Electronic – Understanding of Relays and associated circuitry

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 are 2 devices that can generate spikes.

1. The relay coils. The diodes in anti-parallel will help suppress the spikes when the relays are turned off (The stored energy in the magnetic field is converted back into voltage/current causing a spike that may be inductively and capacitively coupled back in to the mains). Assuming these are DC relays, feed by DC this could be a good solution But may still need filtering.

2. The motor of the fan and the contacts of the relays. Here the motor can be switched on at any point in the mains cycle. If the relays close or open when the maximum current is being drawn you have a similar situation as turning off the relay coils. (The stored energy in the magnetic field is converted back into voltage/current causing a spike that is conductively and may be inductively and capacitively coupled back in to the mains) The solution is to switch at the zero point in the mains cycle. +1 @Brain Drummond comment for devices to search for that can use Zero Point Crossing ( zero crossing switch, or zero crossing SSR (Solid State Relay) )

Keeping the current loops as small as possible and moving cables away from noisy circuits can also help depending on how sensitive the audio equipment is.

After that it is inductors ,resistors and capacitors to construct filters to damp any spikes left.