I got a "problem" and it is because of my lack of knowledge so I need your help.

First, the current design is like the attached below this paragraph. The power comes from a car battery (24v or 12v), then I power a DC-DC Converter T1 and this the my IC. So the converter T1 has a pull up pin, internally, EN that enables the converter.


simulate this circuit – Schematic created using CircuitLab

What I want to achieve is when the car turns on, turning the key, the circuit powers up and when the cars shuts down, the circuit too. In my design I'm using a relay NO with the key contact and the battery on the coil and a GND on the common pin of the relay, on NO the EN pin and NC not connected.

The key contact has 12 or 24v depending the battery.

The problem I got is the relay for 12 or 24V usually is a "Relay cube" that it is big (19mm x 15mm). I saw solid state relay for 3.3 or less voltage that are very small but for this I need another dc-dc converter just to drive this relay and this is more messy.

What will be an improvement for this circuit? A transistor?, but I don't know what kind because the specs for the voltage from base to emitter are low (base 24/12v to 0v) Or are there a some IC for this use case?

On a second iteration of the design I will have another event (key contact and IC pin) so I will need some logic gate for drive the EN pin. Just to know if there is a better implementation.

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    \$\begingroup\$ You have bigger problems than your relay, if you're using a transformer there. You need a DC-DC converter; transformers only work with AC. \$\endgroup\$ – Hearth Nov 15 '19 at 16:27
  • \$\begingroup\$ @Hearth: Obviously, it IS a DC-DC converter. Magnetic transformers don't have an "enable pin". \$\endgroup\$ – Dave Tweed Nov 15 '19 at 16:29
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    \$\begingroup\$ Why don't you permanently enable your "transformer" and connect it to a power source (usually called "Accessory") that is switched on and off with the ignition? \$\endgroup\$ – Dave Tweed Nov 15 '19 at 16:31
  • \$\begingroup\$ Just connect EN to the key, why the relay? Most DCDC the enable is active high with an internal pull-down. \$\endgroup\$ – Jason Morgan Nov 15 '19 at 17:22
  • \$\begingroup\$ Don't need a DC-DC converter to drive it, a zener and a resistor should probably be fine. \$\endgroup\$ – Cristobol Polychronopolis Nov 15 '19 at 20:05

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