Saturday, December 17, 2016

Purchase of the CJ SHOP Obstacle Avoidance Sensor in place the OP-AMP and Potentiometer Circuit for the Photogate

We ended up purchasing the "C.J. SHOP® 5PCS IR Infrared Obstacle Avoidance Sensor Module for Arduino Smart Car Robot 3-wire"  on Amazon [1] for construction of our photogate.  To build our photogate, we will need two in tandem. These are represented as the red boxes in the illustration below:

The C.J. Shop Obstacle Avoidance Sensor Module looks like the op-amp receiver circuit and the potentiometer transmitter circuit , except that a comparator is used instead of an op-amp.

According to Wikipedia, the potentiometer works a bit better than an op-amp since the op-amp tends to have a bit more lag. 

In practice, using an operational amplifier as a comparator presents several disadvantages as compared to using a dedicated comparator:[5]
  1. Op-amps are designed to operate in the linear mode with negative feedback. Hence, an op-amp typically has a lengthy recovery time from saturation. Almost all op-amps have an internal compensation capacitor which imposes slew rate limitations for high frequency signals. Consequently, an op-amp makes a sloppy comparator with propagation delays that can be as long as tens of microseconds. "

Having the comparator is good because we would need to compare the voltage we sampled from the op-amp circuit when a potato is passing and when it is not. The comparator performs this function, outputs a binary 0 or 1 to the arduino (?), and since it allows for high frequency signals it may be able to measure things that happen quickly.

For further documentation for use with the Arduino, compare this obstacle avoidance sensor to the Sunfounder Obstacle Avoidance Sensor [2]. 

If the CJ Avoidance Sensor does not work well, try replacing the LED with a 940 nm IR LED:

Standard LEDs - Through Hole LED PMI 696-SSL-LX5099IEW

This uses the logic from the quote in Makezine [3]:

"Older remote controllers used gallium arsenide compensated with silicon (GaAs:Si). These LEDs emit at about 940nm, which makes them ideal for detecting water vapor, but they’ve become very difficult to find."

An ejected potato has a lot of water vapor.

We tried other LEDs like the ones from RadioShack (pictured on the side of the breadboad) and the ones from the Parallax SUMO Bot Kit (pictured in the circuit) . See below:

It may not be the LED that was the problem. It may be that we did not use a potentiometer on the transmitter LED and an op-amp or comparator on the receiving LED. The C.J. SHOP purchase provides us with this.




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