Hello
Lately I’ve got my hands on this little toy:
It’s a 1981 vintage CRT camera viewfinder that has a huge resolution even if it’s very small. I had to modify it to make it work without the camera: I’ve added a voltage regulator for the 6V rail that was provided by the camera, and created a constant current sync for the CRT heater – oddly enough it seem to be also in the camera body.
the current limit is simple : a NMOS pass transistor , a 10K pull-up resistor from gate to VCC, a sense resistor from source to gnd and a BJT with collector to mosfet gate, emitter to ground and base to the sense resistor -> the entire think tries to keep 0.6V across the sense resistor. The filament current is at ~ 80mA – enough to make the CRT tube work with a very faint glow from the heater. The entire thing work from 9V and uses ~ 300mA