Processor-less instrument consisting of a ToprCmp2FPGA dual high-speed comparator with an FPGA configured as a divide-by-2 counter. Output is a ToprDout2 board, a simple 5V digital driver with 50 Ohms of source impedance.
Comparator thresholds are adjustable from -1V to +1V with the blue trimpots. You can monitor the threshold by putting a voltmeter on one of the MON pins. You can see white lines connecting the MON pins to their respective input channels. For the voltmeter's ground, use pretty much any exposed metal – BNC shields, screw holes, etc.
Power is through a 12V wall-wart.
The reason for the divide-by-two is that it makes a cheap pulse stretcher. The resulting pulse stream is usually fat enough to be processed by low-speed counters.