555 Timer - Astable
August 6, 2013
We're now going to look at a second mode of the 555 timer. In this case, it outputs a continuous stream of pulses. This can be used to drive logic gates and create binary counters amongst other things.
555 Astable
Following on from http://drumcoder.co.uk/blog/2013/aug/06/555-timer-monostable/, we're using the same chip but configuring it slightly differently. Here, a continuous stream of pulses are output from the chip and we can use this to flash an LED. We can control how fast the LED flashes using a couple of resistors and capacitor, and we can therefore adjust the speed by adding in a potentiometer.
The chip is identical to that used in monostable mode:
I've always driven my 555 timer circuits off a 9 volt PP3 battery, so that should be connected to pins 1 and 8.
Pin 2 is the trigger - this is what tells the timer to do something. When the voltage on this pin is below 1/3 of the power supply voltage, then this triggers the chip. The difference for this mode is that we connect the trigger, pin 2, directly to pin 6. This causes the chip to trigger itself.
Pin 3 is the output - this will turn on and off based on the resistor and capacitors used elsewhere in the circuit.
Pin 4 is the reset pin - if this has less than 1/3 the power supply voltage then the timer is reset, and the output is set low. Here, we don't want a reset, so we connect this directly to positive.
Pin 5 is the control pin - we don't need to use this so we isolate it using a small 0.1uF capacitor between it and ground.
Pins 6 and 7 control the oscillation speed.
Frequency is in pulses per second and can be calculated using the following equation:
Frequency = 1440 / ((R1 + 2R2) x C1)
where R1 and R2 are in kilohms and C1 is in microfarads (uF). R1 is often held at 1K, and R2 could be a variable resistor allowing the speed to be varied.
Good values for a visible LED flash are R1 = 1K resistor, R2 = 100K linear potentiometer, C1 = 4.7uF.
Using this configuration, C1 charges through R1 and R2 but discharges through only R2. This means that the on time on pin 3 is always longer than the off time.
To counter this, add a diode:
In this configuration, the on time can be controlled with R1 and the off time with R2 meaning they can vary independently. If R1 == R2, you will get (almost!) equal on/off time.
