November 6, 1997

RS-232C link monitors animal activity

W Stephen Woodward, University of North Carolina--Chapel Hill

A common need in biological experimentation is the ability to unobtrusively monitor the activity of small caged animals, such as mice, rats, and rabbits, as they move about their enclosures. This monitoring must be over long periods and around the clock. The circuit in Figure 1 allows you to effect long-term, unattended collection of animal-behavior data using any standard PC with an RS-232C port. The circuit detects changes in the proximity of the animal subjects to a capacitive sensor electrode. The multivibrator comprising IC_1D, IC_1E, and IC_1F drives the electrode with a 5V p-p, 200-kHz square wave.

Potentiometer R₂ adjusts the amplitude of the excitation, thus adjusting the overall sensitivity. Because any displacement currents sourced to the electrode must pass through the half-wave rectifier, Q₁-D₁, the average dc collector in Q₁ is proportional to the product of the excitation current and C_X, the sensor-electrode capacitance. Therefore, as the animals move about their cage, they alter C_X and produce corresponding variations in I_C.

The current-to-voltage converter comprising IC_1A, IC_1B, and IC_1C provides I_C information to the PC. C₂ stores the integrated charge from I_C. As C₂ charges, the voltage on Pin 1 of IC₁ rises until it reaches the logic-0 to -1 threshold. The multivibrator then switches, forward-biasing D₂ and discharging C₂, and generates an approximately 400-µsec pulse. Meanwhile, the RCD (received data) signal of the RS-232C interface goes high for an interval consistent with a valid RS-232C Start pulse. This high signal results in the port's assembling a correctly framed although meaningless character. The frequency of character transmission is proportional to the rate of charge of C₂; therefore, it is proportional to I_C and C_X.

With the circuit values in Figure 1, the constant of proportionality is approximately 10 Hz/pF. By measuring character frequency, software running on the PC can detect and tally fluctuations in proximity-electrode capacitance and, therefore, animal movement relative to the electrode (Listing 1). The Basic routine provides average proximity data at a little less than one datum per minute to produce a graphing-convenient of 1280 points per day. This monitoring scheme uses no moving parts, such as mechanical sensor switches; therefore, the scheme is inexpensive and robust. And, unlike clicking switches and photoelectric-sensor light sources, it produces no unwanted or disruptive stimulation of the subject animals. Because the circuit's power demands are low and unfussy, you can conveniently "borrow" all necessary power from the DTR (data-terminal-ready) and TXD (transmitted-data) lines of the port. (DI #2107)

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