The Pocket Science Lab Hardware
PSLab is a USB powered, multi-purpose data acquisition and control instrument that can be used to automate various test and measurement tasks via its companion android app, or desktop software. It is primarily intended for use in Physics and electronics laboratories in order to enable students to perform more advanced experiments by leveraging the powerful analytical and visualization tools that the PSLab’s frontend software includes.
Real time measurement instruments require specialized analog signal processors, and dedicated digital circuitry that can handle time critical tasks without glitches. The PSLab has a 64MHz processor which runs a dedicated state machine that accepts commands sent by the host software, and responds according to a predefined set of rules.
It does not deviate from this fixed workflow, and therefore can very reliably measure time intervals at the microsecond length scales, or output precise voltage pulses.
In contrast, a GHz range desktop CPU running an OS is not capable of such time critical tasks under normal conditions because a multitude of tasks/programs are being simultaneously handled by the scheduler, and delays of the order of milliseconds might occur between one instruction and the next in a given piece of software. The PSLab combines the flexibility and reliability of its dedicated processor, and the high computational and visualization abilities of the host computer/phone’s processor to create a very advanced end product.
And now, a flow diagram to illustrate the end product:
An outline of how this state machine works
But first, you might be interested in the complete set of features of this instrument, and maybe screenshots from a few experiments . Here’s a link to a blog post by Praveen Patil outlining the oscilloscope, controls , and the data logger.
From the flow diagram above, it is apparent that the Hardware communicates to the host PC via a bidirectional communication channel, carries out instructions, and might even communicate to a secondary slave via additional communication channels.
The default state of the PSLab hardware is to listen to the host. The host usually sends a 2- byte header first, the first byte is a broad category classifier, and the second refers to a specific command. Once the header is received , the PSLab either starts executing the task , or listens for further data that may contain configuration parameters
An example for configuring the state of the digital outputs [These values are stored in header files common to the host as well as the hardware:
- Bytes sent by the host :
- Byte #1 : 8 #DOUT
- Byte #2 : 1 #SET_STATE
- Byte #3 : One byte representing the outputs to be modified, and the nature of the modification (HIGH / LOW ). Four MSB bits contain information regarding the digital outputs SQR1 to SQR4 that need to be toggled, and four LSBs contain information regarding the state that each selected output needs to be set to.
- Action taken by the hardware:
- Move to the set_state routine
- Set the output state of the relevant output pins (SQR1-4) if required.
- Respond with an acknowledgement
- Move back to listening state
- Bytes Returned by the hardware:
- Byte #1 : 254 ACKNOWLEDGE. SUCCESSFUL.
In a similar manner, instruments ranging from oscilloscopes, frequency counters, capacitance meters, data buses etc are all handled.
For function calls that are time consuming, the communication process might be split into separate exchanges for initialization, and data download. One such example is the Oscilloscope capture routine. The first information exchange sets the parameters for data acquisition, and the second occurs when the acquisition process is complete.
Host-Side Scripts and Software
The software running on the host runs either a dedicated script that sequentially acquires data or executes control tasks , or it runs an event loop where user inputs are used to determine the acquisition task to be executed.
An example of a pulse sensor designed with just the voltmeter and a digital output of the PSLab.
A photo transistor is connected to the SEN input of the PSLab, and the host software reads the voltage on SEN at fixed intervals.
The conductance of the photo transistor fluctuates along with the incident light intensity, and this is translated into a voltage value by the internal signal processor of the SEN input.
When the photo sensor is covered with a finger, and a bright light is passed through the finger, a time linked plot of these voltage fluctuations reflects the fluctuations in blood pressure, and therefore has the same frequency of the heart beat of the owner of this finger.
* The digital output is used to power a white LED being used as the light source here
 : Original content developed for the SEELablet’s first revision, from which PSLab is derived.