wide range
of notes. You can play tunes on it that sound like an
e
lectronic organ, but it takes some practice.
To play a tune, modify the control to the proper note
and press the key. Readjust the control for the next
note and press the key again.
When you close the key the first time, the base
current flows around the loop formed by the battery,
the 10kW resistor, the 50kW resistor, the transistor
base and emitter, and the key.
The base current causes the collector current to
flow around the loop formed by the 3V supply, the
lower half of the transformer winding, the transistor
collector and emitter, and the key.
The current through the transformer causes a
current to flow around the loop formed by the top
transformer winding, the 0.05mF capacitor, the
transistor base and emitter, the key, the battery and
back to the transformer’s center terminal (terminal
4). This current quickly (in less than 0.0001
seconds) charges the 0.05mF to about 4V or so with
a polarity negative on the transformer side and
positive on the transistor base lead side. The
speaker is only activated while the current flows in
the transformer.
When the induced voltage from the top half of the
transformer winding stops, the charging of the
0.05mF capacitor stops, then the capacitor begins to
charge again. As soon as the discharge begins, the
capacitor voltage becomes higher than the battery
voltage. The reverse polarity voltage is applied to
the base and the transistor turns off. Now, all
transistor junctions act as open circuits. The
capacitor discharges around the loop formed by the
top transformer winding, the 10kW resistor, and the
50kW resistor. When you reduce the control setting,
the discharge is faster, so the process is repeated at
a faster rate causing a higher frequency. The cycle
repeats when the 0.05mF capacitor discharges to
slightly below the 3V of the battery.
N
otes:
EXPERIMENT #125: PULSE TONE GENERATOR
Schematic
Wi
r
i
ng Seque
nce:
o 1-
29
o 2-
30
o 3-
108-
110
o
4
-
82-
1
20
o
27-
40
-
107
o
28-
81
o
5-
41-
109
o
42-
137
o
119-
138
EP-130_62315RevC.qxp_EP-130_062812 6/23/15 11:18 AM Page 147