Radio Shack used to make a nice small 9 volt, low current coil, relay that would probably kick in at 6-8 volts which is early in the pump turn on stage for most progressive controllers.
You could use that to send +12 out of the relays switched contacts to the power injection bulb.
Or use the pressure transducer above and when you hit 10-20psi. of alky. pressure use a voltage comparator circuit comprising of the transducers output and a reference voltage, to drive a transistor that turns on a relay to do the same thing. Easy simple stuff really.
You could use comparators or an LM3914 ladder circuit, to output a row of LED's that would show exact pressure in stages, like a Caspers knock gauge, or provide as much detail as you want pressure wise.
Hook it up to a digital gauge to read direct pressure too. |