Build this low cost, novel Hall Effect pedal accessory
A conventional volume pedal consists of an ordinary potentiometer connected in the usual volume control fashion, and operated from the foot pedal via a rack and pinion mechanism. This system works very well, but with a lot of use the potentiometer's track can become worn with consequent noise being generated as the pedal is operated.
The problem is overcome in this pedal, which uses a magnet and a Hall effect device instead of a potentiometer. As the pedal is depressed the magnet is brought closer to the Hall effect device, and the increased magnetic field is converted into an increase in voltage.
The input signal is passed to the output by way of a voltage controlled attenuator (VCA), and, like a volume control, this can provide a level of attenuation of anything from zero to around 80dB. However, it is of course controlled by means of a voltage applied to its control terminal.
The output voltage of the Hall effect device is slightly too high in terms of its quiescent level, and too low in terms of voltage change produced by the varying magnetic field, and so the device cannot directly control the VCA. A level shifter and low gain DC amplifier are therefore used to process the output of the Hall effect sensor and give a suitable control voltage for the VCA.
Pre-emphasis (treble boost) at the input of the VCA and de-emphasis (treble cut) at the output are used to give a slight improvement in the signal to noise ratio of the unit. The ratio is actually about 80dB, and the background noise should be completely insignificant provided the unit is not used with a very low level signal. The circuit can take a maximum input level of about 2 volts RMS at most frequencies without serious distortion being produced. The circuit has an input impedance of about 50k and an output impedance of approximately 350 ohms.
Figure 1 shows the complete circuit diagram of the Volume Pedal, and IC1 is the Hall effect sensor. This is not one of the usual Hall effect switches but is a linear device, of recent origin, which has differential outputs.
IC2 is an operational amplifier used in the inverting mode. This gives an output voltage swing that is sufficient to give a wide attenuation range from the VCA, and a single resistor and diode chain in the feedback network of IC2 is adequate to give the desired tailoring of the amplifier's transfer characteristic.
The VCA is based on IC3 which is the well known MC3340P device. This can provide a voltage gain of about 13dB, but in this application a maximum gain of only unity is required and this is achieved by using R7 and R8 to provide about 13dB of attenuation at the input of IC3.
The current consumption of the circuit is about 12mA, and it is switched on automatically by insertion of the input jack.
A 0.1" matrix stripboard measuring 37 holes by 13 copper strips accommodates practically all the components, and Figure 2 provides details of this board and wiring of the unit.
IC2 is a CMOS device and requires the normal MOS handling precautions to avoid possible damage by static charges.
The pedal itself is a modified Maplin volume pedal. The output lead should be removed, but retain the potentiometer to give friction to the pedal mechanism. Open out the recessed hole on the right hand side of the case to 11mm, and drill another the same size nearby to accommodate the jack sockets.
The magnet passes through a hole about 18 to 20mm in diameter which is made in the top panel, midway along and about 75 mm out from the side of the case on which JK1 and JK2 are mounted. The magnet cannot be fitted direct to the underside of the pedal as it is too short to reach down into the case. Therefore a piece of timber or chipboard about 18 to 20mm thick is glued to the underside of the pedal immediately above the hole in the top panel of the case, and the magnet is then glued to this. A good quality adhesive must be used, and an epoxy type is probably the best choice for this application.
However, it is essential to mount the magnet the right way up or it will produce an output voltage of the wrong polarity from the Hall effect device. Probably the easiest way of finding the correct polarity for the magnet is to first wire the component panel to the off-board components. Connect the battery, and connect the unit into a signal path. It should be possible to control the gain of the circuit by adjusting RV1, and this component is adjusted just far enough in an anticlockwise direction to severely attenuate the signal as it passes through the unit. Apply each end of the magnet to IC1 in turn, and note which end produces an increase in gain from the circuit. The other end of the magnet is glued to the piece of timber or chipboard on the underside of the pedal.
The component board is mounted on the base panel of the case, and it must be positioned so that IC1 is aligned reasonably accurately with the magnet. The easiest way of achieving this is to hold the component board in position with IC1 over the magnet, and then measure the positions of the two mounting holes in the board relative to the sides of the case, and drill the mounting holes in the base panel of the case accordingly. Spacers a quarter of an inch long are used over the mounting bolts for the panel so that when the pedal is in the down position the magnet is in close proximity to IC1. If necessary, one or two extra nuts or some washers can be used to give further spacing and reduce the minimum gap between IC1 and the magnet, but do not use so much spacing that the two come into contact when the pedal is fully in the down position as this could possibly result in damage occurring.
The setting of RV1 controls the maximum amount of attenuation that the unit can provide, and it will probably be possible to obtain a maximum level of around 90dB. This is more than is normally necessary, and a lower level of around 60dB should be more than adequate and would give more precise control of the attenuation level. The magnet will have a slight effect on the unit even with the pedal in the fully up position, and to allow for this RV1 should be adjusted for a few dBs more attenuation than is required (since RV1 cannot be adjusted while the base panel is in position).
Feature by Robert Penfold
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