I have a question for the electronics experts. My Honda's alternator has two charging modes. A high-output mode of 14.5 volts and a low-output mode of 12.5 volts for better MPG. The ECU selects one of these two modes based on a few different inputs. One of these is vehicle speed. The alternator is always in high-output mode over 40 MPH. My speedometer and odometer are cable-driven, the VSS in the speedometer is only for the ECU. So what I want to do is cut the frequency of the VSS in half to get the low-output mode at speeds up to 80 MPH. Even 60 MPH would be fine. I know this can be done with a 555 timer in monostable mode as shown in
this link. What I don't know are the values for the resistors and capacitors in the circuit. I guess the VSS signal is 8200 counts per mile, based on the MPGuino Wiki page. It's a reed switch that grounds itself four times per cable revolution. I just want to space those ground pulses apart a little more. Any advice on what component values to start with would be great.