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If you need a low-cost speed control or low-voltage light dimmer then take a look at this little circuit. It uses just one IC & will control 12V DC motors or lights with rated currents of up to 1 amp. By DARREN YATES A simple low-voltage speed controller If you make your own PC boards, then no doubt you own one of those small PC board drills. These call for an external 12V DC supply capable of delivering at least 1A. With a supply of this rating, the drill will run at about 10,000 RPM. However, you don’t always want to run the drill at full speed and so ideally you need a variable power supply. This simple circuit will do the job efficiently and at low cost. It uses only a handful of components and will comfortably control DC motors rated up to 1A. It is a switching circuit which delivers bursts of current to the motor to provide an efficient means of varying the speed. The circuit can also be used to dim 12V lights and, in fact, its principle of operation is the same as used in the 50  Silicon Chip dimmers for automotive dashboard lamps. Circuit diagram As you can see in Fig.1, the Mini Drill Speed Controller uses very few components and only one IC – a CMOS 555 timer. The 10kΩ and 5.6kΩ resistors, along with the 0.1µF capacitor at pins 2 and 6, set the output frequency of the 555 to about 6.8kHz, although this will vary according to the speed setting. The duty cycle of the pulse output at pin 3 is set by VR1 which has its wiper connected to the control input (pin 5). The higher the control voltage to pin 5 of IC1, the higher the duty cycle and visa versa. The output signal is taken from pin 3 and drives Darlington transistor Q1 via a 1kΩ resistor. Q1 in turn drives the drill motor while diode D2 prevents Q1 from being damaged due to the back-EMF generated each time the motor switches off. The voltage supply to the 555 IC is regulated to 9.1V by zener diode ZD1 and its associated a 220Ω limiting resistor. While the 555 is fairly tolerant of supply variations, the zener diode and its accompanying 10µF filter capacitor are desirable to filter out hash and spikes which can be generated by the motor’s commutator. A 330µF capacitor at the supply input provides extra fil­tering, while diode D1 protects the circuit against incorrect supply polarity. Construction All of the components for the Mini Drill Speed Controller are installed D1 1N5404 220  10 16VW ZD1 9.1V 400mW MOTOR D2 1N4004 10k 4 8 7 5.6k 6 1k 3 IC1 LMC555 2 C B 1 Q1 BD679 E SPEED VR1 10k 5 12VDC 1A PLUG-PACK 330 25VW M D & K WILSON ELECTRONICS PLASTIC SIDE 0.1 E CB MINI PCB DRILL SPEED CONTROLLER Fig.1: the circuit uses 7555 timer IC1 to drive transistor Q1 & this pulses the motor on & off at a frequency of about 6.8kHz. VR1 controls the width of the output pulses on pin 3 of IC1 to set the speed of the motor. The circuit can control 12V DC motors rated up to 1A. on a small PC board measuring 61 x 43mm and coded 09111931. Before you begin any assembly work, check the board carefully for any shorts or breaks in the tracks by comparing it with Fig.3. If you do find any faults, fix them before proceeding further. Once the board has been checked, you can begin construction by installing the wire link, followed by the resistors, the diodes, capacitors and the IC. Follow the overlay wiring diagram to make sure that they are correctly located – see Fig.2. The transistor needs to be fitted with a small heatsink. The 10kΩ pot is a miniature 16mm type which solders directly to the board. If you prefer, you can use one of the larger types with some flying leads to the lid of a box for easier control. Before you hook up your 12V motor or lamp, apply power to the circuit and measure the current drain with your multimeter switched to amps. PARTS LIST 1 PC board, code 09111931, 61 x 43mm 1 miniature TO-126/220 heatsink 1 10kΩ 16mm potentiometer (VR1) Semiconductors 1 LMC555/7555 CMOS timer (IC1) 1 BD679 NPN Darlington transistor (Q1) 1 1N5404 3A diode (D1) 1 1N4004 rectifier diode (D2) 1 9.1V 400mW zener diode (ZD1) Capacitors 1 330µF 25VW PC electrolytic 1 10µF 16VW PC electrolytic 1 0.1µF 63VW MKT polyester Resistors (0.25W, 1%) 1 10kΩ 1 1kΩ 1 5.6kΩ 1 220Ω TO MOTOR 10uF IC1 LMC555 VR1 ZD1 1 2/87a Queen St, St Marys, NSW 2760. Phone (02) 833 1342 Fax (02) 673 4212 When switched on and with no load connected, the circuit should consume about 15mA or so. If this is OK, connected up a small 12VDC motor or lamp and adjust the control. You should see the motor speed or globe brightness vary as you turn the pot. The choice of case for the project is left up to you. You could install it inside a small zippy case or diecast SC box. 330uF D2 5.6k 10k 220  Have you found those components yet? We know that it can be difficult, frustrating and a waste of your valuable time. So why haven’t you contacted us? We specialise in hunting down and locating components – old, obsolete, leading edge, normally available but now scarce due to allocation by overseas manufacturers. Integrated circuits, resistors, capacitors, transistors, diodes, valves, varistors, etc. Any brands Let us save your valuable time Contact us now on 833 1342 We are also distributors for Electrolube lubricants and chemi­cals Hakko - desoldering & soldering irons; SMD tools; replacement parts NTE - replacements semiconductors 1k Q1 D1 12V PLUGPACK 0.1 Fig.2: install the parts on the PC board exactly as shown in this layout diagram. Fig.3: check your PC board against this fullsize pattern before mounting any parts. January 1994  51