Pulse Hubs

A project log for Bullet Movies

Using red, green, and blue LEDs to capture short movies of very fast objects

ted-yapoTed Yapo 06/18/2017 at 23:130 Comments

I assembled the remaining pulse hubs. These accept the pulse signals from the flash controller and distribute them to up to 18 flash boards. Since I only have 9 boards (3 each R, G, and B), I only populated three output jacks on each hub. SMA connectors are pretty but expensive.

The circuit on each board is simple: a NCP81074B MOSFET gate driver IC serves as a line receiver and line driver for up to (6) 50-ohm outputs. The "B" version of the IC has input thresholds near ground and the gate driver supply voltage (plenty of hysteresis) to ensure no false triggering. The output impedance is a fraction of an ohm, so 47.8-ohm resistors back-terminate each output. The IC has rise-times of around 4ns, so I could generate really narrow flashes, but this also means the transmission lines need to be terminated to avoid reflection problems. Here's the schematic of each board:

I didn't populate the input AC termination components; I doubt they're necessary. I am considering adding some input protection components in their place. In retrospect, that's what's really missing on these boards. I'll just have to be careful with them for now.

I really like these driver IC's - they're not just for MOSFETs anymore! There is also an "A" version of the driver IC that I will use on the controller board. The "A" version has input thresholds than can be driven by 3.3V logic, while the output voltage swings rail-to-rail.