Caldwell Ballistic Precision Chronograph Theory of Operation and Signal Path

Written 2018-12-04

Tags:Caldwell Chronograph 

Inside my chronograph, we find this simple PCB. With only two chips and a handful of passive components, this device can measure speeds from 5 to nearly 10000 feet per second. Here is how it works. The chronograph waits for an object to pass the first light-gate, starts a timer, waits for the object to pass the second light gate, records the finish time, and divides the distance between light gates by the time, and converts to appropriate human-readable units for display.

Caldwell Chronograph

PCB

Caldwell Precision Ballistic Chronograph Teardown

Sensors

At each end of the device we see a large, black sensor module.

Caldwell Precision Ballistic Chronograph Teardown

Due to the presence of three wires running to each pod, it is likely either a phototransistor, or a photodiode with a bias circuit to help return a signal over the long wires.

Sensor Amplifier

Connected to the sensors is a TI LM324 quad operational amplifier. This common component adds a lot of flexibility in the analog domain, but its general purpose is to amplify the signal coming from the sensors over a range of frequencies.

Analog to Digital Converter - ADC

Connected to the output of the LM324 is the Silicon Labs C8051F300 microcontroller. From the datasheet we can see that the ADC is 10-bit resolution, up to 500kilosamples per second.

ADC

This is a pretty simple, basic ADC design. The window-compare feature may be used to allow the 8051 core to sleep while waiting for a signal pulse to arrive, but given that the chrono does nothing else, it may not be utilized. One nice thing about this ADC is that it features differential inputs. This means that instead of attempting to read the signals from both sensors we can instead subtract one from the other and feed it into the single ADC channel(this may also be done with the OpAmp).

Signal Processing - what a puzzler

I frankly have no idea what the signal of an arrow or bullet passing over a phototransistor when amplified by an unknown amplifier, but we can figure out some basic requirements from the device specifications.

This is a problem, because the ADC can only run at 0.5MHz. What does that mean? With a sample-rate of 500kHz, we can work out the error over speed like so:

error_plot

This means that either the device doesn't operate quite as it appears to, or Battenfeld Technologies / Caldwell cheated a little on their specifications. Not that anyone would notice - no bullet travels anywhere near 10000 feet per second, rare bullets travel approximately 5000 feet per second, but most are much, much lower than that. A 220 Swift might be 4000 feet per second, and the chrono would be around 0.8 percent accurate. Sadly, 0.25 percent accuracy is achieved only for 1000 feet per second and slower velocities.