Hi Scott-You'll need a device called an analog comparator, which is very similar to an operational amplifier, but is optimized for comparing two voltages. I presume you would like one that comes in a DIP package, because that will be easier for you to connect. Let's start with an LM311.We need to drop the voltage down a bit, maybe to 12 volts or so, to run the comparator and to serve as a voltage reference. 35 volts is a bit awkward as a starting voltage, because many linear voltage regulators have a maximum input voltage of 35 volts (there are options of higher input voltage if we use a surface mount device, but I'm guessing you would prefer to avoid that). Use an LM7812 with a 12 volt Zener diode in series with the input to drop the voltage down into the safe range. Use the 12 volts from the regulator to power the comparator. Take a voltage divider form the output of the comparator to create a reference voltage. 2- 10 K resistors in series will make 6 volts. connect this to the non-inverting input of the comparator.Now take a voltage divider from your 36 volt input and divide down to 6 volts (100K in series with 21K ohms). Tie this to the inverting input of the comparator. I presume you want a relay output. Pick a relay with a 12 volt coil and tie it between the 12 volt source coming from the voltage regulator and the output of the comparator. See figure 6 of the LM311 spec sheet.One last thing- you should probably add a high value resistor between the comparator output and the non-inverting input. This adds a bit of hysteresis. to avoid having the relay "chatter" when the input voltage is near the threshold. See the LM311 spec sheet page 12 paragraph 6.
If you have a go at building this, post your progress here and I'll be happy to help with any questions.-Rick
