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Tuesday, October 19, 2010

Electric Guitar Preamplifier

Here is the circuit diagram of a guitar preamplifier that would accept any standard guitar pickup. It is also versatile in that it has two signal outputs. A typical example of using a pick-up attached to a guitar headstock is shown inFig. 1. The pickup device has a transducer on one end and a jack on the other end. The jack can be plugged into a preamplifier circuit and then to a power amplifier system. The pickup device captures mechanical vibrations, usually from stringed instruments such as guitar or violin, and converts them into an electrical signal, which can then be amplified by an audio amplifier. It is most often mounted on the body of the instrument, but can also be attached to the bridge, neck, pick-guard or headstock.

Photo Of Electric Guitar PreamplifierThe first part of this preamplifier circuit shown in Fig. 2 is a single-transistor common-emitter amplifier with degenerative feedback in the emitter and a boot-strapped bias divider to secure optimal input impedance. With the component values shown here, the input impedance is above 50 kilo-ohms and the peak output voltage is about 2V RMS. Master-level-control potentiometer VR1 should be adjusted for minimal distortion. The input from guitar pickup is fed to this preamplifier at J1 terminal. The signal is buffered and processed by the op-amp circuit wired around IC TL071 (IC1). Set the gain using preset VR2. The circuit has a master and a slave control. RCA socket J2 is the master signal output socket and socket J3 is the slave.

Electric Guitar Preamplifier Circuit DiagramIt is much better to take the signal from J2 as the input to the power amplifier system or sound mixer. Output signals from J3 can be used to drive a standard headphone amplifier. Using potentiometer VR3, set the slave output signal level at J3. House the circuit in a metallic case. VR1 and VR3 should preferably be the types with metal enclosures. To prevent hum, ground the case and the enclosures. A well-regulated 9V DC power supply is crucial for this circuit. However, a standard 9V alkaline manganese battery can also be used to power the circuit. Switch S1 is a power on/off switch.

Source:EFY Mag

High-Intensity, Energy-Efficient LED Light

Here is a rechargeable LED lamp that gives you bright light for a long duration of time as it consumes little power. The circuit presented here is compact, automatic, reliable, low-cost and easy to assemble. The circuit comprises power supply, battery charging and switching sections. The power supply section takes power from 230V AC mains supply without using a transformer.

High-Intensity, Energy-Efficient LED Light Circuit Diagram

Capacitor C1 is used as an AC voltage dropper a well-known transformer-less solution. This helps to make the circuit compact without generating heat, as capacitor C1 dissipates negligible power. Capacitor C1 also protects against fluctuations in mains.

Current required for the battery charging circuit is provided by capacitor C1. Capacitor C1 discharges through resistor R1 when the circuit is disconnected from the mains voltage. This helps to prevent a fatal shock due to any voltage remaining in the input terminals. Capacitor C1 must be rated at least 440V AC, with mains application class X2. The AC mains voltage after capacitor C1 is given to bridge rectifier diodes D1 through D4 to convert alternating current into direct current and filtered by capacitor C2. The voltage from point B+ is given to positive terminal of the battery (BATT), anodes of LEDs (LED2 through LED21) and transistor base-bias resistor R3 through slide switch S1.

The circuit is operated in three modes (AC/charge, off and batt) by using three-position switch S1. When switch S1 is in middle position, the circuit is off. When S1 is towards right, white LEDs glow by drawing power from 4V battery. When S1 is towards left, the circuit connects to AC mains and battery starts charging. The presence of AC mains voltage and battery charging is indicated by LED1. White LEDs remain off if AC mains supply is available and glow in the absence of AC mains. When switch S1 is towards left position and AC mains is available, the battery charges through diode D6 and the white LEDs don’t glow. The negative DC path through diode D5 makes the transistor cut-off, preventing the battery current from LEDs to the negative terminal through the transistor.

Thus the white LEDs don’t glow. On the other hand, if AC mains is not available, charging stops and the base of transistor SS8050 gets positive voltage from the battery through slide switch S1 and resistor R3. The transistor conducts and the current flows from the battery’s positive terminal to the negative terminal of the battery through the LEDs (LED2 through LED21), collector to emitter of transistor T1 and switch S1. Thus the white LEDs glow. When the switch is in ‘batt’ position, the white LEDs (LED2 through LED21) get the supply directly from 4V battery through switch S1 and therefore all the white LEDs glow. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Fix the mains power cord on the back of the cabinet and slide switch and LEDs on the front side.

Source:EFY Mag

Automatic Room Power Control

An ordinary automatic room power control circuit has only one light sensor. So when a person enters the room it gets one pulse and the lights come ‘on.’ When the person goes out it gets another pulse and the lights go ‘off.’ But what happens when two persons enter the room, one after the other? It gets two pulses and the lights remain in ‘off’ state.

Automatic Room Power Control Circuit Diagram

The circuit described here overcomes the above-mentioned problem. It has a small memory which enables it to automatically switch ‘on’ and switch ‘off’ the lights in a desired fashion. The circuit uses two LDRs which are placed one after another (separated by a distance of say half a meter) so that they may separately sense a person going into the room or coming out of the room.
Outputs of the two LDR sensors, after processing, are used in conjunction with a bicolour LED in such a fashion that when a person gets into the room it emits green light and when a person goes out of the room it emits red light, and vice versa. These outputs are simultaneously applied to two counters. One of the counters will count as +1, +2, +3 etc when persons are coming into the room and the other will count as -1, -2, -3 etc when persons are going out of the room. These counters make use of Johnson decade counter CD4017 ICs. The next stage comprises two logic ICs which can combine the outputs of the two counters and determine if there is any person still left in the room or not. Since in the circuit LDRs have been used, care should be taken to protect them from ambient light.
If desired, one may use readily available IR sensor modules to replace the LDRs. The sensors are installed in such a way that when a person enters or leaves the room, he intercepts the light falling on them sequentially—one after the other. When a person enters the room, first he would obstruct the light falling on LDR1, followed by that falling on LDR2. When a person leaves the room it will be the other way round. In the normal case light keeps falling on both the LDRs, and as such their resistance is low (about 5 kilo-ohms). As a result, pin 2 of both timers (IC1 and IC2), which have been configured as monostable flip-flops, are held near the supply voltage (+9V). When the light falling on the LDRs is obstructed, their resistance becomes very high and pin 2 voltages drop to near ground potential, thereby triggering the flip-flops.
Capacitors across pin 2 and ground have been added to avoid false triggering due to electrical noise. When a person enters the room, LDR1 is triggered first and it results in triggering of monostable IC1. The short output pulse immediately charges up capacitor C5, forward biasing transistor pair T1-T2. But at this instant the collectors of transistors T1 and T2 are in high impedance state as IC2 pin 3 is at low potential and diode D4 is not conducting. But when the same person passes LDR2, IC2 monostable flip-flop is triggered. Its pin 3 goes high and this potential is coupled to transistor pair T1-T2 via diode D4. As a result transistor pair T1-T2 conducts because capacitor C5 retains the charge for some time as its discharge time is controlled by resistor R5 (and R7 to an extent).
Thus green LED portion of bi-color LED is lit momentarily. The same output is also coupled to IC3 for which it acts as a clock. With entry of each person IC3 output (high state) keeps advancing. At this stage transistor pair T3-T4 cannot conduct because output pin 3 of IC1 is no longer positive as its output pulse duration is quite short and hence transistor collectors are in high impedance state. When persons leave the room, LDR2 is triggered first, followed by LDR1. Since the bottom half portion of circuit is identical to top half, this time, with the departure of each person, red portion of bi-color LED is lit momentarily and output of IC4 advances in the same fashion as in case of IC3. The outputs of IC3 and those of IC4 (after inversion by inverter gates N1 through N4) are ANDed by AND gates (A1 through A4) and then wire ORed (using diodes D5 through D8).
The net effect is that when persons are entering, the output of at least one of the AND gates is high, causing transistor T5 to conduct and energize relay RL1. The bulb connected to the supply via N/O contact o relay RL1 also lights up. When persons are leaving the room, and till all the persons who entered the room have left, the wired OR output continues to remain high, i.e. the bulb continues to remains ‘on,’ until all persons who entered the room have left. The maximum number of persons that this circuit can handle is limited to four since on receipt of fifth clock pulse the counters are reset. The capacity of the circuit can be easily extended to handle up to nine persons by removing the connection of pin 1 from reset pin (15 and utilizing Q1 to Q9 outputs of CD4017 counters. Additional inverters, AND gates and diodes will, however, be required.



Digital Main Voltage Indicator

Continuous monitoring of the mains voltage is required in many applications such as manual voltage stabilisers and motor pumps. An analogue voltmeter, though cheap, has many disadvantages as it has moving parts and is sensitive to vibrations. The solidstate voltmeter circuit described here indicates the mains voltage with a resolution that is comparable to that of a general-purpose analogue voltmeter.

Digital Main Voltage Indicator Circuit Diagram

The status of the mains voltage is available in the form of an LED bar graph. Presets VR1 through VR16 are used to set the DC voltages corresponding to the 16 voltage levels over the 50-250V range as marked on LED1 through LED16, respectively, in the figure. The LED bar graph is multiplexed from the bottom to the top with the help of ICs CD4067B (16-channel multiplexer) and CD4029B (counter).The counter clocked by NE555 timer-based astable multivibrator generates 4-bit binary address for multiplexer-demultiplexer pair of CD4067B and CD4514B. The voltage from the wipers of presets are multiplexed by CD4067B and the output from pin 1 of CD4067B is fed to the non-inverting input of comparator A2 (half of op-amp LM358) after being buffered by A1 (the other half of IC2). The unregulated voltage sensed from rectifier output is fed to the inverting input of comparator A2. The output of comparator A2 is low until the sensed voltage is greater than the reference input applied at the non-inverting pins of comparator A2 via buffer A1.When the sensed voltage goes below the reference voltage, the output of comparator A2 goes high. The high output from comparator A2 inhibits the decoder (CD4514) that is used to decode the output of IC4029 and drive the LEDs. This ensures that the LEDs of the bar graph are ‘on’ up to the sensed voltage-level proportional to the mains voltage.The initial adjustment of each of the presets can be done by feeding a known AC voltage through an auto-transformer and then adjusting the corresponding preset to ensure that only those LEDs that are up to the applied voltage glow.


It is advisable to use additional transformer, rectifier, filter, and regulator arrangements for obtaining a regulated supply for the functioning of the circuit so that performance of the circuit is not affected even when the mains voltage falls as low as 50V or goes as high as 280V. During Lab testing regulated 12-volt supply for circuit operation was used.)



1KHz Sine wave Generator

This circuit generates a good 1KHz sine wave adopting the inverted Wien bridge configuration (C1-R3 & C2-R4). It features a variable output, low distortion and low output impedance in order to obtain good overload capability. A small filament bulb ensures a stable long term output amplitude waveform. Useful to test the Precision Audio Millivoltmeter, Three-Level Audio Power Indicator and other audio circuits posted to this website.

1KHz Sinewave generator circuit diagram

R1____________5K6  1/4W Resistor
R2____________1K8  1/4W Resistor
R3,R4________15K   1/4W Resistors
R5__________500R   1/2W Trimmer Cermet
R6__________330R   1/4W Resistor
R7__________470R   Linear Potentiometer

C1,C2________10nF  63V Polyester Capacitors
C3__________100µF  25V Electrolytic Capacitor
C4__________470nF  63V Polyester Capacitor

Q1,Q2_______BC238  25V 100mA NPN Transistors

LP1___________12V  40mA Filament Lamp Bulb (See Notes)

J1__________Phono chassis Socket

SW1__________SPST  Slider Switch

B1_____________9V  PP3

Clip for 9V PP3 Battery


  • The bulb must be a low current type (12V 40-50mA or 6V 50mA) in order to obtain good long term stability and low distortion.

  • Distortion @ 1V RMS output is 0.15% using a 12V 40mA bulb, raising to 0.5% with a 12V 100mA one.

  • Using a bulb differing from specifications may require a change of R6 value to 220 or 150 Ohms to ensure proper circuit's oscillation.

  • Set R5 to read 1V RMS on an Audio Millivoltmeter connected to the output with R7 rotated fully clockwise, or to view a sinewave of 2.828V Peak-to-Peak amplitude on the oscilloscope.

  • With C1, C2 = 100nF the frequency generated is 100Hz and with C1, C2 = 1nF frequency is 10KHz but R5 requires adjustment.

  • High gain transistors are preferred for better performance.


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