Build a PC-Based Timer Circuit Diagram

Timers are very useful both for industrial applications and household appliances. Here is a PC-based timer that can be used for controlling the appliances for up to 18 hours. For control, the timer uses a simple program and interface circuit. It is very cost-effective and efficient for those who have a PC at workplace or home. The tolerance is ±1 second.

The circuit for interfacing the PC’s parallel port with the load is very simple. It uses only one IC MCT2E, which isolates the PC and the relay driver circuits. The IC prevents the PC from any short circuit that may occur in the relay driver circuit or appliance. The glowing of LED1 indicates that the appliance is turned on. Transistor BC548 is used as the relay driver.

The program code is written in ‘C’ language and compiled using ‘Turbo C’ compiler. When the program is run, it prompts the user to input the time duration in seconds or minutes to control the appliance. After entering the required timing, press any key from the keyboard.

Suppose you input the total duration as ‘x’ minutes, of which ‘on’ and ‘off’ durations are ‘y’ and ‘z’ minutes, respectively. The program will repeat the on-off cycle for x/(y+z) number of times. After completion of the total time, to repeat the cycle, you will have to reset the time in the program to activate the circuit.

PC-Based Timer Circuit Diagram

The program uses two bytes for storing integer type data. So when input is given in terms of seconds or minutes, it can hold 216–1=65,535 seconds or 18 hours at the maximum. The sleep() function in the program is used to hold the appliance in ‘on’ or ‘off’ condition for the ‘on’ and ‘off’ periods as entered by the user against prompts. The sound() function is used to give a beep during ‘on’ condition of the appliance.

EFY note. The source code and executable file of this program have been included in this month’s EFY-CD.

Sourced By: EFY Author Akshy

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Whistle to Call Dogs Dog Caller Circuit Diagram

Dog trainers use a whistle to call dogs. But why blow that irritating, loud whistle when the dog can hear a sound inaudible to the humans? We the humans can hear up to 20 kHz, but dogs can hear ultrasound (sound ranging between 20 and 30 kHz) also. Here’s a circuit that generates 21 to 22 kHz (frequencies just above the audible range), so it can be used to call your pets by generating ultrasonic sound.

Whistle to Call Dogs Dog Caller Circuit Diagram

IC 555 is used as an oscillator. By adjusting the preset, ultrasonic sound of 21-22kHz frequency can be generated. Whistle effectiveness depends on the speaker used. Use of a low-wattage tweeter is recommended. (Don’t use an ultrasonic transducer, because it is designed for 40 kHz only.)

The circuit works off 9V. For portability, use a 9V PP3 battery and house the unit inside a pocket radio cabinet.

Author: Pradeep G  www.electronicsforu.com

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Simple IC 555 Timer Tester Circuit Diagram

This is a Simple IC 555 Timer Tester Circuit Diagram. This simple and easy-to-use gadget not only tests the IC 555 timer in all its basic configurations but also tests the functionality of each pin of the timer. Once a timer is declared fit by this gadget, it will function satisfactorily in whatever mode or configuration you may try it. The two basic configurations in which a timer IC 555 can be used are the astable and the monostable modes of operation. 

When the DPDT switch (S2) is in position 1-1, the timer under test automatically gets wired as a monostable multivibrator. In this case, the monoshot can be triggered by the microswitch (S1). The debouncing circuit constituted by the two NAND gates of IC1 (N1 and N2) produces a clean rectangular pulse when the microswitch is pressed. Resistor R3, capacitor C1 and diode D1 ensure that the trigger terminal of timer IC 555 (pin 2 is the trigger terminal) gets the desired positive-to-ground trigger pulse. This differentiator circuit also ensures that the width of the trigger pulse is less than the expected monoshot output pulse. 

Simple IC 555 Timer Tester Circuit Diagram

The monoshot output pulse width is a function of the series combination of resistor R8 and potentiometer VR2, and capacitor C4. When DPDT switch S2 is in position 2-2, the timer gets configured for the astable mode of operation. The output is a pulse train with the high time period determined by the series combination of resistors R8, potentiometer VR2, resistor R9 and capacitor C4, whereas the low time period is determined by resistor R9 and capacitor C4.

The reset terminal of timer IC (pin 4) should be tied to Vcc normally. More precisely, the voltage at pin 4 should be greater than 0.8V. A voltage less than that resets the output. Whether you have connected the timer in the monoshot or astable mode of operation, the output goes low the moment you bring the reset terminal below 0.8V.

The control terminal (pin 5) can be used to change the high time (‘on’ time) of the output pulse train in the astable mode and the output pulse width in the monoshot mode by applying an external voltage. This external voltage basically changes the reference voltage levels of the comparators inside the IC. The levels are set by three identical resistors of usually 5 kilo-ohms inside the IC connected from Vcc to ground, at 2/3Vcc for pin 5 and 1/3Vcc for pin 2. These levels can be changed by connecting an external resistor between pin 5 and ground. Resistor R10 and potentiometer VR3 have been connected for this purpose.

The pulse width in the monoshotmode is given by:
1.1×total charging resistance×charging capacitance

This expression is valid when there is no external resistor connected at pin 5. The pulse width can be reduced by connecting an external resistor.

The high and low time periods in the astable mode are:
High time period = 0.69×chargingresistance×charging capacitance
Low time period = 0.69×dischargeresistance×capacitance

Again the expressions are true with no external resistor at pin 5. The high time period can be made to decrease by connecting an external resistor between pin 5 and ground.

The circuit can thus be used to check:
1. The timer IC in astable configuration.
2. The timer IC in monostable configuration.
3. The capability of the reset terminal to override all functions and rest the output to low.
4. The function of the control terminal to change the ‘on’ or the ‘high’ time of the output waveform in astable mode of operation and the output pulse width in monostable mode of operation.

The circuit operates off a 9V battery, which makes the gadget portable. You can construct it easily on any general-purpose PCB along with the 8-pin socket.

To test an IC 555:
1. Insert it into the socket.
2. Set switch S2 in position 1-1.
3. Switch on the power supply by flipping switch S3 to ‘on’ position. Power-indicator LED (LED3) glows to indicate that the circuit is ready to test the IC timer.
4. If the IC is okay, LED1 glows because the IC is wired as a monoshot and in the absence of any trigger, its output is low.
5. Apply the trigger pulse by momentarily pressing switch S1. LED1 stops glowing and, in turn, LED2 glows. This confirms that the output of the monoshot has gone high. After the predetermined time period, LED2 goes off and LED1 again glows. Vary preset VR2 and trigger the monoshot again through switch S1. You will find that LED2 glows this time for a longer or a smaller time period depending upon whether you increased or decreased VR2 resistance.
6. For checking the reset function of the timer, trigger the monoshot again, and before the expected time is over, quickly decrease the potmeter VR1 resistance so as to bring the voltage at pin 4 below 0.8V. You will observe the output going low (indicated by glowing LED1 and extinguished LED2).
7. For checking the control function of the timer IC, set potmeter VR1 again in the maximum resistance position. Also set preset VR3 in the minimum resistance position. Trigger the monoshot using switch S1. You’ll observe its output going high for a time period that is much less than that determined from the series combination of R8 and VR2, and capacitor C4. In fact, for any fixed setting of this series combination, the output pulse width can be observed to vary for different values of potmeter VR3 resistance—by triggering the monoshot several times, once for each setting of VR3.
8. Now set the DPDT switch in position 2-2. LED1 and LED2 glow alternatively with the timing determined by the resistances in the charge and discharge paths. This means the timer IC is okay and wired in astable mode.
9. The functions of reset and control pins can be checked in astable configuration too in the same way as discussed above for the monoshot configuration.

Sourced By: EFY. Author:  Raj K. Gorkhali

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One Condition Trimming Circuit Diagram

One Condition Trimming Circuit Diagram. This relatively simple, inexpensive circuit requiring one trimming operation can multiply or divide with a consistent accuracy of greater than 1 part in 1,000. An inexpensive CMOS version of standard 555 timer chip T, in conjunction with low-drift LMll error amplifier A3, an inexpensive analog chopper switch SW, form a unique voltage-to-duty-cycle converter to produce the difficult transfer function necessary for accurate conversion. Read: Use 555 Build Spaceship Alarm

 One Condition Trimming Circuit Diagram

An unknown multiplicand voltage applied to the A3 error op amp circuit`s Y input controls the duty cycle of the timer through its pin 5 modulation input. The network between the sink-and-source output of the timer, pin 3, and the state trigger inputs, pins 2 and 6, cause the timer to oscillate. An error feedback signal from the timer`s discharge output, pin 7, represents the duty cycle. Integrating this duty-cycle signal with voltage reference REF representing full scale, and applying the result to the inverting input of A3, closes the feedback loop and insures high accuracy. Read: Rf Probe Circuit Diagram For vtvm

Multiplier X feeds into another LMll op amp, A1, which acts as a input buffer and scaler. A third LMll, A2, filters and buffers the Z output. Between A1 and A2, the timer`s duty-cycle output modulates the analog switches of a CD4066 to achieve the desired multiplier output. To perform division instead of multiplication, reconfigure the op amp A1 circuit with the use of jumpers. Amplifier A2 isn`t required in the division configuration. To calibrate the circuit, connect the X andY inputs together and apply 10 V. Read: DC to AC Inverter by IC 555

Then adjust the 10-turn span potentiometer to achieve a 10-V output at Z for multiplication, or 1 V for the division configuration. Also check for zero output at a zero multiplier input. The circuit is scaled for 0 -10 V inputs and outputs with a small overrage capability, but other scalings are possible. Star grounding or a heavy ground bus should be used to reduce offset problems that are unavoidable in this design. Sourced By : Circuitsstream

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