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A simple control loop and added synchronous rectification could make this circuit useful for driving LEDs. Other applications await. Note that another way to achieve a similar result would be to use an op amp monitoring the voltage across a shunt in series with the load, and using negative feedback to provide the required drive voltage to match the shunt voltage to a reference sinewave. However, such a circuit may suffer from stability problems, necessitating added compensation components which would reduce its precision. Mauri Lampi, Glenroy, Vic. ($90) Controlling model railway points with a servo This controller was created to operate a set of points (or turnout) on a OO/ HO model train layout using a small 9g model servo (eg, Jaycar YM2758). For simplicity, each set of points has its own small microcontroller with just three inputs. Potentiometers VR1 and VR2 set the two positions, while switch S1 selects between them. It runs from a DC supply of at least 9V and 1A. This is reduced to 5V by 7805 regulator REG1, which is adequate to operate a 9g servo. Power for the microcontroller is decoupled by schottky diode D2 and a 220µF filter capacitor, to prevent motor current surges affecting its operation. The controller should be close to the servo and the points, due to the weak drive and to minimise power losses in the wires. So switch S1 may be several metres away, and thus its wiring is susceptible to interference. Circuit Ideas Wanted siliconchip.com.au The 1kW/100nF RC low-pass filter between S1 and the GP3 input of IC1 (pin 4) reduces the effects of EMI, and it is advisable to use twisted pair wires and/or grounded shielding to further reduce the chance of interference. Potentiometers VR1 and VR2 are wired across the micro's supply, and the wiper voltages are stabilised by 100nF capacitors which perform two functions. They reduce the effects of stray electric fields and also provide the low source impedance required by the micro's internal analog-to-digital converter (ADC). The servo signal has a 330W series resistor to protect IC1 from accidental shorts at CON1. A heartbeat LED, LED1, flashes to indicate when the circuit is operational. Setup is simple. With the power off, set VR1 & VR2 to their mid positions and the points also in their mid positions. Turn the power on and adjust one potentiometer to set the points to "ahead" or "turn". Then change the position of switch S1 and adjust the other potentiometer. The points are then operational, controlled by S1. Note that if the points are hard against either end position and the servo is trying to move the points more, the servo will be destroyed in little time. To prevent this, the mechanical link between the servo and the points should not be rigid. You can use an open-coil spring or provide a U-shaped loop so that there is some compression or extension of the link. The software was written in PICBASIC Pro. The Servo_Dual_Posn_ SC.BAS and Servo_Dual_Posn_ SC.HEX files are available from siliconchip.com.au/Shop/6/5638, along with a PDF of the PCB pattern. George Ramsay, Holland Park, Qld. ($80) Got an interesting original circuit that you have cleverly devised? We will pay good money to feature it in Circuit Notebook. We can pay you by electronic funds transfer, cheque or direct to your PayPal account. Or you can use the funds to purchase anything from the SILICON CHIP Online Store, including PCBs and components, back issues, subscriptions or whatever. Email your circuit and descriptive text to editor<at>siliconchip.com.au Australia’s electronics magazine December 2020  89