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An amplifier to drive low to medium impedance headphones built using discrete components.

Both halves of the circuit are identical. Both inputs have a dc path to ground via the input 47k control which should be a dual log type potentiometer. The balance control is a single 47k linear potentiometer, which at center adjustment prevents even attenuation to both left and right input signals. If the balance control is moved towards the left side, the left input track has less resistance than the right track and the left channel is reduced more than the right side and vice versa. The preceding 10k resitors ensure that neither input can be "shorted" to earth.

Amplification of the audio signal is provided by a single stage common emitter amplifier and then via a direct coupled emitter follower. Overall gain is less than 10 but the final emitter follower stage will directly drive 8 ohm headphones. Higher impedance headphones will work equally well. Note the final 2k2 resistor at each output. This removes the dc potential from the 2200u coupling capacitors and prevents any "thump" being heard when headphones are plugged in. The circuit is self biasing and designed to work with any power supply from 6 to 20 Volts DC.

Why Class A ? Because , when biased to class A, the transistors are always turned on, always ready to respond instantaneously to an input signal. Class B and Class AB output stages require a microsecond or more to turn on. The Class A operation permits cleaner operation under the high-current slewing conditions that occur when transient audio signal are fed difficult loads. His amplifier is basically simple, as can be seen from the block diagram.

The circuit described here counts the number of times that an infrared beam is interrupted. It could be used to count the number of people entering a room, for instance, or how often a ball or another object passes through an opening (handy for playing shuffleboard). The heart of the circuit consists of - you guessed it - a light gate! Diode D1 is an IR diode that normally illuminates IR transistor T1. The light falling on T1 causes it to conduct to a certain extent. The resulting voltage on the collector of T1 should be just low enough to prevent the following transistor (T2) from conducting. This voltage can be adjusted within certain limits using P1.

As soon as an object comes between D1 and T1, the light shining on T1 will be partially or fully blocked, causing the IR transistor to conduct less current. As a result, the voltage on its collector will increase, producing a brief rise in the voltage on the base of T2. This will cause T2 to conduct and generate a negative edge at IC1. This negative edge will trigger the monostable multivibrator, which will then hold the output signal on pin 3 ‘high’ for a certain length of time (in this case, one second). At this point, two things will occur. First, a buzzer will be energized by the output of IC1 and produce a tone for approximately one second.

When the buzzer stops, a negative edge will be applied to the clock input of IC2, causing the counter in IC2 to be incremented by 1. IC2 is conveniently equipped with an internal binary-to-BCD decoder, so its outputs only have to be buffered by IC3 and T3 to allow the state of the counter to be shown on the 7-segment display. Switch S1 can be used to reset the counter to zero. If a one-second interval does not suit your wishes, you can modify the values of R3 or C1 to adjust the time. Increasing the value of R3 lengthens the interval, and decreasing it naturally shortens the interval.

The same is true of C1. When building the circuit, make sure that T1 is well illuminated by the light from D1, while at the same time ensuring that T1 ‘sees’ as little ambient light as possible. This can best be done by fitting T1 in a small tube that is precisely aimed toward D1. The longer the tube, the less ambient light will reach T1. The sensitivity of the circuit can be adjusted using P1.

This circuit diagram shows you how to make a 5V to 12V variable DC power supply from a fixed 5V regulator IC 7805. This is attained by adding two resistors R1 and R2 as shown in figure. When the resistors R1 and R2 are added the equation for the output voltage of 7805 becomes Vout= Vfixed + { R2 [ (V fixed/R1) + Istandby] } ,where Vfixed=5V and Istandby=Vfixed/R1.By varying the POT R2 you can adjust the output voltage between 5V and 12V.

Notes. * The circuit can be assembled on a vero board. * T1 can be a 230V primary, 9V/5A secondary stepdown transformer. * 7805 must be fitted with a heat sink. * F1 can be a 1A fuse.

The 8051 microcontroller has two
independent 16 bit up counting timers named Timer 0 and Timer 1 and this
article is about generating time delays using the 8051 timers.
Generating delay using pure software loops have been already discussed
here but such delays are poor in accuracy and cannot be used in
sensitive applications. Delay using timer is the most accurate and
surely the best method.

Designing a delay program using 8051 timers.

While
designing delay programs in 8051, calculating the initial value that
has to be loaded inot TH and TL registers forms a very important thing.
Let us see how it is done.

• Assume the processor is clocked by a 12MHz crystal.
• That means, the timer clock input will be 12MHz/12 = 1MHz
• That means, the time taken for the timer to make one increment = 1/1MHz = 1uS
• For a time delay of “X” uS the timer has to make “X” increments.
• 2^16 = 65536 is the maximim number of counts possible for a 16 bit timer.
• Let TH be the value value that has to be loaded to TH registed and TL be the value that has to be loaded to TL register.
• Then, THTL =  Hexadecimal equivalent of (65536-X) where (65536-X) is considered in decimal.

Example.

Let the required delay be 1000uS (ie; 1mS).
That means X = 1000
65536 – X =  65536 – 1000 = 64536.
64536 is considered in decimal and converting it t0 hexadecimal gives FC18
That means THTL = FC18
Therefore TH=FC and TL=18

Program for generating 1mS delay using 8051 timer.

DELAY: MOV TMOD,#00000001B // Sets Timer 0 to MODE1 (16 bit timer). Timer 1 is not used
       MOV TH0,#0FCH // Loads TH0 register with FCH
       MOV TL0,#018H // LOads TL0 register with 18H
       SETB TR0 // Starts the Timer 0
HERE: JNB TF0,HERE // Loops here until TF0 is set (ie;until roll over)
      CLR TR0 // Stops Timer 0
      CLR TF0 // Clears TF0 flag
      RET

The above delay routine can be looped twice in order to get a 2mS delay and it is shown in the program below.

MAIN: MOV R6,#2D
LOOP: ACALL DELAY
      DJNZ R6,LOOP
      SJMP MAIN

DELAY: MOV TMOD,#00000001B
       MOV TH0,#0FCH
       MOV TL0,#018H
       SETB TR0
HERE: JNB TF0,HERE
      CLR TR0
      CLR TF0
      RET

Few points to remember while using timers.

• Once timer flag (TF) is set, the programmer must clear it before it can be set again.
• The timer does not stop after the timer flag is set. The programmer must clear the TR bit in order to stop the timer.
• Once
  the timer overflows, the programmer must reload the initial start
  values to the TH and TL registers to begin counting up from.
• We can configure the desired timer to create an interrupt when the TF flag is set.
• If  interrupt is not used, then we have to check the timer flag (TF) is set using some conditional branching instruction. 
• Maximum
  delay possible using a single 8051 timer is 65536µS and minimum is 1µS
  provided that you are using a 12MHz crystal for clocking the
  microcontroller.

Square wave generation using 8051 timer.

1KHz Square wave using 8051 timer.

MOV P1,#00000000B
MOV TMOD,#00000001B
MAIN: SETB P1.0
      ACALL DELAY
      CLR P1.0
      ACALL DELAY
      SJMP MAIN
DELAY: MOV TH0,#0FEH
       MOV TL0,#00CH
       SETB TR0
HERE: JNB TF0,HERE
      CLR TR0
      CLR TF0
      SETB P1.0
      RET
      END 

2 KHz Square wave using 8051 timer.

MOV P1,#00000000B
MOV TMOD,#00000001B
MAIN: SETB P1.0
      ACALL DELAY
      CLR P1.0
      ACALL DELAY
      SJMP MAIN
DELAY: MOV TH0,#0FCH
       MOV TL0,#018H
       SETB TR0
HERE:JNB TF0,HERE
     CLR TR0
     CLR TF0
     SETB P1.0
RET
END

10 KHz square wave using 8051 timer.

MOV P1,#00000000B
MOV TMOD,#00000001B
MAIN: SETB P1.0
      ACALL DELAY
      CLR P1.0
      ACALL DELAY
      SJMP MAIN
DELAY: MOV TH0,#0FFH
       MOV TL0,#0CEH
       SETB TR0
HERE:JNB TF0,HERE
     CLR TR0
     CLR TF0
     SETB P1.0
RET

 END