Output: Motor control
Zip File ©Christopher Leigh
Simple Motor
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This block is a very simple one: motor between the output of a driver and the positive voltage line. The 10R resistor is in the circuit to reduce the sudden rush of current when the motor is turned on and is not always necessary. The 220nF capacitor should be soldered straight across the motor terminals and is to stop noise from the commutator getting back into your circuit; very important if you are using a microcontroller.
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Speed Control
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It's easy to turn a motor on and off, but how do you control its speed? By chopping the voltage on and off very quickly so that the average voltage is varied. If you vary the time the voltage is high, then you control the power going to the motor and hence its speed. Use a simple 555 astable with variable mark-space ratio and you have a speed controller. You will need to use a driver block between this and the motor. Instead of using a 555 you can use a PICAXE with its pwmout command; see my Picaxe Outputs page.
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Reversible Motor
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This block allows you to connect a motor between outputs 4 and 5 so that you can reverse it.
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Relay Reversing
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You need a double pole double throw relay to change the way the power supply is connected to the motor - then you can reverse the motor.
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Transistor Bridge Reversing
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Rather than using a relay, which takes a lot of power, you can use transistors in an H bridge arrangement. The transistors form the two sides of the H with the motor forming the cross piece. If the top transistor on the left side and the bottom transistor on the right side are turned on then the motor will turn one way. If the bottom left and top right transistors are on the motor will turn the other way. It is not a good idea to have both transistors on a side switched on; they form a short-circuit across the power and will burn out!
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FET Bridge Reversing
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This H Bridge just replaces the transistors in the previous block with low on resistance FETs. This allows your transistors to pass more current without getting too hot and robbing the motor of power. If you are using a high power motor, then as well as using high power transistors you will need to add diode protection across the transistors because of the back emf generated by the motor.
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L293D Chip Reversing
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The L293D is one of a range of integrated circuits specially designed for motor control. This one contains a bridge circuit which can handle about 1amp. The chip actually contains two bridge circuits which I have wired in parallel for better current handling. Note that this chip is designed to run hot - but not too hot!
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Linear Actuator
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The linear actuator consists of a motor which drives a screw thread. Moving along this screw thread is a block or whatever you need to move. At each end are microswitches which detect when the block gets to the end of its travel. They can simply interrupt the power to the motor, or they could be used as inputs to the controlling PIC; that's what this block is designed for. A linear actuator could be used to bolt and unbolt a door - for a safe perhaps.
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Servo Motor
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A servo motor often used in radio control is simply a motor connected to a gearbox, with a potentiometer connected to the output shaft. This means that the potentiometer provides feedback about what angle the shaft is at. So a servo motor can be made to move to, and stay at, a particular angle. One way to control it is using a microcontroller - see my Picaxe Outputs page.
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RC Pulser
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An alternative to Picaxe control is to use a variable mark-space ratio astable. The one shown here uses a 555 to produce a pulse width between 1mS and 2mS. The pulse controls the positionning of the servo. The pulse repetition rate is 50Hz so there is roughly 20mS between pulses.
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