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Check out three new Circuit Design Ideas that have been added to the site - examples to give you inspiration for designing your own circuits. And they have an international flavour...
- White-noise generator has no flicker-noise component, courtesy of Alfredo Saab and Randall White, Maxim Integrated Products, Sunnyvale, California
- Red LEDs function as light sensors, courtesy of Geoff Nicholls, Glinde, Germany
- Precision voltage-controlled current sink tests power supplies, courtesy of Luca Bruno, ITIS Hensemberger Monza, Lissone, Italy
White-noise generator
White-noise generators generate a flat graph of output-power density versus frequency. These generators are useful for testing circuits that have an extended low-frequency or dc response. However, the presence of pink, or flicker, noise complicates the design of white-noise generators for frequency ranges that extend to a few hertz or below.
A semiconductor device generates noise that always has the characteristic signature of pink noise: Its output-power-density amplitude increases as frequency decreases, with a corner frequency of 10s of hertz to a few kilohertz.
A high-value resistor generates noise with its own flicker-noise component, whose value and characteristics vary with the resistor's technology. If, on the other hand, the resistor has a low value and the device uses low-noise technology, then the noise is almost completely white with power density that is constant with frequency.
Red LEDs function as light sensors
Ordinary red LEDs normally function as light emitters, but they can also function as photosensors. A single LED can even function as both a light emitter and a light detector in the same circuit (Reference 1). The basic idea is to pulse the LED, using the on-time to light it and the off-time to sense the photovoltaic current from the ambient light that the LED sees.
Figure 1's circuit functions as a night-light. The LED stays off during daylight and turns on when the ambient-light level drops. The 7555 CMOS timer is a monostable one-shot, which triggers when Pin 2's voltage is less than one-third of the supply voltage. R1 and R2 form a voltage divider, which keeps the cathode of the LED just below the trigger voltage.
Precision voltage-controlled current sink tests power supplies
To discover potential power-supply problems, you must run dynamic and static tests. This simple current sink tests low- to medium-power supplies and voltage sources. In this application, the current sink can draw current of 0 to 1.5A for an input-voltage range of 0 to 5V with a supply voltage as high as 20V.
The basis of the circuit is precision op-amp IC1, an OPA277 from Texas Instruments, which features a maximum input-offset voltage of only 100 µV, maximum input-bias current of 4 nA, and low drift over the temperature range of -40 to +85°C (Figure 1). The op-amp IC compares its positive input voltage with the voltage across sense resistor RSENSE.
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