Hi
The schematic at:
is a reference (TLV431) controlled by the Key signal that is present in a car when the ignition is on.
Transistor Q5 if turned on will activate Q6 and turn on the reference for the SMPS and get things running
The trick is that the Key signal is off when below 4V and on when above 7V. But I have used a zener D30 and the BJT Q5 to set the threshold level. This is not precise since this threshold will be influenced by the current in Q5, the B+ voltage (between 9V and 32V), the current in the zener and the temperature drift of these components.
The temperature is from -40 to 125 degrees. Can anyone suggest a better way to do this without using a dedicated opamp/comparator?
Thanks
Klaus