The 555 timer has been a hobbyist favorite since its introduction in the 1970s. Inexpensive, versatile and easy to use, you can configure this general-purpose pulse generator for astable, bistable or monostable operation. The best way to learn use the 555 is by building circuits. The IC's datasheet has the basic operating parameters, formulas and schematics needed to get you started. Besides the 555 itself and its datasheet, all you need are a few outboard parts, a breadboard and a simple DC power supply.
Instructions
1. Read the 555's datasheet and learn about its layout, operating characteristics and applications. Being a pulse generator, it gets used in many digital circuits. You can also use it as an audio tone source or an LED flasher. Note where the 555 is connected for power and ground. Also see the timing network formed by two resistors and a capacitor. This network determines the 555's time period and duty cycle.
2. Pick out two resistors with values between 1K and 1M ohms and a capacitor between .001 and 10 microfarads. You will use these parts for the 555's timing network. Calculate the period using the following formula:
T = .693 x (Ra + 2Rb) x C
Where T is time in seconds, Ra and Rb are resistance in ohms, and C is capacitance in farads. Notice that you double the value of one resistor, Rb. Ra and Rb have special locations in the 555's timing circuit, so if the values of the two resistors are different, their effect on the circuit depends on which ones you use for Ra and Rb.
3. Set up a prototype breadboard by wiring positive and ground from a 9-volt DC supply to the breadboard's power busses. Insert the 555 timer in the breadboard so it straddles the special slot in the plastic. This slot is spaced for dual-inline package (DIP) integrated circuits. Be careful so the pins go straight into the holes and don't bend.
With the DC power off, wire positive power to pins 4 and 8 of the IC and ground to pin 1. Insert the resistor for Ra and wire one side to pin 7 and the other to positive power. Insert Rb into the board so that one side connects pin 7 and the other connects pin 6. Insert the capacitor so that one side connects to pin 6 and the other goes to power ground. Wire pin 2 to pin 6.
Connect a probe to your oscilloscope and clip its ground wire to your power supply ground. Insert a wire so it connects to pin 3, the 555's output, and clip the oscilloscope to this wire. Turn the DC power supply on. You should see a repeating pulse waveform at the oscilloscope, and its period should match what you calculated in step 2. This is astable operation.
4. Experiment with other resistor and capacitor values to see their effect on frequency, period and duty cycle. Try the 555's monostable and bistable modes. These will involve some rearrangement of parts and IC pin connections, though the timing network changes very little.
Tags: power supply, timing network, connects other, duty cycle, ground Insert, period duty, period duty cycle