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Electronics-World-1959-05

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www.americanradiohistory.com<br />

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

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