Over Voltage Protector Circuit Using SCR

A silicon-controlled rectifier is installed in parallel with 12v line and connected to a normally-closed 12v relay, RL1. The SCR’s gate circuit is used to sample the applied voltage. As long as the applied voltage stays below a given value, SCR1 remains off and RL1’s contacts remain closed, thereby supplying power to the load. When the source voltage rises above 12V, sufficient current is applied to the gate of SCR1 to trigger it into conduction. The trigger point of SCR1 is dependent on the setting of R1. Once SCR1 is triggered (activating the relay), RL1’s contacts open, halting current flow to the load.

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Infrared Remote Control Switch Circuit

Remote controls, specially cordless type, are very popular nowadays. Here is a simple and cost-effective cordless remote control circuit which is based on infrared rays.

Figs 1and 2 shows transmitter and receiver circuit respectively.The transmitter produces infrared rays that can be easily transmitted up to four metres with a special convex lens or a twin LED arrangement.

IC1 (741) in the transmitter is wired as a high frequency square wave oscillator which provides the gate pulse for SCR1. As soon as output current of IC1 flows through SCR1 (SN050 or equivalent), it conducts and enables the LED to emit infrared rays. Output frequency of IC1 can be varied with the help of VR1, which in turn varies the output radiations of the LED.

When infrared rays fall on phototransistor T1 of the receiver, it produces charge carriers at a rate depending on the rate of arrival of incident radiations at the pn junction of the transistor. The resulting emitter voltage is amplified by using IC2 and rectified by D2. The signal is amplified by T2 to drive the relay.

By vary VR2, one can match detection freguency of the receiver with transmiting frequency of the transmitter.

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SCR TESTER CIRCUIT

The Device under test cathode, anode and gate are connected to the unit’s CATHODE, ANODE and GATE terminals, respectively.

Pressing switch S1 feeds a gate current to the DUT (Device Under Test), which triggers it on. Resistor R1 limits the gate current to the appropriate level. Resistor R3 limits the current through the LED to about 20 mA which, with the  current through R2, results in a latching current of about 110 mA. The LED is used to monitor the latching current. IF the DUT is good, once the gate is triggered with S1, the LED will remain light, indicating that the device is conducting. To end the test, turn off the device by interrupting the latching current flow using switch S2. The LED should turn off and remain off. The preceding procedure will work SCRs and Triacs. To check LED and other diodes, connect the anode and cathode leads to the anode and cathode of the diode; LED should light. When the leads are reversed, the LED1 should remain off.

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Capacitance Meter Circuit Using 555

It directly reads capacitance in the range 100pF to 10µF. IC1 and IC2 operate as an astable (with frequency above 80Hz) and as a monostable multivibrator respectively. Time period of the monostable multivibrator is determined by the resistors (R3 to R7 and VR1 to VR5) selected by switch S1 and unknown capacitance Cx.

By using high accuracy multi-turn presets the capacitance meter can be calibrated for the different ranges. Adjust VR6 so that the meter gives full scale deflection. Now the meter is ready for the calibration.

Cx should be 3.3µF, 0.33µF, 0.033µF, 3300pF, 330pF in ranges 1, 2, 3, 4 and 5 respectively for calibration. In each range the corresponding preset is adjusted to read 3.3 on the meter. Once presets have been set they should not be disturbed.

Electrolytic capacitor should not be used because of their wide tolerances. Supply should be well regulated.

Unknown capacitor is placed in the position of Cx and S1 is rotated to get a clear reading. It is not possible to go above 10µF as value of presets goes below IC555 specifications.

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Toy DC Motor Control Circuit With SPEED, INERTIA, BRAKE, CRUISING Controller

ere is a versatile project to control the speed of a small electric motor and also to bring it to a dead stop instantaneously. Provision is also made to let it cruise along at a slow speed if so desired.

The circuit is built around two of the most popular ICs : 556 and 741. IC556 is a dual timer whereas IC741 is an Operational Amplifier.

The voltage to start the motor is provided by the charge on C6. The rate at which this capacitor gets charged is determined by the INERTIA control VR4. So, to start the motor, decrease the resistance of the speed control VR1. The Motor will accelerate at a rate determined by the inertia control. If the speed control is suddenly increased to its maximum value, the motor speed will keep decreasing till it comes to rest after a minute or so. To bring the motor to rest quickly, the speed control resistance is set at maximum, and then use the BRAKE control. As the brake control is adjusted the rate of deceleration will depend upon the inertia control.

The slow cruising speed of the motor can be adjusted by varying the CRUISING speed control.

thus, the gadget has the following controls : SPEED (VR1), INERTIA (VR4), BRAKE (VR2) and CRUISING mode (VR3).

How it works?

The voltage on C8 is determined by VR1 via D1 and VR4. The voltage decreases slowly via R5 and R6 when the speed control is brought to its minimum position, but this voltage will decrease quickly via VR4 and VR2.

The switch S1 is provided to bring the brake control in circuit. When the motor is running at full speed, keep S1 open.

The dual timer 556 (IC1) functions in two modes. In the first mode it functions as a bistable oscillator producing 100Hz negative pulses at pin 5. In the second mode it functions as a monostable oscillator which is triggered by voltage pulses at pin 8. Pin 5 is connected to pin 8. Therefore the output of the timer consist of positive pulses and are available at pin 9. The width of these pulses is controlled by VR3.

Diodes D2 and D3 form an OR GATE which allows either the voltage on C6 or the positive pulse voltages of the timer (pin 9), to IC2, depending on which of two types of voltages happens to be higher in magnitude.

IC2 is the operational amplifier 741. It functions as a unity-gain voltage follower. The output of this IC is applied to the Darlington-Pair amplifier Q1 and Q2. This amplifier provides current about 800mA to operate the motor.

Setting up is very simple. Connect a suitable voltmeter to the output (pin6) of IC2. The meter should read about 1.5V. Slowly increase the speed control and the voltage on pin 6 should rise gradually to a maximum of about 12 volts. Next, set the speed control to minimum : the output of IC2 should remain at 12 volts for a minute or so. Now operate the brake control fully; the voltage on pin 6 should decrease to about 1 volt.

To check the operation of the slow cruising speed control, shut the speed control, and apply full brakes to ensure that C6 is fully discharged. Now, operate VR3 slowly. The Voltage at pin 6 of IC2 should rise from about 1 volt to 5 volts.

Ensure that Q1 and Q2 are mounted on proper heat sinks, since these are power transistors.

Note: If the voltage settings indicated above are not easily discernible, change the values of C2 and C5 to 100 microfarad temporarily. After setting up, replace the original capacitors as shown in figure.

This project can be utilized to operate an electric motor-driven toy train as well. Connect the output of Q1 and Q2 one rail and the negative terminal to the other rail. The motor connection, then, should also be made correctly to the rails.

The power supply is designed to separate the Control Supply from the traction supply by means of IC3.

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Voltage Controlled Volume circuit

his volume control circuit offers an unusual approach to the well-known problem of distortion in active-device attenuators. The zero-output in this case is obtained by allowing equal signals of opposite phase to cancel each other.

The input transistor operates as a ‘concertina’ phase-splitter, producing equal-amplitude opposite-phase voltages at its collector and emitter. The two signals can be brought into complete cancellation, at the summing point, by means of preset p1. The harmonic content of the two signals is very small, but not quite identical.

There will therefore actually be an even smaller distortion output at the nominally ‘zero’ point.

If something now happens to the amplitude ratio of the signals at the summing point, there will be output passed to the buffer-stage. The necessary unbalance is achieved by means of the JFET and capacitor C2. The gate bias on the FET is set by the DC control voltage applied to point A. With this voltage close to zero the FET will be cut off, so that the above mentioned cancellation takes place. As this voltage is increased there will come a point at which the channel starts to ‘bleed off’ AC collector current from the splitter; this will upset the  balance and so cause an output signal to appear. The more conductive the FET, The more output. Unfortunately, the more channel current there flows the lower will be negative gate bias and so the greater will be the distortion of the ‘regulated’ summing component.

The trick is now to employ only a moderate degree of unbalance – so that the FET operates at low distortion percentages. The process is helped also by the always present ‘clean’ summing component. The buffer stage provides gain, so that a sufficient output level is obtained. The Circuit’s frequency response extends from 50Hz to 35kHz (-3dB points). The input voltage should be limited to 100mV p-p: the output can be varied from ‘0′ to 1V p-p (by using the appropriate range for the control voltage at A).

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MOSFET Power Amplifier Circuit diagrams

Two complementary MOSFETs are used to deliver 20W into 8Ω.

T3

T4

The MOSFETs should be heatsinked with a heatsink of better than 5°C/W capability.

A TL071 op amp is used as an input amplifier.

THD is less than 0.15% from 100Hz to 10kHz.

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Parabola Calculator for Parabolic Satellite Dish Antenna Design
This Freeware program was written to help you design solar collector or wifi projects using parabolic reflectors. This program calculates the focal length and (x, y) coordinates for a parabola of any diameter and depth. It can help you determine what size and shape to make your parabola very quickly. Version 2 includes Wifi calculations for centered or offset feedhorn dishes.

Download

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Bass Treble Tone Control Circuit

Bass and treble circuits can be combined to form a two control tone adjust circuit, as shown here.

VR1 for Bass Control

VR2 for Treble Control

Bass and Treble controls of about ±10 dB boost or cut. It should be useful in a wide variety of situations.

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TDA7000 FM Radio Receiver Circuit Using Tuning Capacitor
GENERAL DESCRIPTION The TDA7000 is a monolithic integrated circuit for mono FM portable radios or receivers where a minimum on peripheral components is important (small dimensions and low costs).
The IC has an FLL (Frequency-Locked-Loop) system with an intermediate frequency of 70 kHz. The i.f. selectivity is obtained by active RC filters. The only function which needs alignment is the resonant circuit for the oscillator, thus selecting the reception frequency. Spurious reception is avoided by means of a mute circuit, which also eliminates too noisy input signals. Special precautions are taken to meet the radiation requirements.

The TDA7000 includes the following functions:
· R.F. input stage
· Mixer
· Local oscillator
· I.F. amplifier/limiter
· Phase demodulator
· Mute detector
· Mute switch

This circuit is typical using with a  LM386 for the audio power amplifier.

With a minimum on peripheral components we can build a high performance and small FM radio receiver .

TDA7000 FM Receiver Coils

L1:Tuning Coil, 5¾ (5.75) Turns of 23 swg enamelled copper wire close-wound on a 3mm diameter.	L2: Antenna Coil, 4¾ (4.75) Turns of 23 swg enamelled copper wire close-wound on a 3mm diameter.

TDA7000 IC

TDA7000 FM Radio Using Tuning Capacitor

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