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The LED driver comprises a PNP bipolar transistor, Q1, and an NPN bipolar transistor, Q2. Q1 and Q2 form a switchable current source. At a high logic level at the cathode of Schottky diode D2S, a constant current flows through the LED with a value of roughly IO≈0.7V/RS, or about 10 mA in this circuit. Series-connected silicon diodes D1 and D2 provide strong nonlinear negative feedback. If for any reason the voltage drop across the sensing resistor, RS, rises, the D1-to-D2 connection will force almost the same increase in voltage at the emitter of Q2. This increase reduces the collector current of Q2 and, therefore, the base current of Q1 and closes the loop; the net result is a reduction of the collector current of Q1 to maintain a constant value. Note that when the output of IC1 goes low, the current through D2S and resistor RB is negligible. This is due to the fact that, with the output of IC1 low, the base of Q2 is held low, turning it and the current source off. With the current source off, the LED is off as well, and only microamps of leakage current flow through D2S and RB. If you use all surface-mount devices, you can build the circuit on a board no larger than 16×16 mm. This work was supported by the Slovak Research and Development Agency under contract no. APVV-0062-11.EDN Rotary encoder with absolute readout offers high resolution and low cost Michael Korntheuer, Vrije Universiteit Brussel, Brussels, Belgium Rotary encoders are typically used in positioning systems with servo feedback in which the cost of the encoder usually is of minor importance. Encoders, however, are also used in user interfaces to encode the positions of knobs—the volume knob on an audio system, for example. For those knobs, you have the choice between either a potentiometer boasting low cost, high resolution, and absolute readout but only limited travel—typically less than 340°—or a mechanical-optical rotary encoder, which has endless travel but a higher cost, low resolution, and only relative readout. The Design Idea presented here attempts to combine the advantages of the potentiometer with the endless operation of the mechanical optical rotary encoder. The encoder uses standard potentiometer construction techniques and is thus easily produced. It basically is a dual-wiper quadrature endless pot. It consists of a full ring of resistive material, which is powered from opposite sides and on which two electrically independent wipers move. The wipers are mechanically connected to each other at an angle of 90° (Figure 1). An ADC on a microcontroller reads out the two signals; firmware uses both signals to determine in which quadrant the axis is located. Once the quadrant is known, the signal of both wipers can be used to calculate the position of the axis. When a wiper reaches the top or bottom power connections, its signal should be ignored because of nonlinear response (Figure 2). Both wipers cannot be in this nonlinear position at the same time because of the 90° angle between the wipers. Today, even the most basic microcontrollers offer a 10-bit ADC, so the combined signals give an 11-bit resolution, or better than 0.2°. The microcontroller can ignore the absolute readout if the application does not require it or when a software reset is useful. This quadrature endless pot provides a user experience similar to the old tuning knob of a classical analog radio. It offers new possibilities in human-interface design and can give a quality feel in consumer products at low cost.EDN V+ MICROCONTROLLER RESISTIVE MATERIAL P1 P2 AIN1 AIN0 40 EDN Europe | MARCH 2013 www.edn-europe.com DI5262_FIG1 V+ P1 0° 0 180° 360° 540° P2 NOT LINEAR DI5262_FIG2 Figure 1 The encoder is a dual-wiper quadrature endless potentiometer that consists of a full ring of resistive material, which is powered from opposite sides and on which two electrically independent wipers move. Figure 2 When a wiper reaches the top or bottom power connections, its signal should be ignored because of nonlinear response. designideas


EDNE MARCH 2013
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