Beware Logic Levels

I had a very annoying time tracking down a problem with my poke detector circuit.  For some reason, it recently started acting up.  Now, the circuit is basically just a phototransistor feeding into an AND logic gate.  The weird behavior was that occasionally the output of the logic would get lots of noise on it and there would be strong crosstalk between different channels, i.e. different pokes.

After poking around with a scope for a while and re-reading documentation on the AND gate, I figured out the problem:  the phototransistor did not activate fully in the "no poke" condition and this caused a small voltage drop across the transistor.  This voltage drop was right about 0.8 volts, the TTL low level (TTL levels are 0.8V-=LOW and 2V+=HIGH).  Apparently if the voltage is between LOW and HIGH, strange things happen.

I fixed the problem by installing a higher value resistor prior to the phototransistor.  This served to drop more volts before the phototransistor.  Now the reading is only 0.3 volts when there is no poke and the system is working just fine.

Measuring Olfactometer Output in Odor Port

Odors are hard to measure and typical humans have little intuition for how they behave.  However, there are now ways to [relatively] easily measure the concentration of odors in clean air.  These devices are called photoionization detectors (PIDs) and they are used heavily in industry to detect specific volatiles.  Many of these devices are good at quantifying a stable odor source, but lack temporal resolution.  Some companies, however, have elected to make particularly fast PIDs.  One of these is the minPID from Aurora Scientific.  This device gives a measure of odor concentration in clean air with millisecond precision.

We use it routinely to calibrate our odor presentation and it is really the only reason we have solid idea about what odor concentration is being presented to the animal in our odor port.

Routine testing: miniPID (bottom) inserted into one of the odor ports.

Behavioral Box

After a delay, I now have the new box running.  Here are some pictures of my setup:

Dual Teflon Rodent Odor Port

In my new rat behavior box, I am putting in a dual odor port.  This lets me do comparisons between two independent odor lines.  I just assembled the new odor ports with beam break detectors that will activate when the rat pokes its nose into the port.  The ports are designed to be small so that odor onset is fast. Here is a picture of the new ports:

 In the middle there are some LTE 302 emitters facing out and on the sides there are LTR-301 receivers.  There are several ports on the back for odor injection.  On the top is a vacuum suction.

I am now making a controller for these ports with an Arduino Nano.