Electronic Fundamentals Essay Example | Topics and Well Written Essays - 2000 words

Electronic Fundamentals - Essay Example The inverting input of the first comparator is attached to the voltage divider at the level of two thirds VCC, and the non-inverting input carries the THRESHOLD signal. The non-inverting input of the second comparator is connected to one third of VCC, and the inverting input carries the TRIGGER signal. The functions of THRESHOLD and TRIGGER will be explained in the operations section. The use of the voltage divider means that the device can operate across a range of supply voltages. The outputs of the comparators are connected to the reset and set terminals, respectively, of a flip-flop gate. This is the heart of the device and is responsible for the switching behaviour utilized in most of the device’s applications. It is important to note that the rest of the circuit is connected to the complement of the flip-flop logic. The base of bi-polar junction transistor (BJT) is connected to the output of the flip-flop, and is made available to discharge (hence the signal name DISCH) external capacitors. Finally, there is an output stage that increases the available output current, and inverts the logic of the flip-flop output. 1.2 Operation The 555 has three modes of operation: astable, monostable, and bistable. Each of these modes of operation requires the connection of a slightly different set of external components. In each application, however, the basic functioning of the device remains the same: THRESHOLD Signal THRESHOLD Comparator Output TRIGGER Signal TRIGGER Comparator Output Flip-Flop Output (Complement) 555 Output > ? VCC HIGH > ? VCC LOW HIGH LOW > ? VCC HIGH ? VCC LOW Keep state Keep state < ?VCC LOW < ? VCC HIGH LOW HIGH 1.2.1 Astable In astable mode the 555 puts out a continuous stream of rectangular pulses at a frequency determined by the external circuit. The circuit is said to be ‘astable’ because it does not stay in a given output state, but continuously switches back and forth between high and low states. Figure 2 - Astable ci rcuit diagram [3] The external resistor R1 is connected between VCC and the DISCHARGE pin (7), and R2 is connected between DISCHARGE and the input signals. The input signals, TRIGGER (2) and THRESHOLD (6), are connected together as well as to the external capacitor C1. This capacitor is charged through R1 and R2 when the discharge pin is isolated from ground (flip-flop output is LOW), and discharged through R2 when the discharge pin has a low impedance path to ground (flip-flop output is HIGH). The continuous oscillation of the device can be seen as follows: -C1 is initially discharged, TRIGGER is below 1/3 VCC, THRESHOLD is below 2/3 VCC, the F-F output is HIGH, and the discharge transistor is off. -C1 charges to ? VCC and the flip-flop output switches to low. The discharge transistor opens and the capacitor discharges to ground through R2 and pin 7. -Once the capacitor discharges to ? VCC, the logic in the flip-flop reverses, and the circuit is back to its initial state. The versa tility of the 555 is demonstrated by the fact that the time