This project is all about LEDs. It gives basic guideline of how LEDs can be used to design something new in a better way. The circuit entitled; ‘DOT/BAR DISPLAY DRIVER’ accepts variable input voltage and turns on one or more LEDs corresponding to the instantaneous voltage of the input. We can thus conclude that LEDs are here used to provide visual display of the magnitude of the input signal. Depending on the requirement, the circuit offers two modes of operation, out of which one produces a single moving dot .i.e. one LED visible at a time and the other develops a bar graph display .i.e. multiple LEDs visible at a time.
Talking about the construction, this circuit employs nothing but just a set of comparators and a number of voltage divider. The voltage divider sets a stable voltage at one input of each comparator and each comparator also has its own unique voltage level. When the input signal is much less than the unique value of comparator, LED remains in ‘off’ state but as the input value starts to rise and a condition is reached where the input reaches the level fixed at a given comparator, the comparator output will switch on its LED (each comparator drives a single LED). As the input voltage increases, more comparators and LEDs are switched on. In case of dot mode, the LED that represents the highest value of input will be on, whereas in bar mode, the highest value LED including all lower value LEDs will be on.
This Driver device can handle or drive normally up to ten LEDs. To drive large number of LEDs, the device can be cascaded to provide a longer string of LEDs. Since the display drivers available can respond to the input signal in a linear, VU, or logarithmic fashion, modifications can be made to achieve desired effect.
The basic key parameters of this circuit are described hereby:
- LED current drive: – The level of drive current for the LEDs can be programmed. Because of which, need of numerous resistors corresponding to the number of LEDs in the circuit can be avoided. The drive current is set by a single external resistor. The resistor sets the current in the 2 to 30 mA range.
- Reference voltage: – Reference voltage is the supply to voltage divider which determines the state of combinations of comparators and LEDs. An internal fixed voltage reference is available (1.25 V is typical). However, the voltage divider is available via external pins, so that a reference voltage of any value up to 1.5 V less than the supply voltage can be used. The reference voltage in the circuit is chosen to correspond to the magnitude of the variable input signal.
The chip can be used to drive LEDs, LCDs, or vacuum fluorescent displays to provide a visual representation of a variable input signal. Since the circuit can be modified as per our need, it is also possible to create interesting displays by modulating LED brightness and its arrangement.