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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number.

Maximite stepper motor interface

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This simple circuit and program listing allows the Maximite microcomputer (SILICON CHIP, March-May 2011) to control a stepper motor. It could be expanded to allow for the control of multiple motors, with four of the Maximite’s external I/O pins used to control each motor with identical driver circuits.

A ULN2003 Darlington transistor array (IC1) switches current through the stepper motor’s two windings in either direction. When one of the four Maximite output pins (8, 12, 16 & 20, corresponding to I/Os 19, 17, 15 & 13) goes high, the corresponding output pin on IC1 goes low, sinking current through a motor winding. Conversely, when these pins are high, the corresponding Darlington transistor is off and so no current flows through that portion of the winding.

The centre tap of each motor winding is connected to a current source comprising PNP Darlington transistor Q1 and some resistors. The maximum current is determined by the resistive divider driving its high-impedance base, setting the base voltage to around 9.1V when it is fully on.

By adding Q1’s base-emitter voltage (1.4V at 0.5A, as per the data sheet) we can determine that there will be around 1.5V across the 3.3Ω resistor (12V - 10.5V), resulting in a current of 1.5V ÷ 3.3Ω = ~450mA.

Transistor Q1 must be fitted with a medium-sized flag heatsink (Jaycar HH8504, Altronics H0637) or larger to handle its maximum dissipation of (10.5V - 4.9V) x 450mA = 2.5W.

When one of the Darlington transistors switches off and current flow through the corresponding motor winding ceases, the inductive winding generates a back-EMF current which causes the voltage across that winding to spike. IC1 has internal “free-wheeling” diodes from each output to the COM pin, which is connected to the +12V supply.

The back-EMF current flows back into the power supply and the voltage spikes are clamped at about 12.7V, so that the Darlington transistors do not suffer collector reverse breakdown, which might damage them.

A 470µF capacitor provides supply bypassing for the motor while a 47kΩ pull-up resistor and toggle switch/pushbutton S1 drives input pin 9 of the Maximite, allowing manual control of the motor direction.

Table 1 shows the sequence in which the output pins are driven to turn the motor forward; the steps are run backwards for reverse operation. The delay between the steps determines the speed at which the motor rotates.

The source code of the sample program is available for download from the SILICON CHIP website (maximite_stepper_motor.bas).

L. Kerr,
Ashby, NSW. ($60)

Solar tracking with a standard motor speed controller

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While commercial solar tracking devices are available, they tend to cost more than typical motor speed controllers and are not as common. This circuit can drive most standard motor speed controllers to keep a solar panel, solar heater or similar device aimed at the Sun throughout the day.

Two photocells (ie, solar cells) are used to sense the Sun’s position. For the prototype, these were scrounged from two identical solar-powered calculators. One is mounted on the east side of the rig and one on the west, as shown. When more light falls on the west sensor than the east sensor, after a short delay the motor is driven forwards to tilt the platform until the light levels on the two sensors are again equal.

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