Crocodile Clips - Guides

555 Timers

Introduction
The NE 555 timer chip is one of the most versatile intergrated circuits available. If you have read Tutorial 3, then will already know how to make the circuits, but if you haven't read it, I suggest that you do so first.

Monostable Circuit
The monostable circuit works by having an LED lit for a certain time before turning it off. The circuit looks like this below:

When pin 2 is set to 0v (because of the action of the switch), the chip sets pin 3 to 9v. This means that the potential difference across the LED is 9v so it switches on. Normally current is diverted through pins 7 and 1 from the 5.6k resistor, but now this route is blocked off and the capaciter starts to charge up. When it reaches 2/3s of the supply voltage, the current is diverted back through pins 7 & 1, pin 3 becomes 0v and the LED turns off. The chip then discharges the capaciter and the circuit is reset.

You can observe what is happening to the voltages at the pins using this graph.

To calculate the time delay, there are two formulas that can be used. You can use the time constant for an 'RC' chain formula (T=RxC) which is used with the 5.6k resistor and the capacitor (remember it from the capacitors page?). Or you can use T=1.1 CR, which is specific to a monostable circuit, the latter is more accurate, but the former easier to use and will do for most cases. Remember to use R in Mohms and C in µF.

Variations to the Monostable circuit
Here are some basics alterations that can be made to a 555 Monostable circuit:

LED Arrangement		Two LEDs
Place the LED & Resistor connecting to the +ve rail to make the LED go off for a certain time and then turn on.		Use two LEDs to make the top one turn off and the bottom one on when the circuit is triggered, and then have them switch states when the time runs out.
Trigger when power on		Convert pulse time
Using the circuit, the monostable timer is triggered when the power turns on. Replace the switch with a 1µF capacitor.		To convert a long pulse to a short pulse to avoid re-triggering the circuit (more useful in real world than CC).

Astable Circuit
The astable circuit, as you will already know, provides a regular clock pulse from its pin 3. The circuit looks like this:

The astable circuit is a dual-in-line (DIL) package, with the pins arranged like this:

Formulas for the 555 Astable
The formula for working out the frequency of the 'flashing' is (R1 is the top resistor, e.g. the 1K in our circuit):

1.44  /  ( C ( R1+2R2 ) )

For example, the speed for the circuit above would be:

Although this formula gives quite exact answers, it is often a lengthy and boring process to go through this sum many times, especially when trying to find a set value for, say 6 seconds. For this, you can use the graph below:

Please note that this graph is only a very rough guide and as values start to reach the top of the table (a large capacitance) the accuracy decreases considerably. However, it is useful for getting a feeling for whether the values are about okay.

Mark to Space Ratio
However, there is also another interesting point to be made about the timing of the circuit. Look at this graph below:

The purple line shows the voltage of pins 6 & 2. The size of the 'space' is measured by the size of R2 (68K in our circuit), and this is the time when the capacitor is discharging. The size of the 'mark' is measured by the size of R1+R2, meaning that it will always be larger. For an even mark to space ratio (therefore an even frequency), R1 must be very low and R2 must be comparatively higher. Note that at the start there is a double 'mark' as the capacitor charges to 2/3s of the supply voltage, but it then only ever drops as low as 1/3 of the supply voltage.

Note also that a cycle is one mark and one space, and it is the frequency of the cycles that creates the frequency for the flashes.

Variations on the Astable circuit
Here are some basic alterations that can be made to the astable circuit:

Using an LDR		High Current Outputs
Try replacing R2 with an LDR. The output frequency will then depend on the light falling on the sensor.		Add a single transistor, Darlington Pair (two transistors) or a relay to pulse outputs which require higher current.
Output to a loudspeaker
Set up the astable like this. It will then generate a very fast frequency, enough to allow a loudspeaker to produce a resonable tone. Turn on your speakers to here the tone.