Hardware status and details
Posted: Mon Jul 11, 2011 10:56 am
Pickups and sensors:
Because there seems to be some interest in the non contact sensor for ped/lev position sensing, that also works as an individual string pickup system (also all strings like the standard pickups), I thought that I would try and explain the approach.
The basic sensor element is a Honeywell HMC1501. It is a magnetic field direction sensor that consists of a magnetic Wheatstone bridge. It can be used with an external magnet, or just an external magnetically conductive material, like a screw or a string.
It is followed by a gain stage in the form of an “inamp”…instrumentation amp. My choice is an Analog Devices AD627, or an AD8223…a matter of taste and cost.
The combined elements may be powered by a single supply, or a dual supply system. I am using a Radio Shack available AA battery holder that holds two AA batteries = 3VDC. I add a wire to it from the junction of the two batteries so that it gives +1.5VDC and +3VDC = either single +3VDC source, or +/- 1.5 VDC source.
The reason for the choice is that subsequent circuitry may want a signal that works from “ground” up, or centered about “ground”.
There are two ways to set up the interface between the HMC1501 and the ADxx…AC or DC coupled. If DC coupling is used, the offset in the HMC1501 will be amplified along as any externally sensed signal…if AC the offset is ignored. DC is good for the PED/LEV position sensor, AC is best for the pickup. The board can be laid out so that how it is populated and powered can serve both/either the offset and the “grounding” issues.
The boards that I have now do not have the AC coupling as they were aimed at the PED/LEV sensing, I.E. meant to deliver a ramp voltage as a function of cross shaft rotation. The reason for using the cross shaft instead of the ped/lev is familiar appearance for the first approach.
The boards were laid out to have 6 channels that can be split apart via perforations. They can be used as either pickups, or position sensors. The next generation will have the choice of AC or DC coupling in the front end, depending upon how the board is populated.
The sensors are spaced for 11/32” string separation = most PSGs. Two boards gets you 12 strings, or 10 strings and some other function like maybe a volume sensor etc.
An alternative would have been to use 3/8” string spacing as found on standard guitars and slant the board for use on PSGs.
Connection to the board, or the channels is by 100 mil centered connectors (6 pins for the channels). The power source is common for all channels if used together as a pickup, or has connectors for each channel is split apart. A 50 mil centered connector (10 pins) is added to handle all power and signals for the pickup approach.
A secondary split off section is included for added coupling flexibility. If populated and split, the 50 mil centered connectors will plug together to route power and signals to and from the pickup board to the outside world. The signals in this case are thru resistors that can be ganged at the output to make the individual string pickup function into a combined function as well as individual.
The output signal is from pin 6 on the ADxx, and the signal “ground” is from the “REF” pin 5 on the ADxx. To switch between single supply mode and dual supply mode just interchange the 0VDC and +1.5VDC input wires on the power in connection.
External magnet(s) may be used to bias the offset in the HMC1501 into, or out of saturation. This will interact with the gain setting of the ADxx to provide some interesting overdrive effects.
For PSG ped/lev sensing, an adjustable spring system will be needed to allow adjusting for “feel” to suit the user. If using the cross shafts as the sensor location, the ped to shaft rods will make the unit look “conventional. If the ped sensors are located at the ped bar, the rods will not be needed. In either case the tuning rods and bell cranks (and associated changer mechanism will not be needed….this assumes the DSP pitch(spectral) shift functions are attained. Till then, the position sensing ramps can be used to control other mechanical string tensioning devices, and the pickup used as the common wound magnet pickups are used.
There appears to be about 50dB of separation between strings at the 11/32” string spacing when used in the individual string mode…without tinkering to optimize.
Individual string outputs look/sound quite like those of the common wound magnet units.
Higher supply voltages can be used (say 5 or 6 VDC) to play around with the offset to gain interaction.
Because there seems to be some interest in the non contact sensor for ped/lev position sensing, that also works as an individual string pickup system (also all strings like the standard pickups), I thought that I would try and explain the approach.
The basic sensor element is a Honeywell HMC1501. It is a magnetic field direction sensor that consists of a magnetic Wheatstone bridge. It can be used with an external magnet, or just an external magnetically conductive material, like a screw or a string.
It is followed by a gain stage in the form of an “inamp”…instrumentation amp. My choice is an Analog Devices AD627, or an AD8223…a matter of taste and cost.
The combined elements may be powered by a single supply, or a dual supply system. I am using a Radio Shack available AA battery holder that holds two AA batteries = 3VDC. I add a wire to it from the junction of the two batteries so that it gives +1.5VDC and +3VDC = either single +3VDC source, or +/- 1.5 VDC source.
The reason for the choice is that subsequent circuitry may want a signal that works from “ground” up, or centered about “ground”.
There are two ways to set up the interface between the HMC1501 and the ADxx…AC or DC coupled. If DC coupling is used, the offset in the HMC1501 will be amplified along as any externally sensed signal…if AC the offset is ignored. DC is good for the PED/LEV position sensor, AC is best for the pickup. The board can be laid out so that how it is populated and powered can serve both/either the offset and the “grounding” issues.
The boards that I have now do not have the AC coupling as they were aimed at the PED/LEV sensing, I.E. meant to deliver a ramp voltage as a function of cross shaft rotation. The reason for using the cross shaft instead of the ped/lev is familiar appearance for the first approach.
The boards were laid out to have 6 channels that can be split apart via perforations. They can be used as either pickups, or position sensors. The next generation will have the choice of AC or DC coupling in the front end, depending upon how the board is populated.
The sensors are spaced for 11/32” string separation = most PSGs. Two boards gets you 12 strings, or 10 strings and some other function like maybe a volume sensor etc.
An alternative would have been to use 3/8” string spacing as found on standard guitars and slant the board for use on PSGs.
Connection to the board, or the channels is by 100 mil centered connectors (6 pins for the channels). The power source is common for all channels if used together as a pickup, or has connectors for each channel is split apart. A 50 mil centered connector (10 pins) is added to handle all power and signals for the pickup approach.
A secondary split off section is included for added coupling flexibility. If populated and split, the 50 mil centered connectors will plug together to route power and signals to and from the pickup board to the outside world. The signals in this case are thru resistors that can be ganged at the output to make the individual string pickup function into a combined function as well as individual.
The output signal is from pin 6 on the ADxx, and the signal “ground” is from the “REF” pin 5 on the ADxx. To switch between single supply mode and dual supply mode just interchange the 0VDC and +1.5VDC input wires on the power in connection.
External magnet(s) may be used to bias the offset in the HMC1501 into, or out of saturation. This will interact with the gain setting of the ADxx to provide some interesting overdrive effects.
For PSG ped/lev sensing, an adjustable spring system will be needed to allow adjusting for “feel” to suit the user. If using the cross shafts as the sensor location, the ped to shaft rods will make the unit look “conventional. If the ped sensors are located at the ped bar, the rods will not be needed. In either case the tuning rods and bell cranks (and associated changer mechanism will not be needed….this assumes the DSP pitch(spectral) shift functions are attained. Till then, the position sensing ramps can be used to control other mechanical string tensioning devices, and the pickup used as the common wound magnet pickups are used.
There appears to be about 50dB of separation between strings at the 11/32” string spacing when used in the individual string mode…without tinkering to optimize.
Individual string outputs look/sound quite like those of the common wound magnet units.
Higher supply voltages can be used (say 5 or 6 VDC) to play around with the offset to gain interaction.