TIMER PROJECTS

Timer circuit has been used in many projects and there are basically 2 types that are used these days. One of them is the use of analog RC circuit where charging of the capacitor circuit determined the T(time) of the circuitry. This type of circuitry has larger tolerance and is used in applications where the T is not so critical as the T is affected by the tolerance of the RC components used.
The other is the use of crystal or ceramic resonators together with microprocessor, microcontroller or application specific integrated circuit that need higher precision T in the tolerance of up to 5 ppm (parts per million).



555 IC
One commonly used circuit is the 555 IC which is a highly stable controller capable of producing timing pulses. With a monostable operation, the T(time) delay is controlled by one external resistor and one capacitor. With an astable operation, the frequency and duty cycle are accurately controlled by two external resistors and one capacitor. The application of this integrated circuit is in the areas of PRECISION TIMING, PULSE GENERATION, TIMING DELAY GENERATION and SEQUENTIAL TIMING.
A typical 555 IC block diagram is as shown below.

555/556 Monostable

555 monostable output, a single pulse

555 monostable circuit with manual trigger

A monostable circuit produces a single output pulse when triggered. It is called a monostable because it is stable in just one state: 'output low'. The 'output high' state is temporary. The duration of the pulse is called the time period (T) and this is determined by resistor R1 and capacitor C1:

time period, T = 1.1 × R1 × C1

T   = time period in seconds (s)
R1 = resistance in ohms ()
C1 = capacitance in farads (F)
The maximum reliable time period is about 10 minutes.
Why 1.1? The capacitor charges to ²/₃ = 67% so it is a bit longer than the time constant (R1 × C1) which is the time taken to charge to 63%.

• Choose C1 first (there are relatively few values available).
• Choose R1 to give the time period you need. R1 should be in the range 1k to 1M, so use a fixed resistor of at least 1k in series if R1 is variable.
• Beware that electrolytic capacitor values are not accurate, errors of at least 20% are common.
• Beware that electrolytic capacitors leak charge which substantially increases the time period if you are using a high value resistor - use the formula as only a very rough guide!
  

  Astable operation

  With the output high (+Vs) the capacitor C1 is charged by current flowing through R1 and R2. The threshold and trigger inputs monitor the capacitor voltage and when it reaches ²/₃Vs (threshold voltage) the output becomes low and the discharge pin is connected to 0V. The capacitor now discharges with current flowing through R2 into the discharge pin. When the voltage falls to ¹/₃Vs (trigger voltage) the output becomes high again and the discharge pin is disconnected, allowing the capacitor to start charging again.
  This cycle repeats continuously unless the reset input is connected to 0V which forces the output low while reset is 0V.
  An astable can be used to provide the clock signal for circuits such as counters.
  A low frequency astable (< 10Hz) can be used to flash an LED on and off, higher frequency flashes are too fast to be seen clearly. Driving a loudspeaker or piezo transducer with a low frequency of less than 20Hz will produce a series of 'clicks' (one for each low/high transition) and this can be used to make a simple metronome.
  An audio frequency astable (20Hz to 20kHz) can be used to produce a sound from a loudspeaker or piezo transducer. The sound is suitable for buzzes and beeps. The natural (resonant) frequency of most piezo transducers is about 3kHz and this will make them produce a particularly loud sound.
  
  
  
  

  Duty cycle

  The duty cycle of an astable circuit is the proportion of the complete cycle for which the output is high (the mark time). It is usually given as a percentage. For a standard 555/556 astable circuit the mark time (Tm) must be greater than the space time (Ts), so the duty cycle must be at least 50%:
  
  

  Duty cycle  =	Tm	=	R1 + R2
  	Tm + Ts		R1 + 2R2

  
  
  

  555 astable circuit with diode across R2

  To achieve a duty cycle of less than 50% a diode can be added in parallel with R2 as shown in the diagram. This bypasses R2 during the charging (mark) part of the cycle so that Tm depends only on R1 and C1: Tm = 0.7 × R1 × C1   (ignoring 0.7V across diode)
  Ts  = 0.7 × R2 × C1   (unchanged)
  
  
  

  Duty cycle with diode  =	Tm	=	R1
  	Tm + Ts		R1 + R2

  Use a signal diode such as 1N4148.
  
  
  
  
  
  OPAMP AS TIMER
  
  
  At switch on, the voltage across the capacitor is zero and the output is at +12 volts.
  The buzzer is not energised.
  After a time, determined by the values of C and R3, the voltage of the inverting input rises above that of the non inverting input.
  The output goes to minus 12 volts and the buzzer is energised.