Guitar Effect Pedal Power

A small box is fitted to the rear of the amplifier providing a 9V output for the effect pedal. The amplifier section gets 9V through a pedal switch. This power output and guitar signal input lines are combined into a single unit with multi-way cable connecting points as shown in the following figure.

The circuit can be divided into two sections: power supply and signal handling. The power supply section is built around transformer X1, regulators 7805 and 7905, bridge rectifier comprising diodes D1 through D4, and a few discrete components. The signal-handling circuit is built around two OP27 op-amps (IC3 and IC4).

The power supply of about 9V for the effect pedals is derived from step-down transformer X1. MOV1 is a metal-oxide varistor that absorbs any large spike in mains power. 

IC 7905 (IC1) is a -5V low-power regulator. By using a 3.9V zener diode (ZD1) at its ground terminal, you get -8.9V output. The same technique is also applied to IC 7805 (IC2)-a +5V regulator to get 8.9V. Use good-quality components and heat-sinks for the regulators. This supply is more than enough for the five effect pedals.

The greater the voltage drop across the regulator, the lower the output current potential. Resistors R1 and R2 provide a constant load to ensure that the regulators keep regulating. Capacitors C3 through C8 ensure that the supplies are as clean as possible. It is very important to use proper heat-sinks for IC1 and IC2. Otherwise, these could heat up.

Working of the circuit is simple. The input signal stage uses a basic differentiation amplifier to accept the incoming signal and a voltage follower to buffer the output to the power amplifier. The differential amplifier is built around IC3. It works by effectively looking at the signals presented to its inputs. If the input signals are of different amplitudes, IC3 amplifies the difference by a factor determined by R4/R3 (where R4=R6 and R3=R5). If the input signals have same amplitudes, these are attenuated by the common-mode rejection ratio (CMRR) of the circuit. The value of CMRR is determined by the choice of the op-amp the auxiliary components used and circuit topology. You can use standard resistors. With the values shown, you get an overall gain of unity. 

The combination of resistor R7 and C13 serves as a passive low-pass filter, progressively attenuating unwanted high-frequency signals. The second op-amp (IC4) forms a simple voltage follower (its output follows its input), providing a low output impedance to drive into the standard power amplifier. 

Assemble the circuit on a general-purpose PCB and fit it to the rear of an amplifier. The unit must be compact, yet robust. So use a very sturdy aluminium extrusion for the cabinet in order to neatly house the assembled PCB. 

To ensure simple operation, there are only three connections to the unit. First, mains power is tapped from the transformer. The second lead carries the 9V output to the amplifier. The third is the guitar signal input at the five-way socket for connection to the effect pedal.

SAFE CIGARETTE LIGHTER POWER SOCKET

Cigarette lighter sockets available on the dashboards of some vehicles act as a heat source to light cigarettes. Besides, these can power electronic gadgets like tablets, laptops, portable video players, MP3 players and cellphones. Some of these devices can be plugged directly into the cigarette lighter socket, while others may need an inverter (DC-AC converter). 

However, using the cigarette lighter socket to power electronic gadgets while the car engine is not running may drain the car’s battery. This circuit lets you use the cigarette lighter socket to power your electronic gadgets without the fear of draining the car’s battery.

The circuit supplies power to electronic devices with low-voltage protection mechanism. A moulded cigarette lighter socket extension cable (refer Fig.3) is necessary for the circuit. Cut the extension cable and solder its leads to the input of the circuit (J1). The circuit receives 12V DC at J1 through polarity protection diode D1.

As shown in Fig.1, the heart of the circuit is IC TL431 (IC1)—a voltage regulator wired as a comparator. When the voltage at the reference terminal (Ref) of IC1 exceeds 2.5 V , its cathode (K) goes low, which provides base emitter biasing of transistor T1. Transistor T1 provides regenerative feedback via the combination of resistor R6 and diode D2 to turn on transistor T2, which is an n-channel power MOSFET. As a result, the DC supply from input pin J1 is routed to the output load connected at socket J2.

 

Fig.1: Safe cigarette lighter socket

However, if the battery voltage drops below 10V, the reference terminal voltage of IC1 falls below 2.5V and its cathode goes high. Transistor T1 unbiases to turn off transistor T2 and the output load disconnects to prevent deep discharge of the storage battery. The red LED (LED1) is used as a simple output-power status indicator. SPST toggle switch S1 is the power-on/ off-cum-reset switch. For testing the circuit, apply 12V DC to input point J1 and adjust trimmer VR1 (22-kilo-ohm) such that LED1 lights up and the output voltage is available at J2. Recheck the calibration to ensure that the output supply is disabled when the input voltage falls to a value of about 10V DC.

 

Fig.2: Pin configurations of  IRF540, SK100 and TL431

IC1 is a three-terminal adjustable shunt regulator. Its output voltage may be set at any level greater than 2.5V (VRef) and up to 3.6V by selecting resistors R1 and R2 that act as a voltage-divider network. T2 is a general-purpose n-channel power MOSFET. Any MOSFET with similar characteristics can also be used in place of T2. Use a heat sink for T2 to allow heat dissipation.

After construction, enclose the circuit in a small metallic or plastic box with holes to mount the power switch (S1) and indicator (LED1). Output jack J2 should match with the appliance to be used.