Using an External Input Source

Overview

There are times when you want StimTracker to detect the onset of an external input, e.g. a switch, a foot pedal, or even a TTL signal that’s coming from another computer. There are two ways to do this.

Using Light Sensor Inputs

The light sensor inputs can also be used to connect a switch or TTL input on StimTracker Duo and StimTracker Quad. You will need a cable with a 2.5mm connector, available from Amazon and many others:

Here are some important notes:

You cannot use a switch or TTL signal with the audio input or microphone input. If you do connect a switch or TTL signal, it may seem to work but it’s not predictable enough that you would want to use it.
The original StimTracker ST-100 model has 6 digital inputs on the RJ45 connector that can be used as TTL inputs or with a switch. But the light sensor inputs on that model can be used only with a switch.

Using the External Response Connector

The Quad model includes a built-in connector that allows up to 8 lines of digital input. You can provide digital signals from a computer. You can also connect switches if you use an external power source like a battery. Internally, these lines are wired in parallel with input from a response pad and are pulled down by 10kohm resistors. It is not possible to use the external input connector and a response pad at the same time.

Cedrus has developed a circuit board that would make it easy to take advantage of this connector. It is offered in three variations:

Bare PCB for soldering wires.
PCB with screw terminals
A kit for connecting output from a parallel port to StimTracker Quad

The inputs are active high and have 10kΩ pulldown resistors to prevent floating inputs when not connected.

Adjusting Filters

When a mechanical switch is pressed, its contacts will “bounce” a few times, causing several signals to be produced. The size and quality of the switch determines how long it takes for the bouncing to die down. You can “debounce” the switches using a hold-on filter. See XID Filters and Signal Flow.

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Last Revision: Nov 16, 2023