Harp FlyPAD Controller
Introduction
The Harp FlyPAD Controller is a Harp Standard device developed at the Champalimaud Foundation. It is the acquisition and multiplexing board of the flyPAD (fly Proboscis and Activity Detector) system, a capacitance based method to automatically monitor and quantify feeding behaviour in Drosophila, originally described in Itskov et al., Nature Communications 5:4560 (2014).
In the original version, the behavioral arenas were multiplexed by an FPGA based board (a Terasic DE0-Nano carrying an Altera Cyclone IV) which polled the arenas capacitance values and forwarded the readings to the computer over a serial USB interface. The Harp FlyPAD Controller is the functional equivalent of that board: it polls the same behavioral arenas, but exposes the data as a Harp device. This adds hardware time-stamping of every capacitance sample, clock synchronization with the remaining Harp devices of the rig, and digital inputs and outputs that can be used to align the feeding data with other events of the experiment, for example.
Architecture
The flyPAD system is organized in two layers: the behavioral arenas, which perform the capacitance measurement, and the Harp FlyPAD Controller, which multiplexes and streams the readings.
The supported arenas are composed by a single board with four independent arenas each one with two independent food channels. Each channel is a pair of concentric electrodes:
- an outer annular fly electrode (10 mm outer diameter, 3 mm inner diameter), on which the animal stands;
- an inner food electrode, a 1.75 mm through-plated via accessed from the bottom of the PCB, on which the food is placed.
When the fly touches the food with its proboscis or with a leg, it changes the dielectric constant between the two electrodes, and therefore the capacitance measured between them. Each arena carries an AD7150 capacitance-to-digital converter (Analog Devices), which digitizes the two channels at 100 Hz.
Up to 32 dual-channel arenas are connected to the Harp FlyPAD Controller. The controller
addresses each converter simultaneously and time-stamps the resulting set of readings with
the Harp clock, streaming the data to the computer over USB as a single
CapacitanceValues event of 64 values
(32 arenas × 2 channels).
Mode of Operation
Acquisition is idle until the EnableAcquisition register is
set to Enabled. From that moment the controller continuously scans all connected arenas and emits
one CapacitanceValues event per acquisition cycle, containing the current reading of every
channel. Setting EnableAcquisition back to Disabled stops the scan and the event stream.
The two digital inputs and four digital outputs are independent from the acquisition state. Digital
inputs report every transition through the DigitalInputState event, which can be used to align the
capacitance traces with external triggers, for example the onset of a trial or an optogenetic
stimulus. Digital outputs are controlled by software and can be used to drive external hardware from
the same time base.
As with every Harp device, all events are time-stamped by the device itself, so the feeding traces can be aligned offline with the data of any other Harp device sharing the same clock line.
Key Features
- Capacitance Channels: up to 64 (32 dual-channel behavioral arenas)
- Sampling Rate: 100 Hz per channel
- Sensitivity: 1 fF, provided by the AD7150 capacitance-to-digital converter on each arena
- Two Food Sources per Fly: each arena monitors two independent food channels, allowing preference experiments within a single animal
- Harp Time-stamping: every capacitance and digital event is time-stamped by the device and synchronized with the remaining Harp devices of the rig
- Digital IO: 2x digital inputs and 4x digital outputs for synchronization with external hardware
Connectivity
- State LED [STATE]: Green LED Harp status indicator
- Digital IN/OUT [IN/OUT] 1x Screw terminal for 2x outputs and 2x inputs (digital 5V tolerant)
- Clock Sync Input [CLKIN]: 1x Stereo jack clock input
- Computer Interface [USB]: 1x USB Type-C
- Arenas Connectors: 8x IDC connectors for each one of the quad arenas
Technical Specifications
| Parameter | Specification |
|---|---|
| Behavioral Arenas | Up to 8 quad flypad boards |
| Capacitance Channels | Up to 64 (2 per arena) |
| Sampling Rate | 100 Hz per channel |
| Capacitance Resolution | 1 fF (AD7150, 12 bit) |
| Digital Inputs | 2x |
| Digital Outputs | 4x |
| Computer Interface | USB |