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Advertising Index Altronics.................................69-76 Ampec Technologies.................. 13 ADI Maxim Integrated................. 11 Dave Thompson.......................... 95 Dick Smith Contest..................... 77 Digi-Key Electronics...................... 5 Emona Instruments.................. IBC Hare & Forbes............................ 2-3 Jaycar.............................. IFC,45-52 Keith Rippon Kit Assembly......... 95 LD Electronics............................. 95 LEDsales..................................... 95 Microchip Technology......... 7, OBC Mouser Electronics....................... 9 Ocean Controls........................... 10 Phipps Electronics...................... 85 PMD Way..................................... 95 SC Christmas Decorations......... 63 Silicon Chip Shop.................90-91 Silicon Chip Subscriptions........ 68 Solder Master............................. 15 Switchmode Power Supplies..... 12 The Loudspeaker Kit.com.......... 87 Tronixlabs.................................... 95 Vintage Radio Repairs................ 95 Wagner Electronics..................... 14 Next issue release The December 2021 issue is due on sale in newsagents by Thursday, November 25th. Expect postal delivery of subscription copies in Australia between November 23rd and December 13th. 96 Silicon Chip Controlling motor speed with an Arduino I want to build your June 2011 Motor Speed Controller (siliconchip.com.au/ Article/1035) using the Jaycar KC5502 kit. Can the external potentiometer be replaced by connecting to an Arduino for speed control? I have been purchasing motor controller boards from the website: www. dimensionengineering.com (Sabertooth 2x12 R/C boards). This alternative would be of great interest to my group of friends that also build Star Wars robots. I am scratch-building robots having left aircraft engineering after 36 years. One of my builds was in DIYODE magazine issue 3. (F. H., Engadine, NSW) • You can use an Arduino to control the June 2011 Motor Speed Controller via a pulse width modulated (PWM) output. The PWM signal is filtered to give a varying voltage to feed the Speed Controller. Disconnect the trimpot or potentiometer connected to the +IN1 input (pin 1) of IC1 and instead, connect the PWM output of the Arduino via a 10kW resistor. The supply ground of the Arduino must connect to the Motor Controller ground. PWM frequency should be set for at least 500Hz so that the 10kW resistor and existing 1μF capacitor at the +IN1 input provide a smooth DC voltage. The motor speed can then adjusted by varying the duty cycle of the PWM signal from the Arduino. A 50% duty will result in 2.5V DC at the Motor Controller input and will set the motor at full speed. A 0% duty cycle will turn the motor off. If you want the motor to run at maximum speed when the duty is set at 100%, change the 10kW resistor to 22kW and connect another 22kW resistor from the +IN1 input to ground. This will effectively halve the applied voltage. Details on using PWM with Arduino is covered in the tutorial at siliconchip. com.au/link/abb7 Transformer for 20W Class-A Amplifier Where can I obtain the 160VA 16-016V shielded toroidal power transformer for the 20W Class-A Amplifier Module project (May-September 2007; siliconchip.com.au/Series/58)? Australia’s electronics magazine In Leo’s April 2011 article titled “Fixing transformer buzz in the Class A amplifier”, he suggests using an 18-0-18V transformer when the mains is reduced to 230V AC (it’s around 239V AC here in Melbourne suburbia) and adding chokes which reduce the DC output voltage of the power supply by more than a volt. As ordinary 16-0-16V toroidal transformers are not off-the-shelf items (it’s either 15-0-15V or 18-0-18V), should I be looking to get an 18-0-18V transformer? In the 1998 15W Class-A amplifier version, its designers had lots of trouble with hum, even when using the transformer inside a steel box inside the amplifier chassis. Would an ordinary unshielded 160VA toroidal transformer be OK if placed about 400mm below the Class A amplifier PCB, as in the separate power supply version of 1998? (I. H., Essendon, Vic) • We checked all four articles and couldn’t find any mention of a source for the transformer. These were supplied with the contemporary kits but they have long since sold out. It was probably made by either Harbuch or Tortech. Regardless, either of them should be able to wind one for you. Email sales<at>harbuch.com.au or see siliconchip.com.au/link/abba Don’t forget to specify the electrostatic shielding. You should be able to use the 18-018V transformer. If the voltage ends up being too high, wind on extra turns in the opposing direction to the original winding for each 18V winding to reduce the voltage. But the chokes will probably drop the voltages enough. Placing the transformer in a separate box will reduce the hum significantly. 400mm spacing from the amplifier should be sufficient. If hum is still heard, try rotating the toroid to get the lowest hum or move the toroid further away. Finally, note that the 20W Class-A amplifier is essentially made obsolete by the Ultra-LD Mk.3 (July-September 2011; siliconchip.com.au/Series/286) and Mk.4 amplifiers (August-October 2015; siliconchip.com.au/Series/289), which have similar distortion (lower in the case of the Mk.4), significantly more power and higher efficiency. Importantly, they also do not suffer from the same hum problems that plague Class-A amplifiers. SC siliconchip.com.au