DXARTS 472 A 555 TIMER 462 Mechatronic Art, Design & Fabrication II DXARTS 472 A 555 TIMER IC ‣ The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. ‣ Introduced in 1972 by Signetics, the 555 is still in widespread use due to its low price, ease of use, and stability. It is now made by many companies in the original bipolar and in low-power CMOS technologies. As of 2003, it was estimated that 1 billion units were manufactured every year. The 555 is the most popular integrated circuit ever manufactured. DXARTS 472 A DXARTS 472 A 555 INSIDE FROM EVIL MAD SCIENTIST LAB DXARTS 472 A 555 PINOUT DIAGRAM WHAT ARE THE 555 PINS’ FUNCTIONS? DXARTS 472 A ‣ ‣ ‣ ‣ 1 Ground (GND) - Pin 1 connects the 555 timer chip to ground. 2 Trigger - Pin 2 is the trigger pin. It works like a starter pistol to start the 555 timer running. The trigger is an active low trigger, which means that the timer starts when voltage on pin 2 drops to below 1/3 of the supply voltage. When the 555 is triggered via pin 2, the output on pin 3 goes high. 3 Output - Pin 3 is the output pin. 555 timer's output is digital in nature. It is either high or low. The output is either low, which is very close to 0V, or high, which is close to the supply voltage that's placed on pin 8. The output pin is where you would connect the load that you want the 555 timer to power. This may be an LED, in the case of a 555 timer LED flasher circuit. 4 Reset - Pin 4 is the reset pin. This pin can be used to restart the 555 timer's timing operation. This is an active low input, just like the trigger input. Thus, pin 4 must be connected to the supply voltage of the 555 timer to operate. If it is momentarily grounded, the 555 timer's operation is interrupted and won't start again until it's triggered again via pin 2. WHAT ARE THE 555 PINS’ FUNCTIONS? DXARTS 472 A ‣ ‣ ‣ ‣ 5 Control Voltage - Pin 5 is the control pin. In most 555 timer circuits, this pin is simply connected to ground, usually through a small capacitor, about 0.01 µF capacitor. This capacitor serves to level out any fluctuations in the power supply voltage that might affect the operation of the timer. Some circuits (though rare) do use a resistor between the control pin and Vcc to apply a small voltage to pin 5. This voltage alters the threshold voltage, which in turn changes the timing interval. Most circuits do not use this capability, though. 6 Threshold - Pin 6 is the threshold pin. The purpose of this pin is to monitor the voltage across the capacitor that's discharged by pin 7. When this voltage reaches 2/3 of the supply voltage (Vcc), the timing cycle ends, and the output on pin 3 goes low. 7 Discharge - Pin 7 is the discharge pin. This pin is used to discharge an external capacitor that works in conjunction with a resistor to control the timing interval. In most circuits, pin 7 is connected to the supply voltage through a resistor and to ground through a capacitor. 8 Power Supply (VCC) - Pin 8 is connected to the positive power supply voltage. 555 timer ICs need DC voltage in order to operate. This is the pin which connects to the DC voltage to power the 555 chip. The voltage must be at least 4.5V and no greater than 15V. It's common to run 555 timer circuits using 4 AA or AAA batteries for 6V or a single 9V battery. DXARTS 472 A BLINKING AN LED WITH A 555 TIMER IC ‣ The 555 timer chip is a very versatile IC, because when connected correctly, it can it can create pulses of current at specific time intervals decided by the resistor-capacitor (RC) network. When a 555 timer creates pulses in this way, the LED doesn't stay constantly on. It only turns on at a pulse and then shuts off after the pulse has passed. And it does this in a neverending cycle, which creates a blinking effect. ‣ To make the 555 timer chip create pulses, it must be placed in astable mode. Astable mode simply means that the 555 timer has no stable state. It switches constantly between high and low, or on and off. This is why this mode is also called oscillator mode, because it uses the 555 timer as an oscillator, which creates square wave signals. DXARTS 472 A UNDERSTANDING THE 555 LED BLINKING CIRCUIT ‣ ‣ ‣ ‣ The output of a 555 timer are square waves. There are 3 important time measurements for a square wave. There is the total length of the square wave (the time it is on and off, or high and low), there is the length of time it is high (Thigh), and the length of time it is low (Tlow). The total time of a square wave is equal to the sum of Tlow and Thigh. The amount of time that the square wave is high is its duty cycle. So, for example, if the total time of a square wave is 1 second and it's high for 0.2s, it has a duty cycle of 20%, because it's on for only 20% of the cycle. The duty cycle is very important for an application like this LED blinking circuit. The duty cycle we choose determines how long the LED will stay on for compared to how long it is off for. Again, as an example, if we set our duty cycle to be 20%, this means the LED will flash on for 20% of the cycle and be off for 80% of the cycle. If we choose a duty cycle of 80%, the LED will be on for 80% of the cycle and off for 20% of the cycle. Thus, our duty cycle is very important. The formulas to calculate these values in our circuit is: ‣ T = 0.7 * (R1 + 2R2) * C1 ‣ Thigh = 0.7 * (R1 + R2) * C1 ‣ Tlow = 0.7 * R2 * C1 C1 charges through both R1 and R2 but discharges only through R2. That's why we must add the 2 resistor values for the Thigh calculation, but only use R2 for the Tlow calculation. It's also why you must double R2 but not R1 for the total time (T) calculation. UNDERSTANDING THE 555 LED CIRCUIT DXARTS 472 A ‣ ‣ ‣ ‣ ‣ ‣ ‣ ‣ According to the previous formulas, the larger value we use for the resistors and the capacitor, the longer the cycle will be. If we use very large values such as 100KΩ or 100µF, we will have very large cycles. For example, let's say we make R1 and R2 resistors both 1MΩ resistors and C1 100µF. This would produce a total time period of 210 seconds for the cycle. Of this 210s, the LED would be on for 140s and off for 70s. This is not what we want at all. This is way too long. This would not produce a LED flasher. We must choose much lower resistor and capacitor values in order to see a flashing or flickering effect. However, on the other extreme, we do not want to choose values which are too small as well. If we do, the human eye won't be able to detect that it has even turned on. For example, if we choose 1KΩ resistors and a 0.1µF capacitor, the time cycle and the on-off cycle would be too short. It will be as if the LED is constantly on. Therefore, we must choose a precise range of values to see the blinking effect. With these values, each cycle would last just a few millionths of a second. For the values we must use in our circuit, our duty cycle is just about 50%. With a 50% duty cycle, the LEDs are on for about the same time that they are off. With R1= 1KΩ and R2=10KΩ and C1= 10µF, our calculations will be: T= 0.7 * (1KΩ + 2(10KΩ)) * 10µF= 0.147s Thigh= 0.7 * (1KΩ + 10KΩ) * 10µF= 0.077s Tlow= 0.7 * 10KΩ * 10µF= 0.07s These values for resistors R1 and R2 and capacitor C1 allows us to see the flashing of the LED. DXARTS 472 A FLASHING 2 LESS WITH A 555 ‣ We can slightly modify the previous circuit so that it flashes 2 LEDs. The LEDs will flash alternatively. As one flashes on, the other flashes off. This happens pretty quickly but it still can be seen with the human eye. ‣ Additional Parts: ‣ 1KΩ Resistor ‣ Red LED ‣ We need all the parts listed for the first circuit, now with the addition of 1 more 1KΩ resistor and 1 red LED. ‣ This will create a circuit where 2 LEDs now flash, alternative, like police sirens rotating between a blue siren and a red siren. And this is how LED flasher circuits can be built with 555 timers. DXARTS 472 A 555 TONE GENERATOR ‣ This is a circuit for a generating a variable tone. The main component of this circuit is the 555 timer IC, configured as an astable multivibrator. A 555 configured as such outputs a periodic signal with a frequency determined by the values of R1, R2, and C1. ‣ By feeding this 555 output to a speaker as shown in Figure 1, a tone will be produced, the pitch of which depends on the 555 output frequency. The tone generated by this circuit is adjusted by varying the value of potentiometer R1. ‣ The formula to calculate the frequency is defined again by the RC relation: ‣ Frequency = 1.44 / (R1 + 2R2) * C1 DXARTS 472 A ATARI PUNK CONSOLE SYNTHESIZER DXARTS 472 A 555 FM RADIO TRANSMITTER ‣ This FM transmitter circuit, the 555 timer is designed as an astable multivibrator, but the tension control pin is used to connect a piezoelectric element (or electret mic) instead of the capacitor disk. The piezoelectric element generates a voltage and the output pin is connected to a wire antenna 30 inches for the transmission of signals. ‣ Just tap on the Piezo element. You can hear the tapping sound in an FM radio tuned exactly. Slowly turn the tuning knob of the FM radio along with tapping on the Piezo element. At a particular point, the tapping sound will be heard in the radio. The range is relatively short - around 3 feet.
