Paraphase Tone Controller

As opposed to the widespread Baxandall circuit (dating back to 1952!) a ‘paraphrase’ tone control supplies a straight frequency response as long as the bass and treble controls are in the same position. This unique property makes the ‘paraphase’ configuration of interest if only treble or bass needs to be adjusted - it is not possible to adjust both at the same time! Essentially, it’s the difference in setting of the tone controls that determines the slope of the frequency response, and the degree of bass/treble correction. The circuit is simplicity itself, based on two networks C1-C2-C3/R9-R10-R11 and C5-C6-C7/R12-R13-R14.
Picture of the project:
The first is for the high frequencies (treble) response, the second, for the low frequencies (bass). The roll-off points have been selected, in combination with C4 and C8, for the sum of the two output signals to re-appear with a ‘straight’ frequency response again at the output. Roughly equal output levels from the networks are ensured by R6 = 7.15 k and R8 = 6.80 k. However, the operating principle requires the input signals to the two networks to be in anti-phase. For best operation the networks are driven by two buffers providing some extra gain.
Circuit diagram:
The gain of IC1.D is slightly higher than that of IC1.C to ensure the overall response curve remains as flat as possible at equal settings of the tone controls. Because each network introduces a loss of about 1.72 (times), IC1.D and IC1.C first amplify the signal. The gain is set at about 8 (times) allowing input signal levels up to 1 V to pass the circuit at maximum gain and distortion-free. The gain also compensates the attenuation if you prefer to keep the tone controls at the mid positions for a straight response.
Parts and PCB layout:
To audio fans, the circuit is rewarding to experiment with, especially in respect of the crossover point of the two networks. R3 and R4 determine the control range, which may be increased (within limits) by using lower resistor values here. The values shown ensure a tone control range of about 20 dB. IC1.B buffers the summed signal across R15. C9 removes any DC-offset voltage and R16 protects the output buffer from the effects of too high capacitive loads. R17, finally, keeps the output at 0 V. The choice of the quad opamp is relatively uncritical. Here the unassuming TL074 is used but you may even apply rail to rail opamps as long as they are stable at unity gain. Also, watch the supply voltage range. A simple circuit board was designed for the project. Linear-law potentiometers may be fitted directly onto the board. Two boards are required for a stereo application. The relevant connections on the boards are then wired to a stereo control potentiometer.
  • Current consumption (no signal) 8 mA
  • Max. input signal 1 Veff (at max. gain)
  • Gain at 20 Hz +13.1 dB max. –6.9 dB min.
  • at 20 kHz +12.2 dB max. –7.6 dB min
  • Gain (controls at mid position) 2.38 x
  • Distortion (1 Veff, 1 kHz) 0.002% (B = 22kHz) 0.005% (B = 80 kHz)
R1-R4 = 10k
R5,R7 = 1k
R6 = 7k15
R8 = 6k80
R9,R10,R11 = 8k2
R12,R13,R14 = 2k2
R15 = 1M
R16 = 100R
R17 = 100k
P1,P2 = 100k preset or chassis-
mount control potentiometer, linear law
C1,C2,C3 = 47nF MKT, lead pitch 5mm
C4 = 68nF MKT, lead pitch 5mm
C5,C6,C7 = 10nF MKT, lead pitch 5mm
C8,C10,C11 = 100nF MKT, lead pitch 5mm
C9 = 2µF2 MKT, lead pitch 5mm or 7.5mm
IC1 = TL074
K1,K2 = line socket, PCB mount, e.g.
T-709G (Monacor/Monarch)
Author: Ton Giesberts - Copyright: Elektor Electronics
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FM Telephone Bug Circuit

Here is a simple transmitter that when connected to a phone line, will transmit anything on that line (execpt the dial tone) to any FM radio. The frequency can be tuned from 88 to about 94Mhz and the range is about 200 feet. It is extremely easy to build and is therefore a good, useful beginner project.
Circuit diagram:
R1 180 Ohm 1/4 W Resistor
R2 12K 1/4 W Resistor
C1 330pF Capacitor
C2 12pF Capacitor
C3 471pF Capacitor
C4 22pF Capacitor
Q1 2SA933 Transistor
D1, D2, D3, D4 1SS119 Silicon Diode
D5 Red LED
S1 SPDT Switch
L1 Tuning Coil
MISC Wire, Circuit Board
1. L1 is 7 turns of 22 AWG wire wound on a 9/64 drill bit. You may need to experiment with the number of turns.
2. By stretching and compressing the coils of L1, you can change the frequency of the transmitter. The min frequency is about 88 Mhz, while the max frequency is around 94 Mhz.
3. The green wire from the phone line goes to IN1. The red wire from the phone line goes to IN2. The green wire from OUT1 goes to the phone(s), as well as the red wire from OUT2.
4. The antenna is a piece of thin (22 AWG) wire about 5 inches long.
5. All capacitors are rated for 250V or greater.
6. The transmitter is powered by the phone line and is on only when the phone is in use. S1 can be used to turn the transmitter off if it is not needed.
7. If you have problems with the LED burning out, then add a 300 ohm 1/4W resistor in series with it.
Source : electronic
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A Friendly Charger Circuit for Mobile Phones

Most mobile chargers do not have current/voltage regulation or short-circuit protection. These chargers provide raw 6-12V DC for charging the battery pack. Most of the mobile phone battery packs have a rating of 3.6V, 650mAh. For increasing the life of the battery, slow charging at low current is advisable. Six to ten hours of charging at 150-200mA current is a suitable option. This will prevent heating up of the battery and extend its life.
Circuit diagram:
P1 = 10K LOG
R1 = 1K
R2 = 1K
R3 = 1K
R4 = 1K
R5 = 3.3K
R6 = 16R/2W
R7 = 220R
R8 = 3.3R
R9 = 1K
C1 = 470uF/25V
C2 = 10uF/25V
C3 = 1KuF/25V
D1 = Red LED
D2 = Green LED
Q1 = BC547
Q2 = BD677
ZD1 = 12V/1W
ZD2 = 5.6V/1W
IC1 = CA3130
The circuit described here, provides around 180mA current at 5.6V and protects the mobile phone from unexpected voltage fluctuations that develop on the mains line. So the charger can be left ‘on’ over night to replenish the battery charge. The circuit protects the mobile phone as well as the charger by immediately disconnecting the output when it senses a voltage surge or a short circuit in the battery pack or connector. It can be called a ‘middle man’ between the existing charger and the mobile phone.
It has features like voltage and current regulation, over-current protection, and high- and low-voltage cut-off. An added specialty of the circuit is that it incorporates a short delay of ten seconds to switch on when mains resumes following a power failure. This protects the mobile phone from instant voltage spikes. When short-circuit occurs at the battery terminal, resistor R8 senses the over-current, allowing Q1 to conduct and light up D1. Glowing of D2 indicates the charging mode, while D1 indicates short-circuit or over-current status.
The value of resistor R8 is important to get the desired current level to operate the cut-off. With the given value of R8 (3.3 ohms), it is 350 mA. Charging current can also be changed by increasing or decreasing the value of R7 using the ‘I=V/R’ rule. Construct the circuit on a common PCB and house in a small plastic case. Connect the circuit between the output lines of the charger and the input pins of the mobile phone with correct polarity.
Author : D. Mohan Kumar Copyright : E F Y

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Modular Phono Preamplifier

High Quality Moving Magnet Pick-up module, Two-stage Series/Shunt feedback RIAA equalization
Any electronics amateur still in possess of a collection of vinyl recordings and aiming at a high quality reproduction should build this preamp and add it to the Modular Preamplifier chain. This circuit features a very high input overload capability, very low distortion and accurate reproduction of the RIAA equalization curve, thanks to a two-stage op-amp circuitry in which the RIAA equalization network was split in two halves: an input stage (IC1A) wired in a series feedback configuration, implementing the bass-boost part of the RIAA equalization curve and a second stage, implementing the treble-cut part of the curve by means of a second op-amp (IC2A) wired in the shunt feedback configuration.
This module comprises also an independent dual rail power supply identical to that described in the Modular Preamplifier Control Center. As with the other modules of this series, each electronic board can be fitted into a standard enclosure: Hammond extruded aluminum cases are well suited to host the boards of this preamp. In particular, the cases sized 16 x 10.3 x 5.3 cm or 22 x 10.3 x 5.3 cm have a very good look when stacked. See below an example of the possible arrangement of the rear panel of this module.
Circuit diagram :

R1_____________270R 1/4W Resistor
R2_____________100K 1/4W Resistor
R3_____________2K2 1/4W Resistor
R4_____________39K 1/4W Resistor
R5_____________3K9 1/4W Resistor
R6_____________390K 1/4W Resistor
R7_____________33K 1/4W Resistor
R8_____________75K 1/4W Resistor (or two 150K resistors wired in parallel)
R9_____________560R 1/4W Resistor
C1_____________220pF 63V Polystyrene or Ceramic Capacitor
C2_____________1µF 63V Polyester Capacitor
C3_____________47µF 25V Electrolytic Capacitor
C4_____________10nF 63V Polyester Capacitor 5% tolerance or better
C5_____________1nF 63V Polyester Capacitor 5% tolerance or better
C6,C9__________100nF 63V Polyester Capacitors
C7,C10_________22µF 25V Electrolytic Capacitors
C8,C11_________2200µF 25V Electrolytic Capacitors
IC1____________LM833 or NE5532 Low noise Dual Op-amp
IC2____________TL072 Dual BIFET Op-Amp
IC3____________78L15 15V 100mA Positive Regulator IC
IC4____________79L15 15V 100mA Negative Regulator IC
D1,D2_________1N4002 200V 1A Diodes
J1,J2__________RCA audio input sockets
J3_____________Mini DC Power Socket
  • The circuit diagram shows the Left channel only and the power supply
  • Some parts are in common to both channels and must not be doubled. These parts are: IC3, IC4, C6, C7, C8, C9, C10, C11, D1, D2 and J3.
  • IC1 and IC2 are dual Op-Amps, therefore the second half of these devices will be used for the Right channel
  • This module requires an external 15 - 18V ac (50mA minimum) Power Supply Adaptor.
Technical data:
Sensitivity @ 1KHz: 4.3mV RMS input for 200mV RMS output
Max. input voltage @ 100Hz: 53mV RMS
Max. input voltage @ 1KHz: 212mV RMS
Max. input voltage @ 10KHz: 477mV RMS
Frequency response @ 200mV RMS output: flat from 30Hz to 23KHz; -0.5dB @ 20Hz
Total harmonic distortion @ 1KHz and up to 8.8V RMS output: 0.0028%
Total harmonic distortion @10KHz and up to 4.4V RMS output: 0.008%
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555 Timer Circuit With Variable On/Off Times

This circuit enables the on/off times of a 555 timer to be independently varied over a wide range. This is not possible with a conventional 555 circuit with the RC network being charged from the positive supply rail and discharged via pin 7. Instead, the capacitor at pins 2 & 6 of IC1 is charged and discharged from the output at pin 3. Furthermore, the charging and discharging circuits are different, being isolated by diodes D1 & D2.
Circuit diagram:
555-timer-circuit-with-variable -on-off-times d
Therefore the capacitor at pins 2 & 6 is charged via diode D2 and trimpot VR2 and discharged via D1 and trimpot VR1. With this arrangement you can have very long on times combined with very short off times and vice versa, or you can adjust the duty cycle to exactly 50% and so on. This circuit also employs a second 555 timer (IC2) as an inverter so that complementary pulses are available, if required. If not, delete IC2.
Author: A. Davies - Copyright: Silicon Chip Electronics
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12 Volt Battery Guardian

Don't get caught with a flat battery; this easy-to-build circuit can cut off the power to a 12V fridge or car stereo system if the battery voltages drops below critical level. Electric fridges in vans and 4WDs are a great idea but if you are not careful, they can severely discharge the battery and leave you stranded. Maybe the battery will end up with severe damage as well. The same problem applies if you have a big stereo system and you like to play it without the motor running.

Main features:
  • Cuts power to load (eg, fridge) when battery voltage drops below a preset level.
  • 10A rating.
  • Low power drain.
  • Chirping sound during cut-out.
  • Flashing LED indication during cut-out.
  • Automatically reconnects power when battery recharged.
Operation on 12V is fine when the motor is running and battery charge is maintained but if the fridge is allowed to run for too long when the motor is stopped, it can flatten the battery in a relatively short time. This is where the Battery Guardian comes into play. It monitors the battery voltage and disconnects power to the fridge before the battery becomes too flat to allow the engine to be started again.

Parts layout:

PCB layout:

Circuit diagram:

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Zero Gain Mod For Non-Inverting Opamp

Electronics textbooks will tell you that a non-inverting opamp normally cannot be regulated down to 0 dB gain. If zero output is needed then it is usual to employ an inverting amplifier and a buffer amp in front of it, the buffer acting as an impedance step-up device.
The circuit shown here is a trick to make a non-inverting amplifier go down all the way to zero output. The secret is a linear-law stereo potentiometer connected such that when the spindle is turned clockwise the resistance in P1a increases (gain goes up), while the wiper of P1b moves towards the opamp output (more signal). When the wiper is turned anti-clockwise, the resistance of P1a drops, lowering the gain, while P1b also supplies a smaller signal to the load. In this way, the output signal can be made to go down to zero.
Circuit diagram:
Zero Gain Mod For Non-Inverting Opamp

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