Electronic – Relay Spike at Vdd

Comments on your schematic and circuit:

1. A 7805 is not designed to handle the full range of car power, including short high voltage bursts. This has been mentioned to you before. See previous answers for details.

2. You have D1 the right way around, but I would make it a Schottky. Those react faster and will be fine for these relatively low voltages.

3. Fix the schematic to be less of a ratsnest. Traces worming their way all over the place makes it hard to read. See https://electronics.stackexchange.com/a/28255/4512 for details.

4. Fix the schematic to add part values! You seem to have mentioned some of them in the text, but I'm not going to remember everything you said when looking over the schematic. Do it right, especially since you are asking others to look at it.

Without part values, it is difficult to critique anything but the overall topology. That seems OK, within the detail provided by the schematic.

Added after schematic cleaned up:

That's a lot better. It looks basically OK.

I didn't look up the maximum input voltage of the LM2576. Check that it can handle car power glitches.

The 12 V sense lines driving the PIC inputs look fine.

Check that the BC547 can support the relay coil current with a 10 kΩ base resistor driven to 5V. That should provide about 430 µA base current. If the minimum guaranteed gain of the transistor is 50, for example, then this can support up to 21 mA collector current. That's cutting it close if the relay needs 20 mA, and of course is insufficient if it needs more.

There is no reason to use a Schottky in this circuit- the extra few hundred mV will actually help, and if you use a Zener you want it to result in the maximum voltage the transistor can safely handle.

Below are three possible circuits, of which the first is adequate for your purposes. D1 can be a 1N400x or 1N4148, it only needs to be rated for the supply voltage (+5) and to handle the relay coil current (tens of mA) for a short time. The zener (+ series diode drop, if there is one) has to be less than the voltage rating of the transistor, but it might have to be even less depending on the SOA (safe operating area) of the transistor. The first circuit is not as hard on the transistor and any modern jellybean part (don't try an RF transistor even if the ratings look like they might be okay) will work fine.

^{simulate this circuit – Schematic created using CircuitLab}

The two circuits to the right will give you a bit more life from the relay because the contacts will open more quickly (less arcing) but probably not necessary for your application. D2 is necessary because the Zener diode will conduct happily in the forward direction- it breaks down in the reverse direction. The circuit with D7 involves one less part, and is similar if there is a bypass cap nearby.