Our 30 year old Manlift uses descrete components to produce a square wave output (around 150hz) to control a hydraulic valve. As I understand it, instead of outputting a constant voltage from 0 to 12 volts, it outputs a square wave duty cycle so it doesn't heat up or something? To help you understand how it works: The proportional valve is like two relay valves connected in series, with 0 volts and 12 volts at each end with a center tap connecting the two, coming from the control. So neutral is the equivalent of 6 volts, only they tell me it's not really 6 volts, but 12 volts, cycled on/off 50% of the time. Forward, or 100% duty cycle would turn one valve completely on, the other completely off, moving it full speed in that direction. and 0% duty cycle the opposite direction.
This sounds exactly like what an UNO can do, and it won't cost $700, like the new old school PCB! I'm needing to replace one of the joysticks, and thinking it would probably be a simple thing for an atmega328 chip. The attached schematic is the way the original joystick worked. It's powered by a 741 opamp IC.
Couldn't I just have an Atmega produce PWM for a MOSFET to do the same thing? The Atmega would read the joystick POT, and do the PWM accordingly.
In the schematic, there's 2 MOSFETS or transistors being used. PNP & NPN. Would I need both of these? If someone could explain why there's two, I would really like to understand it.
The output has a 6 amp diode on it to keep 12 volt backfeed out from other systems. So could I just just a heavy PNP MOSFET, and PWM from the UNO chip?
I'm thinking something along the line of this power Mosfet by sparkfun (This one is NPN, but they have the PNP equivalent)
It needs to handle about 3 amps of current at 12 volts, so I'd need to beef up the traces on the board I make.
Thanks so much. If this is do-able, it can save $$$!
Squarewave.pdf (14.2 KB)