Electronics-World-1959-05
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less but the necessary starting control<br />
current must be greater.<br />
The circuit may be tested with the<br />
component parts shown within the<br />
dashed box at the input of V,. These<br />
will provide a signal control current of<br />
about 100 microamperes when R, is<br />
3300 ohms as shown in Fig. 1.<br />
Total current drain with the motor<br />
running is 450 milliamperes. This may<br />
be compared with the current required<br />
for a flashlight using two size C batteries.<br />
A lamp used in a flashlight containing<br />
two of these batteries is the<br />
G -E PR2 rated at 500 milliamperes.<br />
Thus, it can be seen that the transistorized<br />
motor control device draws less<br />
current than the flashlight bulb. If the<br />
constructor requires more power for<br />
the motor, B. can be increased to 4%<br />
volts.<br />
A less sensitive circuit that was tried<br />
is given in Fig. 2. It consists of just two<br />
direct -coupled transistors, an n -p -n and<br />
p -n -p type. With this circuit, a signal<br />
control current of 0.3 milliampere is<br />
sufficient to start the motor and when<br />
this current is increased to 0.5 milliampere,<br />
the motor runs at full speed.<br />
It is to be noted that the polarity of<br />
the control signal is the reverse of that<br />
required for the circuit of Fig. 1.<br />
Construction Notes<br />
No particular construction pattern<br />
Fig. 1. Schematic diagram<br />
of the motor<br />
control circuit. The<br />
motor will operate<br />
with an input control<br />
current of less than<br />
100 pa. The section<br />
within the dashed box<br />
is used to test circuit.<br />
Fig. 2. Diagram of a less sensitive motor<br />
control circuit. An input control signal<br />
of 5' milliampere will operate the motor.<br />
The several components within the dashed<br />
lines may be used for testing the circuit.<br />
These will provide an input control current<br />
of 1.2 ma. A penlite cell may be employed.<br />
need be followed. In fact, the physical<br />
layout will depend entirely on the<br />
particular application. It may be desirable<br />
to locate the motor away from<br />
the other component parts, for instance,<br />
when the device is used in<br />
model working activities. The author<br />
built up the circuit of Fig. 1, in the<br />
manner shown in the photograph, for<br />
experimental applications.<br />
All the parts are attached to the<br />
upper half of a 2!á" x 2!4" x 4" aluminum<br />
Bud "Minibox." The three<br />
transistors and motor are mounted on<br />
the top side and the two flashlight batteries<br />
are located on the bottom side.<br />
A small, dual -type battery holder secures<br />
the batteries. The "on -off" switch<br />
and miniature connector for the control<br />
signal are placed on one end of the box.<br />
Two sockets are provided for the<br />
smaller transistors. The 2N256 transistor<br />
is attached directly to the box<br />
with two 6 -32 machine screws. It is to<br />
be remembered that the case of this<br />
transistor is part of the collector circuit<br />
and therefore other connections<br />
must not be made to the metal box.<br />
Other equivalent transistor types may<br />
be used instead of the ones shown.<br />
Two connector inserts, removed from<br />
a miniature tube socket, are slipped<br />
over the base and emitter pins of the<br />
power transistor for electrical connection.<br />
Wires should be soldered on<br />
these clips before placing them on the<br />
transistor pins.<br />
The small pulley on the shaft does<br />
not come with the motor. It is a small<br />
wheel taken from a pulley used for<br />
pull -type curtains and draperies.<br />
Although just two motors have been<br />
mentioned. electronic mail order houses<br />
have advertised miniature motors rated<br />
at various voltages from 1?¿ to 8 volts<br />
and current ratings from 200 to 500<br />
milliamperes. Weights have been given<br />
from 10 grams to 5 ounces at prices<br />
ranging from 85c to over $5.00.<br />
Motors requiring higher voltage ratings<br />
may be used in the circuit by<br />
increasing the value of B. and adjusting<br />
R, as required. Collector voltage<br />
ratings of V,, V.. and V. are -22. 25,<br />
and -12 volts. respectively. Collector<br />
current ratings are -10, 50. and -500<br />
milliamperes. The V. values are those<br />
recommended when employing a heat<br />
sink. Be careful not to exceed the maximum<br />
current ratings of V, and V..<br />
This transistor motor control device<br />
is one of those hobbyist's "delights"<br />
because of its interesting possibilities<br />
and applications. The reader interested<br />
in model control projects will find<br />
many useful applications for the circuit.<br />
It is also a good project for the<br />
newcomer to transistor circuitry, since<br />
it is easy to assemble. has few component<br />
parts, and is simple to operate.<br />
Total cost of the three transistors and<br />
motor is less than $5.00<br />
A<br />
The underside view of the completed unit<br />
Is shown here. Wiring and layout are sim.<br />
ple and only a few connections are needed.<br />
The completed motor control unit. All that<br />
is needed to start the motor is to appiy<br />
a minute control current to white plug.<br />
Photo of all the component parts for the<br />
motor control device. Note that all<br />
holes have been made in the metal box<br />
and everything is ready for the assembly.<br />
May, <strong>1959</strong><br />
47