Low-Cost Intercom

The intercom circuit described here uses two transistors, an audio transformer, and a few passive components in addition to condenser microphone and low-wattage speaker (refer Fig. 1). The complete unit can be made on a general-purpose veroboard. The microphone signals are amplified by a two-stage transistor amplifier, while the speaker is driven through an audio output transformer (similar to the one used in transistor radios).

Low-Cost Intercom
When ring button (push-to-on switch S1) is pressed, capacitor C3 gets connected between the base of transistor T2 and the top end of primary winding of audio output transformer. As a result, the amplifier circuit wired around transistor T2 gets converted into a Hartley oscillator and produces an audible tone for call-bell. To build a two-way intercom set, make two identical units with the speaker of each circuit installed near the other unit as shown in Fig. 2.
Read The Rest...

Microphone For Computer

Buying a microphone for a computer is costly. Especially when there is a need to have two microphones—one for modem and another for sound card—or if the present microphone is not working properly and needs to be replaced, you are likely to feel the burden of extra cost. Here is a low-cost microphone circuit that comes within your budget. All sound cards and modems have a socket for microphone that is in compatible with stereo jack pins. The stereo socket takes condenser microphone as input and provides the necessary positive voltage for a condenser microphone.

Microphone For Computer circuit diagram
Before building the full circuit, connect three wires to the jackpin, switch on the computer, and insert the jack pins; into the socket of the sound card. With the help of a multimeter, find out the positive terminal out of the three wires. There exists a potential difference of 4V or so between the positive and ground terminals. The third terminal will obviously be for the signal input. The positive terminal is used for biasing the condenser microphone. After identifying all the terminals, connect them as shown in the accompanying circuit diagram.
Read The Rest...

Precision Inductance and Capacitance Meter

This circuit for measurement of inductance and capacitance can be used to test whether the values of inductors and capacitors quoted by the manufacturer are correct. The principle used in the circuit is based on the transient voltages produced across inductors and capacitors connected as series R-L and R-C networks, respectively, across a constant voltage source. The time constant for R-C and R-L networks is given by the relationships t=RxC and L/R, respectively, where resistance R is in ohms, capacitance C in Farads, inductance L in Henries, and time t in seconds.

Precision Inductance and Capacitance Meter
The voltage across capacitor in R-C network rises exponentially to 0.632 of the applied voltage and voltage across inductor in R-L network degrades exponentially to 0.368 of the applied voltage in one RxC and one L/R time (referred to as time constant T of the combination), respectively. When the inductor/capacitor under test is connected across terminals A and B shown in the circuit, it is discharged through the normally-closed contacts of two-way push-to-on/off switch S1.

Precision Inductance and Capacitance Meter
When switch S1 is pushed, the capacitor’s voltage begins to grow (or the inductor’s voltage begins to drop). Simultaneously, the output of timer 555 IC, which is wired as an astable multivibrator, is passed through NOR gates N1 and N2 and applied to the counter circuit. When the time constant (one CxR or one L/R, as the case may be) reaches, gate N2 is inhibited as its pin 2 goes high and the counter circuit freezes. Mode switch S2 is to be kept in position ‘a1’ for capacitance measurement and in position ‘a2’ for inductance measurement. As series resistance R1 is 1 kilo-ohm, the capacitance value is given by the relationship C=Tx10–3 while the inductor value is given by the relationship L=Tx103 .

The time period (1/frequency) of timer 555 (IC2) is adjusted for 1 ms and 1 µs in ‘b1’ and ‘b2’ positions, respectively, of the range switch. The values of capacitors and inductors covered in each range, together with displayed values, are shown in the table. From the table it is obvious that this circuit can measure capacitance from 1 nF to 9,999 µF and inductance from 1 mH to 9999 H. While presets VR1 and VR2 are to be adjusted for the in-circuit value of 1.717 kilo-ohm each, the in-circuit value of preset VR3 is close to 4.7 kilo-ohm. If a regulated +5V is not used, the measurement of capacitance and inductance will be imprecise.

Given below are some important points to be taken care of:
  1. The position of mode-select switch S2 and range-select switch S3 should be changed before switch S1 is pressed.
  2. If the circuit is allowed to function until it displays a constant value, the maximum time taken for measurement will be 10 seconds.
  3. When mode-select switch S2 is in position a1, capacitances can be measured, and when it is in position a2, inductances can be measured.
  4. When range-select switch S3 is in position ‘b1’, the output of 555 IC will have a time period of 1 ms (frequency = 1 kHz), and when it is in position ‘b2’, the output of 555 IC will have a time period of 1 µs. (EFY lab note. The guaranteed frequency of NE555 is limited to 500 kHz, and hence it may not be possible to get 1µs period. One may therefore use a 2nF capacitor to get a period of 2 µs and multiply the displayed value by 2, in b2 range.)
  5. Use a breadboard for connecting inductors or capacitors across terminals A and B.
  6. Using both the ranges for measuring an inductor or capacitor enables one to obtain the accurate value. For example, a 4.7µF capacitor will display only 4 µF when measured in range b1 , while in b2 range it will display 4700 nF (or 4.7 µF).
  7. Don’t press switch S1 before inserting the capacitor or the inductor between terminals A and B.
Read The Rest...

Under-Over-Voltage Beep for Manual Stabiliser

Manual stabilisers are still popular because of their simple construction, low cost, and high reliability due to the absence of any relays while covering a wide range of mains AC voltages compared to that handled by automatic voltage stabilisers. These are used mostly in homes and in business centres for loads such as lighting, TV, and fridge, and in certain areas where the mains AC voltage fluctuates between very low (during peak hours) and abnormally high (during non-peak hours). Some manual stabilisers available in the market incorporate the high-voltage auto-cut-off facility to turn off the load when the output voltage of manual stabiliser exceeds a certain preset high voltage limit.

The output voltage may become high due to the rise in AC mains voltage or due to improper selection by the rotary switch on manual stabiliser. One of the major disadvantage of using a manual stabiliser in areas with a wide range of voltage fluctuations is that one has to keep a watch on the manual stabiliser’s output voltage that is displayed on a voltmeter and keep changing the same using its rotary switch. Or else, the output voltage may reach the preset auto cut-off limit to switch off the load without the user’s knowledge. To turn on the load again, one has to readjust the stabiliser voltage using its rotary switch. Such operation is very irritating and inconvenient for the user.

Under-Over-Voltage Beep for Manual Stabiliser
This under-/over-voltage audio alarm circuit designed as an add-on circuit for the existing manual stabilisers overcomes the above problem. Whenever the stabiliser’s output voltage falls below a preset low-level voltage or rises above a preset high-level voltage, it produces different beep sounds for ‘high’ and ‘low’ voltage levels—short-duration beeps with short intervals between successive beeps for ‘high’ voltage level and slightly longer-duration beeps with longer interval between successive beeps for ‘low’ voltage level. By using these two different types of beep sounds one can readily readjust the stabiliser’s AC voltage output with the help of the rotary switch.

There is no need of frequently checking voltmeter reading. It is advisable to preset the high-level voltage 10V to 20V less than the required high-voltage limit for auto-cut-off operation. Similarly, for low level one may preset low-level AC voltage 20V to 30V above minimum operating voltage for a given load. The primary winding terminals of step-down transformer X1 are connected to the output terminals of the manual stabiliser. Thus, 9V DC available across capacitor C1 will vary in accordance with the voltage available at the output terminals of the manual stabiliser, which is used to sense high or low voltage in this circuit.

Transistor T1 in conjunction with zener diode ZD1 and preset VR1 is used to sense and adjust the high-voltage level for beep indication. Similarly, transistor T2 along with zener ZD2 and preset VR2 is used to sense and adjust low voltage level for beep indication. When the DC voltage across capacitor C1 rises above the preset high-level voltage or falls below the preset low-level voltage, the collector of transistor T2 becomes high due to non-conduction of transistor T2, in either case. However, if the DC voltage sampled across C1 is within the preset high- and low-level voltage, transistor T2 conducts and its collector voltage gets pulled to the ground level.

These changes in the collector voltage of transistor T2 are used to start or stop oscillations in the astable multivibrator circuit that is built around transistors T3 and T4. The collector of transistor T4 is connected to the base of buzzer driver transistor T5 through resistor R8. Thus when the collector voltage of transistor T4 goes high, the buzzer sounds. Preset VR3 is used to control the volume of buzzer sound. In normal condition, the DC voltage sampled across capacitor C1 is within the permissible window voltage zone. The base of transistor T3 is pulled low due to conduction of diode D2 and transistor T2.

As a result, capacitor C2 is discharged. The astable multivibrator stops oscillating and transistor T4 starts conducting because transistor T3 is in cut-off state. No beep sound is heard in the buzzer due to conduction of transistor T4 and non-conduction of transistor T5. When the DC voltage across capacitor C1 goes above or below the window voltage level, transistor T2 is cut off. Its collector voltage goes high and diode D2 stops conducting. Thus there is no discharge path for capacitor C2 through diode D2. The astable multivibrator starts oscillating. The time period for which the beep is heard and the time interval between two successive beeps are achieved with the help of the DC supply voltage, which is low during low-level voltage sampling and high during high-level voltage sampling.

The time taken for charging capacitors C2 and C3 is less when the DC voltage is high and slightly greater when the DC voltage is low for astable multivibrator operation. Thus during low-level voltage sensing the buzzer beeps for longer duration with longer interval between successive beeps compared to that during high-voltage level sensing. This circuit can be added to any existing stabiliser (automatic or manual) or UPS to monitor its performance.
Read The Rest...

Home Appliance Control Using TV Remote

This circuit is designed to switch on/off any home or industrial appliance by using the TV/DVD remote controller. The circuit can be operated up to a distance of 5-10 metre depending on the remote used. The circuit consists of a step-down transformer X1 (6V-0-6V, 250mA secondary), 5V regulator 7805 (IC1), two 5V, 1 change-over (C/O) relay, a timer NE555 IC (IC2), an IR receiver module (IRX1 TSOP1738) and some discrete components. The circuit works on regulated 5V, which is derived from X1 and regulated by IC1. Home appliance is controlled either by pressing any key on the remote or by manually pressing switch S1 to ‘on’ state.

Home Appliance Control Using TV Remote
The TV/DVD remote controller produces 38kHz frequency. The IR receiver module operates at this frequency. It is used to control relay RL2. The relay triggers IC2, which is wired in a bistable mode to control the home appliance connected at the contacts of relay RL1. Timer IC2 toggles relay RL1 when switch S1 is pressed momentarily. Threshold and trigger input pins 6 and 2 of IC2 are held at one-half of the power supply voltage (5V) by resistors R2 and R3. When output pin 3 of IC2 is high, capacitor C4 charges through resistor R4, and discharges when the output pin 3 is low.

When switch S1 is pressed, capacitor C4 voltage is applied to pins 2 and 6 of IC2, which causes the output of IC2 to change from low to high, or high to low. When switch S1 is released capacitor C4 charges or discharges to the original level at the output pin 3 of IC2. At normal condition, when IR rays are not incident on TSOP1738, its output at pin 3 remains high. When any TV remote key is pressed, IR rays fall on the TSOP1738 and its output goes low. At the same time relay RL2 energises for a few seconds through pnp transistor T2 (BC558). The working of the circuit is simple.

Initially, when there are no IR rays falling on the IR receiver module, its output remains high. Transistor T2 is in cut-off condition. Relay RL2 does not energise and hence IC2 does not toggle. As a result home appliance connected at the contacts of relay RL1 remains switched off. When you press any remote key for the first time, IR receiver module’s output goes low and collector of the transistor T2 goes high. Relay RL2 energises and triggers IC2. Output of IC2 goes high and relay RL1 energises to switch on the appliance. Once relay RL1 is energised it remains in that state.

So the appliance which is connected at the contacts of relay RL1 remains switched on. Now when you press any remote key the second time, relay RL2 energises and re-triggers IC2. Output of IC2 goes low and relay RL1 de-energises to switch off the appliance. Once relay RL1 de-energises it remains in that state. So the home appliance remains off. This cycle repeats when any key of the TV remote is pressed to switch on/off the home appliance. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Fix TSOP1738 and switch S1 on front side of the cabinet. Place transformer inside the cabinet and mains power cord at the back of the cabinet.
Read The Rest...