Showing posts with label based. Show all posts
Showing posts with label based. Show all posts
Monday, September 23, 2013
Comparator Based Crystal Oscillator
Although a simple crystal oscillator may be built from one comparator of an LT1720/LT1721, this will suffer from a number of inherent shortcomings and design problems. Although the LT1720/LT1721 will give the correct logic output when one input is outside the common mode range, additional delays may occur when it is so operated, opening the possibility of spurious operating modes. Therefore, the DC bias voltages at the inputs have to be set near the center of the LT1720/LT1721’s common mode range and a resistor is required to attenuate the feedback to the non-inverting input. Unfortunately, although the output duty cycle for this circuit is roughly 50%, it is affected by resistor tolerances and, to a lesser extent, by comparator offsets and timings.
If a 50% duty cycle is required, the circuit shown here creates a pair of complementary outputs with a forced 50% duty cycle. Crystals are narrow-band elements, so the feedback to the non-inverting input is a filtered analogue version of the square-wave output. The crystal’s path provides resonant positive feedback and stable oscillation occurs. Changing the non-inverting reference level can vary the duty cycle. The 2k-680Ω resistor pair sets a bias point at the comparator + (Comparator IC1a) and – (Comparator IC1b) input. At the complementary input of each comparator, the 2k-1.8k-0.1µF path sets up an appropriate DC average level based on the output.
IC1b creates a complementary output to IC1a by comparing the same two nodes with the opposite input. IC2 compares band-limited versions of the outputs and biases IC1a’s negative input. IC1a’s only degree of freedom to respond is variation of pulse width; hence the outputs are forced to 50% duty cycle. The circuit operates from 2.7V to 6V. When ‘scoping the oscillator output signal, a slight dependence on comparator loading, will be noted, so equal and resistive loading should be used in critical applications. The circuit works well because of the two matched delays and rail-to-rail outputs of the LT1720.
Friday, April 26, 2013
Arduino Based Capacitive Touch Screen
Arduino based projects are quite complicated and is no cake walk for beginners in this field. Interfacing a tablet with the microcontroller is a daunting task. Let see how its done here.
The primary focus should be on using the appropriate drivers for the tablet. This makes it a USB host. As seen in the video, light can be a medium of communication between the tablet and the microcontroller. A photo diode or photoresistor is what is needed here and this should be pressed against the screen.
Building a touch sensing capability can help in providing communication between the microcontroller and the tablet. Capacitive touch screens function by sensing the capacitance changes on the screen. In this case, a large conductive brass plate is used to simulate touch. The surface area of the conductor used here to simulate the touch sould be large.
Testing can be done by touching the scrren with a wire and holding the other end. This may not work. This is made to work by sticking a piece of aluminum foil on to the screen and connecting the free end to the pin that is available on the arduino. The video shows the working of the capacitive touch screen. The setup is quite fragile and a little more circuitry could improve its reliability. Thus the capacitive touch screen is done.
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