7 months ago

Electronics-World-1959-05 By HAROLD REED Simple. inexpensive circuit for direct or remote control of small motors. Useful for model control. ONE of the many, and perhaps most useful, uses for transistors is in connection with control applications. A minute current applied to the input of a transistor circuit will trigger or operate many different electronic or electro- mechanical devices which require greater operating current. Most of the published transistor control circuits have been designed to operate some very sensitive device, such as a relay, requiring a small input control current and a larger, but still small, operating current. Now that inexpensive power transistors are available, the experimenter may construct useful transistor circuitry for control of devices less sensitive, that is, demanding greater operating current. One such application that occurred to the author is the control of small d.c. motors as described in this article. Miniature Motors The d.c. motor around which this circuit was designed is called the "Supermite" and measures only 1" x 11á" x 1%/4 ". Its dual shaft extends ?x" from each side of its case and the shaft diameter is %.,". It weighs just 2 ounces. It will run on 1!4 to 6 volts d.c. and its rotation is reversible. This motor was designed to power model boats and planes, toys, moving display signs, and other mechanically driven devices. An- other motor that the constructor may use is the "Mighty Midget" which will operate on 3 to 6 volts d.c. It is supplied with 7:1 reduction gears and pulley. Both motors are available from Lafayette Radio. Circuit Analysis The manner in which this motor control idea is applied can be understood by analyzing the schematic diagram of Fig. 1. The complementary characteristics of n -p-n and p -n-p junction transistors, whereby use is made of the opposite action of these devices, is utilized, and therefore, direct coupling is employed between all stages. An original circuit design for direct coupling was given by George Sziklai of RCA, in which a p -n -p transistor preceded an n -p-n unit. The circuit of Fig. 1 uses a small p -n-p junction transistor for the input stage. This is followed by a small n -p-n type which works into another p -n -p unit of the power type. When a control current of the correct polarity is applied to the input circuit of V,, an amplified collector output current is obtained. This current flows through the base -emitter path of V. and a greater amplified current is available at the V. collector circuit. Likewise, this further amplified current flows through the base -emitter circuit of V, which re- sults in increased current in the collector circuit of V. (the power transistor) of sufficient strength to operate the motor. The first transistor, V,, obtains its negative collector voltage through the base -emitter path of V. Also, the second transistor, V., gets its positive collector potential by way of the base -emitter path of V.. This arrangement makes it possible to employ a single battery supply to provide the correct operating polarities for the different transistor types. An input signal control current of less than 100 microamperes will operate the motor, thus the device may be radio -controlled, using a small transmitter and crystal rectifier receiver of the type employed in model control. Also, the circuit may be operated directly from a photo -electric cell with no auxiliary source of voltage. With a small, inexpensive photo- electric plate connected directly to the input, the motor could be controlled with a flashlight from across the room. The tiny selenium plate was not very sensitive. The standby current drain of the circuit and the required input control current vary with the value of resistor R,. If the value of R, is increased, the standby current of the circuit is greater but the required control current is smaller. When the value of R, is decreased, the non -operating current is Experimental Transistor Motor Control 46 less but the necessary starting control current must be greater. The circuit may be tested with the component parts shown within the dashed box at the input of V,. These will provide a signal control current of about 100 microamperes when R, is 3300 ohms as shown in Fig. 1. Total current drain with the motor running is 450 milliamperes. This may be compared with the current required for a flashlight using two size C batteries. A lamp used in a flashlight containing two of these batteries is the G -E PR2 rated at 500 milliamperes. Thus, it can be seen that the transistorized motor control device draws less current than the flashlight bulb. If the constructor requires more power for the motor, B. can be increased to 4% volts. A less sensitive circuit that was tried is given in Fig. 2. It consists of just two direct -coupled transistors, an n -p -n and p -n -p type. With this circuit, a signal control current of 0.3 milliampere is sufficient to start the motor and when this current is increased to 0.5 milliampere, the motor runs at full speed. It is to be noted that the polarity of the control signal is the reverse of that required for the circuit of Fig. 1. Construction Notes No particular construction pattern Fig. 1. Schematic diagram of the motor control circuit. The motor will operate with an input control current of less than 100 pa. The section within the dashed box is used to test circuit. Fig. 2. Diagram of a less sensitive motor control circuit. An input control signal of 5' milliampere will operate the motor. The several components within the dashed lines may be used for testing the circuit. These will provide an input control current of 1.2 ma. A penlite cell may be employed. need be followed. In fact, the physical layout will depend entirely on the particular application. It may be desirable to locate the motor away from the other component parts, for instance, when the device is used in model working activities. The author built up the circuit of Fig. 1, in the manner shown in the photograph, for experimental applications. All the parts are attached to the upper half of a 2!á" x 2!4" x 4" aluminum Bud "Minibox." The three transistors and motor are mounted on the top side and the two flashlight batteries are located on the bottom side. A small, dual -type battery holder secures the batteries. The "on -off" switch and miniature connector for the control signal are placed on one end of the box. Two sockets are provided for the smaller transistors. The 2N256 transistor is attached directly to the box with two 6 -32 machine screws. It is to be remembered that the case of this transistor is part of the collector circuit and therefore other connections must not be made to the metal box. Other equivalent transistor types may be used instead of the ones shown. Two connector inserts, removed from a miniature tube socket, are slipped over the base and emitter pins of the power transistor for electrical connection. Wires should be soldered on these clips before placing them on the transistor pins. The small pulley on the shaft does not come with the motor. It is a small wheel taken from a pulley used for pull -type curtains and draperies. Although just two motors have been mentioned. electronic mail order houses have advertised miniature motors rated at various voltages from 1?¿ to 8 volts and current ratings from 200 to 500 milliamperes. Weights have been given from 10 grams to 5 ounces at prices ranging from 85c to over $5.00. Motors requiring higher voltage ratings may be used in the circuit by increasing the value of B. and adjusting R, as required. Collector voltage ratings of V,, V.. and V. are -22. 25, and -12 volts. respectively. Collector current ratings are -10, 50. and -500 milliamperes. The V. values are those recommended when employing a heat sink. Be careful not to exceed the maximum current ratings of V, and V.. This transistor motor control device is one of those hobbyist's "delights" because of its interesting possibilities and applications. The reader interested in model control projects will find many useful applications for the circuit. It is also a good project for the newcomer to transistor circuitry, since it is easy to assemble. has few component parts, and is simple to operate. Total cost of the three transistors and motor is less than $5.00 A The underside view of the completed unit Is shown here. Wiring and layout are sim. ple and only a few connections are needed. The completed motor control unit. All that is needed to start the motor is to appiy a minute control current to white plug. Photo of all the component parts for the motor control device. Note that all holes have been made in the metal box and everything is ready for the assembly. May, 1959 47

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