The examples includes automobile subwoofer power amplifier, critical speed indicator for high speed ferries, optoisolator for Philips IIC-bus serial interface, logarithmic rectifier with large dynamic range, low power photocell with modulated infrared light, controller for motorized lamp, telephone answering machine for use with external tape recorder, telephone hybrid for radio studio use, transformer balanced audio input amplifier with extremely low distortion.
- Automobile subwoofer amplifier
- Critical speed indicator for high-speed ferry
- Logarithmic rectifier with large dynamic range
- Low power photocell with modulated infrared light
- Optoisolator for the Philips I²C-bus serial interface
- Controller for motorized lamp
- Telephone answering machine with external tape recorder
- Transformer balanced audio input stage
Automobile subwoofer amplifierThe amplifier is designed around a dynamically self centering bridge circuit, so it will deliver almost rail to rail output with an unregulated power supply. The output power is 115 W into a 0.5 ohms load from a 12 V power supply. Another unique feature of this amplifier is a 10 mA idle current consumption over the -40 C to 85 C temperature range.
I have designed the circuit and PCB and built and tested a prototype.
Critical speed indicator for high-speed ferryNewer passenger ferries sails with still higher speeds and this causes some very high stern waves that can be very problematic for the surrounding environment. The size of the stern waves depends on the ferry's speed and the water depth. At a certain speed, the stern waves has a maximum, but decrease rapidly both below and above this speed. The Critical Speed Indicator calculates how far the current speed is from this critical speed and displays this on a LED bar meter. Input to the Critical Speed Indicator comes from the ship's GPS system and echo sounder in form of RS-232 signals using the NMEA 0183 protocol. The Critical Speed Indicator is built around a PIC microprocessor, that decodes the two NMEA 0183 signals, performs the required calculations and drives the display.
I have designed the circuit, the PCB and the mechanics, designed a PC controlled test procedure and manufactured and tested a number of units.
Optoisolator for the Philips I²C-bus serial interface
Optoisolator for the Philips I²C-bus serial interface, PP type II01During the design of audio equipment, it is often desirable to use a PC to emulate the equipment's microcontroller so that new functions and control routines can be easily tested. Many of the special ICs used for audio have an interface compatible with the Philips I²C specification and can be controlled from a PC with a I²C interface card installed.
The noise generated by a PC will often be induced into the device under test and prevent any measurements to be made, or in some cases (mainly with FM tuners) prevent the device from working at all. These noise problems can be solved by inserting a galvanic isolation between the PC and the device under test.
PP II01 datasheet
Logarithmic rectifier with large dynamic rangeI designed this rectifier for use with a combined compressor, expander and noise gate using feed forward control.
The rectifier is designed around a single log element that is placed in front of the rectifier circuit to reduce the gain-bandwidth required by the op-amp in the rectifier. The circuit obtains a 1 dB accuracy from -70 dBu (250 µV) to +20 dBu (7.75 V) in the frequency range 1 Hz to 20 kHz.
Low power photocell with modulated infrared lightThis photocell design is used to count the number of visitors passing through the gate to a bird preserve.
The design is based on detector that is highly sensitive to short infrared light pulses and at the same time rejects ambient light, so that the detector works in direct sunlight. This has been achieved through careful electronics design rather than with expensive optical components. Power saving is achieved by transmitting with a very low duty-cycle - a 10 µS pulse every 200 mS.
The IR transmitter is a separate unit that is powered and controlled from the detector unit. To ease installation, the control signal for the transmitter is modulated onto its power supply line, so that only 2 wires are required between the detector and transmitter.
The IR photo diode amplifier is a bandpass filter design with a transresistance of 3 V/A at DC and 60 MV/A at 1 MHz.
The circuit is controlled with a PIC microcontroller and a 200 mS timer. The microcontroller is normally in sleep mode to save power. Every 200 mS, the timer issues an interrupt to wake up the microcontroller, that powers up the detector, waits for the detector circuit to settle (around 1 mS), sends a trigger pulse to the transmitter and checks if a pulse is received. If the IR beam has changed state to interrupted, the microcontroller increments its counter.
When the gate to the bird preserve is closed, all circuits are disabled reducing the supply current from the normal 1.5 mA to 70 µA.
Controller for motorized lampThe lamp was used to attract attention to a shop window. This compact 4 channel controller mounts on the rear of the lamp and is powered from this. The control of iris, color, lamp position and gobo is performed by a PIC microcontroller and a 4 channel DAC. A random like function is made by using a carbon track potmeter in one of two timers.
I have designed the circuit, the PCB, the mechanics and the software and manufactured and tested 10 units.
Telephone answering machine with external tape recorderThe telephone answering machine records incoming messages on an external tape recorder to enable very long recording times. The tape recorder is controlled via a remotecontrol interface or by powering it when the recording is to begin. One of 4 electronically stored messages can be played to answer incoming calls. In order to be able to recover from a power failure, all the telephone answering machine's settings are stored in EEPROM.
I have designed the circuit, the PCB and the software, developed a PC controlled test procedure and tested 60 units.
Transformer balanced audio input stageThe transformer, operating in current mode, is loaded with a negative resistance of same value as its winding resistance, so that the field in the core is close to 0. The circuit lowers the transformer's lower cut off frequency (defined by maximum acceptable distortion) with up to 2 decades.
An example of the performance of this circuit with an EI 19/5 transformer can be seen in the
PP IA18 datasheet