Timing Circuits

TIMING CIRCUITS There are many electronic applications when we need to generate a time delay. Sometimes we need to provide a regular on-off pulse and sometimes we need to switch an output on or off for a period of time, before returning it to its original state. To enable us to do either of these we need to build timing circuit. Delay and timing circuits are common in electronic systems. Electronic clocks, televisions, video recorders and many other devices, all require timers to control their operation. A monostable timer has one stable state and one unstable state. Normally it is in its stable state but can be switched to the unstable state by applying a trigger pulse. It stays in its unstable state for a period of time before returning to its stable state. When triggered it produces a single pulse or signal. An astable timer has no stable state (hence its name "astable" meaning "not stable"). Its output switches from one state to the other automatically. The rate at which it switches is determined by the circuit components. It generates a continuous stream of pulses usually in the form of a square wave. It is also known as an oscillator or pulse generator. At the heart of most timing circuits you will find a resistor and a capacitor which are used to control the timed period. The capacitor is used to store electrical charge, while the resistor controls how fast or slow the charge is stored.
WHAT IS A CAPACITOR ? A capacitor stores electric charge. In its simplest form it consists of two flat, parallel metal plates (conductors), close together, but seperated by an insulating material called a dielectric.
Capacitance: The more charge a capacitor can store the greater is its capacitance (C). The capacitance is large if the plates have a large area and are close together, but capacitance is also affected by the dialectric used. Voltage Rating: This is the maximum voltage (d.c. or peak a.c.) a capacitor can withstand across its plates before the dialectric begins to break down (becomes damaged). The capacitor is then useless. The voltage rating is often marked on the capacitor. The higher the voltage rating the thicker the dialectric has to be.
TYPES OF CAPACITOR Different types are used for different purposes and situations. Fixed (non-polarised): These have a fixed value and are named after their dialectric (Ceramic, Polyester and Polystyrene for instance). They are made with values ranging from 10pF to 1uF. The tolerances on capacitors are usually quite large (between 20% and 50% higher or larger than the stated figure). There is no agreed way of colour coding so the value is usually printed on them (although some polyester capacitors do use the resistor colour code).
Electrolytic (polarised): This type is used where very large, fixed values (up to 100, 000 uF) are required. They are quite compact but have a wide tolerance and must be connected so that the +ve terminal leads to the +ve supply voltage. Connecting the capacitor the wrong way round destroys the very thin layer of dialectric used in its manufacture.
HOW A CAPACITOR WORKS
Electrons have a -ve charge. When a capacitor is connected between the terminals of a battery a positive charge builds up on plate X (because it loses electrons) and a negative charge builds up on plate Y (because it gains electrons). This process is called charging the capacitor. During this charging process a current will flow. Charging stops (and so does the current flow) when the p.d. across plates X and Y equals the p.d. across the supply voltage. In electronics capacitors are often charged and discharged through a resistor. The resistor controls the rate of flow of electric current (and hence the rate of charge or discharge). This charge will gradually leak away if the battery is disconnected. As the capacitor discharges a small leakage current flows. If this leakage current is larger than the charging current the capacitor will never fully charge.
CHARGING and DISCHARGING A CAPACITOR Charging: When S1 is closed the capacitor charges through resistor R1. The charge rate depends on the product of CxR1 and is called the Time Constant (T) of the circuit. If T is large (because C and/or R1 is also large) then charging occurs slowly. The capacitor will charge to 2/3 of the supply voltage in 1 time constant. It will then charge to 2/3 of what is left and so on. In practice the capacitor is fully charged after a period of 5T* (5 time constants). Unless the leakage current is greater than the charge current. Discharging:* If S1 is opened and S2 is closed the capacitor will discharge through R1. The rate at which the capacitor discharges will also depend on the product of C and R1 and after 5T secs it is considered to be fully discharged.

Remember ! the voltage will rise (or fall) to approx 2/3 (63%) of its final voltage in one time constant. It will then rise or fall to 2/3 of the remainder in the next time constant and so on. The capacitor is considered to be fully charged or discharged in 5 time constants.
The time constant T can be found using the formula: The time T is in secs, R is in (K)ohms and C is in (Micro)farads.
USING CAPACITORS TO CREATE A TIME DELAY The circuit shown on the right demonstrates how a capacitor can be used to generate a time delay between pressing the switch and turning on the buzzer with the transistor. Remember the capacitor (C) charges up through the variable resistor (R). In this circuit the transistor will not switch on until the base emitter voltage V_be is raised to 0.7V. This will only happen when the p.d. across the plates of the capacitor is raised to 0.7V. The time this takes is determined by CxR.
Set the value of R at 10K and press the switch to connect the capacitor and resistor. The buzzer will sound almost immediately. Press the switch to discharge the capacitor. Set the value of R at 50K and press the switch to connect the capacitor and resistor. The buzzer will sound after several seconds delay. Press the switch to discharge the capacitor.
555 TIMING CIRCUITS A 555 timer IC can be used to produce a monostable or astable circuit. As with other delay or timer circuits the 555 timer uses the charge or discharge of a capacitor as a means of timing. The 555 is an analogue timer, but works well with many digital circuits. When used with a 5V supply it is compatible with most TTL circuits, but it can operate from a supply as high as 15V and is also suited to CMOS circuits for that reason. It can sink or source up to 200mA
The basis of the 555 is a flip flop with set and reset inputs controlled by comparators. In its untriggered state the flip flop output connected to the base of the transistor is high. This discharges the capacitor connected between pin 7 and 0V and gives a low output due to the inverter at the output stage. The trigger is normally held high by the internal circuitry. When pin 2 is taken below 1/3 V_CC comparator 1 changes state and sets the flip-flop. The transistor is turned off and the output goes high due to the inverter. This allows the capacitor connected to pin 7 to begin to charge. Charging continues until the voltage across the capacitor reaches 2/3V_CC at which point comparator 2 resets the flip-flop turning the transistor back on and discharging the capacitor.
If the trigger is still held low at the end of this sequence the output will remain high until the trigger is released. The timer can be reset at any time by taking pin 4 momentarily low. With the addition of a suitable resistor and timing capacitor it can provide time delays of about one hour. The limiting factor is the leakage current. For long time delays, charge leaks from the capacitor faster than it is replaced so it never charges fully. The maximum realistic value of charging resistor is 10-20 megohm.
Monostable operation
The basic arrangement is shown right R1 and C1 are external components whose values fix the timeT of the single output voltage pulse (or signal) produced at the ouput (pin3). The output pulse is produced when S is pressed and released. The monostable is triggered by the falling edge of the input voltage pulse at pin2. It can be shown that T is given in seconds by the formula: T =1.1 R1 C1 If R1is in Megohms and C1 is in microfarads then T is in seconds.
		Pin 4 (reset) can be used to reset the monostable at any time by momentarily connecting it to 0V. The timing diagram for a 555 monostable is shown on the left. When the trigger pin detects a falling edge, the output goes high. It remains high for the timed period providing the trigger returns to a high state. The output then goes back to its normally low state.
Astable operation.
The basic arrangement is shown on the right. R1, R2 and C1 are external components whose values fix the frequency of the stream of continuous square wave pulses produced automatically at the output (pin 3). The period T of the square wave is given by T =0.7 (R1+2R2)C1 Where again T is in seconds if R is in megohms and C is in microfarads. The frequency f= 1/T
		The duty cycle or mark to space ratio is the ratio of the time the output is on divided by the time the output is off. For a true square wave this is 1. If you look carefully at the diagram above you will see that the timing capacitor charges through both R₁ and R₂ and discharges through R₂ only. This means that a rectangular wave is produced unless R₁ is very small in comparison to R₂.