Tuesday, October 8, 2013

Low Battery Indicator I

Here is the circuit diagram of low battery indicator from silicon chip electronics. This simple circuit lights LED1 when the battery voltage drops below the setting set by trimpot VR1. In effect, VR1 and associated resistors bias Q1 on which holds Q2 and the LED off. When the voltage drops below the set value, Q1 turns off, allowing Q2 to turn on and light the LED. The circuit is suitable for nominal battery voltages up to 12V.

Low battery indicator circuit schematic
Continue Reading[...]

Sunday, October 6, 2013



IC NE555 works as an astable multivibrator with variation on the frequency. With this circuit, the LED blinks every half second. How long the blink time is, can be adjusted by adjusting the value of capacitor C1. Up to 18 additional LEDs can be attached to this circuit (36 LEDs total).

Components :
Diode D1-D2 : 5mm LED
Resistor R1 : 4K7 ohm
Resistor R2 : 1k ohm
Resistor R3-R4 : 330 ohm
Variable resistor VR1 : 100k ohm
Polar capacitor C1 : 10 uF/10 V
Capacitor C2 : 0.01 uF
IC1 : NE555
6V power supply
Continue Reading[...]

Friday, October 4, 2013

RIAA Phono Preamplifier

Since modern sound systems usually lack inputs for record players, separate MD preamplifiers are becoming increasingly popular. They are needed not only by people who still regularly like to listen to vinyl records, but also by those who want to finally transcribe their LP collections to CD using a CD recorder. The author has built innumerable phono preamplifiers for friends and acquaintances. In many cases, for the sake of simplicity these were based on an old circuit design with two µA741s, which was originally described by B. Wolfenden in a 1976 issue of Wireless World.

In that simple design, the first 741 simply amplified the full range of the frequency spectrum, while the second one was fitted with RIAA frequency compensation — a fairly common configuration at that time. However, a variant on this classic design was recently born after a bit of experimenting. It also uses two opamps, with the difference that the RIAA frequency compensation is distributed over both opamps. The accompanying figure shows the schematic diagram of this preamplifier. The first opamp attenuates the signal at 6 dB/octave starting at 2.2 kHz, while the second opamp looks after the other corner frequency.

RIAA Phono Preamplifier Circuit Diagram
The objective of the new design was to keep the feedback factor as high as possible in both stages. To the considerable surprise of the developer, this modification turned out to have an unexpected side effect: when records were played, certain scratches were no longer audible! The difference between the new and old preamplifiers could be clearly heard; it was certainly not just imagination. What could be the cause of this? A quick calculation showed that a 0.05-mm scratch in a record groove moving past a needle at a speed of 0.5 m/s produces a square-wave pulse with a frequency of 10 kHz. Evidently, there is a lot to be gained by attenuating such pulses with a low-pass filter as early as possible, which means in the first stage, in order to prevent them from over-driving the rest of the circuit.
Continue Reading[...]

Wednesday, October 2, 2013

2 Line Intercom Cum Telephone Line Changeover Circuit

The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom. Usually a single telephone is connected to a telephone line. If another telephone is required at some distance, a parallel line is taken for connecting the other telephone. In this simple parallel line operation, the main problem is loss of privacy besides interference from the other phone. This problem is obviated in the circuit presented here. Under normal condition, two telephones (telephone 1 and 2) can be used as intercom while telephone 3 is connected to the lines from exchange. In changeover mode, exchange line is disconnected from telephone 3 and gets connected to telephone 2. For operation in intercom mode, one has to just lift the handset of phone 1 and then press switch S1.

As a result, buzzer PZ2 sounds. Simultaneously, the side tone is heard in the speaker of handset of phone 1. The person at phone 2 could then lift the handset and start conversation. Similar procedure is to be followed for initiation of the conversation from phone 2 using switch S2. In this mode of operation, a 3-pole, 2-way slide-switch S3 is to be used as shown in the figure. In the changeover mode of operation, switch S3 is used to changeover the telephone line for use by telephone 2. The switch is normally in the intercom mode and telephone 3 is connected to the exchange line. Before changing over the exchange line to telephone 2, the person at telephone 1 may inform the person at telephone 2 (in the intercom mode) that he is going to changeover the line for use by him (the person at telephone 2).

2-Line Intercom-Cum-Telephone Line Changeover Circuit DiagramAs soon as changeover switch S3 is flipped to the other position, 12V supply is cut off and telephones 1 and 3 do not get any voltage or ring via the ring-tone-sensing unit. Once switch S3 is flipped over for use of exchange line by the person at telephone 2, and the same (switch S3) is not flipped back to normal position after a telephone call is over, the next telephone call via exchange lines will go to telephone 2 only and the ringtone-sensing circuit will still work. This enables the person at phone 3 to know that a call has gone through. If the handset of telephone 3 is lifted, it is found to be dead. To make telephone 3 again active, switch S3 should be changed over to its normal position.
Continue Reading[...]