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##Brief responses to your added questions:## For longer work, you'll have to aim more at new questions, this shouldn't become a complex state-full thread themed "Help Sjoerd finish his design from start to finish"

Brief responses to your added questions:

For longer work, you'll have to aim more at new questions, this shouldn't become a complex state-full thread themed "Help Sjoerd finish his design from start to finish"

##Brief responses to your added questions:## For longer work, you'll have to aim more at new questions, this shouldn't become a complex state-full thread themed "Help Sjoerd finish his design from start to finish"

1. The main reason I wouldn't use the 78-types is the high current it draws itself, it can be protected sufficiently by a TVS and a filter. It may be wise for a 24V design to find something a bit more in the direction of 40Vmax, to allow you vehicle voltage to fluctuate upto at least 34V before your protection starts clamping. 1b. Those TVS should work, but make sure you get their clamping voltage a ways beyond your maximum expected normal use voltage, but at least a few volts below the lowest Vmax of your electronics. Try to think about elements that limit the peak pulse to, or preferably way below, 40A. You can use a polyfuse, or the added filter-damping fuse for that.
2. It should be fine without opto's, if you have a nice noise source you can stress test it in-lab. A drill driving a medium-size DC motor with a low-voltage source in series might give you a nice worst-case test signal. Or it might not.
3. The transistors switch in a sense when you let them saturate. With 4.7k they should be able to pull much more than 70mA and still saturate, when the resistors are connected to a 5V signal. If you want more details there's a lot about that on this site.
4. You can add a small resistor as a sort of extra filter, with or without a ferrite bead for the high frequency stuff. But that will require extra capacitance. It is smart to add a 22nF to 100nF bypass capacitor to any digital chip, to supply switching currents locally. There's also a lot about this on this website. Adding a current-limiting resistor to an MCU is a very strange practise. Especially for 200mA on a Tiny45. If your Tiny45 is pulling an average current of 200mA it's broken and your relay can be discarded anyway. Digital chips, if not filtered, get connected directly, because "we" trust them to behave. Unless we know otherwise.

Input filtering and all that:
The capacitors and the inductor in that image get calculated to filter at the appropriate points without causing oscillations. This post is already too long to detail that, but again, there's a lot on filter design on this website already anyway. The reason I mention the C2 to C1 relationship is that if you don't stick to that, there's a risk you are building a boost converter using your alternator ripple, which might boost the voltage to 1.5x, 2x, or even more. This is not the likeliest thing to happen if you're experienced, but when starting out, it's best to stick to the rule for C1 and C2.

The diode is protection from ms-scale cut-outs and input voltage reversal. To note, as well, that if input reversal is a likely possibility, it's best to have all caps before the diode be ceramic or other bipolar types. For longevity.

For the caps you can over-do it, costing money and space and possibly creating high inrush currents, or possibly the opposite: High ESR in the caps, which would dampen the filter even more than the dampening resistor. If you move the dampening resistor before the TVS, when the TVS only protects your electronics, the filter will work better, you can then also look into pi-filtering, but you can also over-do filters quite easily. A simple L-C filter in the ballpark, a small safety resistor and a TVS should get you so close to Automotive spec that it'll probably do well without actual full testing for all but commercial production runs.

• The 78x05 type regulators are hardly ever specified for high-transients (supra-40V), nor are they guaranteed to perform correctly for negative input, while not as likely as it was in 1960's cars, it's not impossible and broken is broken.
• Similarly you want your opto-couplers protected from inversion by an extra, at least 100V capable diode. Nor do you want one switch to turn on another in vehicles, if you do it properly, so adding more 4148's to the other inputs seems best. (Your forward current at 30V seems to stay below limits, so that seems ok).
• Your MOSFETs are Connected the wrong way around. They show P-Type, but their source is connected to the low-side load. This means your lights are always on.
• How do your MOSFETs turn off (once you have flipped them)? There should be a pull-up to their gates, or your lamps will turn on and then maybe, once, in the distant future through leakage turn off.
• Be aware (when you start debugging) that your MOSFETs are connected directly to the dirty voltage, and though it's much less likely to be problematic, there's a chance when pull-up's are there some noise may filter back in. I've never had trouble with that myself with AVRs though, as their output stages are rock solid. If you do use an Opto-Coupler there, beware to calculated the drive currents and turn/on turn off times, you may still need an extra transistor there.
• Your base resistors for your NPNs can be a factor 10 larger, this will still switch exceptionally well, but it's not mandatory (though I'd do it).
• Without any extra capacitance that 22 Ohm on your Vcc will only make your MCU more susceptible to supply noise coupling in. Any switching current to its core will need to go through 22 Ohm + Board inductance without any nearby help.
• Not so much a problem, but a tip: Make sure you hook the power wire up to something that doesn't turn on in any setting you don't want it on in. Since you estimate a pretty appreciable steady-state current use by your relay. Some bikes (mine included) power the indicators by default in more settings than just the one where you're driving / running the engine.

This isn't Ideal, as your clamp is in a high-current path, and who knows. It'd have been better encapsulated in a somewhat safer frame with a limited current. But I wanted to also offer some protection to the down-stream LED lights, as they were read-made after-market back then, and those lights are teh poop with safety. Hence also the need to not dampen too much, for fear of having to swap in a 21W regular along the side of the road. These days almost everything on the bike is home made or maintained, so some tweaks would be possible, had I not encased it in military grade resin.

If you can get it so that C2 is still at least 100nF and C1 is at least 10 times that, or 25 times... That's where at my lab the scope and the bike meet, to see how well my guesstimation worked.

(Mind you, none of the above contains any maths other than quick top-of-head)

• The 78x05 type regulators are hardly ever specified for high-transients (supra-40V), nor are they guaranteed to perform correctly for negative input, while not as likely as it was in 1960's cars, it's not impossible and broken is broken.
• Similarly you want your opto-couplers protected from inversion by an extra, at least 100V capable diode. Nor do you want one switch to turn on another in vehicles, if you do it properly, so adding more 4148's to the other inputs seems best. (Your forward current at 30V seems to stay below limits, so that seems ok).
• Your MOSFETs are Connected the wrong way around. They show P-Type, but their source is connected to the low-side load. This means your lights are always on.
• How do your MOSFETs turn off (once you have flipped them)? There should be a pull-up to their gates, or your lamps will turn on and then maybe, once, in the distant future through leakage turn off.
• Be aware (when you start debugging) that your MOSFETs are connected directly to the dirty voltage, and though it's much less likely to be problematic, there's a chance when pull-up's are there some noise may filter back in. I've never had trouble with that myself with AVRs though, as their output stages are rock solid. If you do use an Opto-Coupler there, beware to calculate the drive currents and turn/on turn off times, you may still need an extra transistor there.
• Your base resistors for your NPNs can be a factor 10 larger, this will still switch exceptionally well, but it's not mandatory (though I'd do it - for the current consumption).
• Without any extra capacitance that 22 Ohm on your Vcc will only make your MCU more susceptible to supply noise that's coupling in. Any switching current to its core will need to go through 22 Ohm + Board inductance without any nearby help.
• Not so much a problem, but a tip: Make sure you hook the power wire up to something that doesn't turn on in any setting you don't want it on in. Since you estimate a pretty appreciable steady-state current use by your relay. Some bikes (mine included) power the indicators by default in more settings than just the one where you're driving / running the engine.

This isn't Ideal, as your clamp is in a high-current path, and who knows. It'd have been better encapsulated in a somewhat safer frame with a limited current. But I wanted to also offer some protection to the down-stream LED lights, as they were ready-made after-market back then, and those lights are teh poop with safety. Hence also the need to not dampen too much, for fear of having to swap in a 21W regular along the side of the road. These days almost everything on the bike is home made or maintained, so some tweaks would be possible, had I not encased it in military grade resin.

If you can get it so that C2 is still at least 100nF and C1 is at least 10 times that, or 25 times... That's where at my lab the scope and the bike meet, to see how well my guesstimation worked.

Of course, all parts between a positive voltage and ground in my design were chosen for low leakage as well, as in all my automotive/bike-o-motive designs.

(Mind you, none of the above contains any maths other than quick top-of-head)

And one more note, before the comments come in: Of course I should remove the point about the virtual magic ground, as with a true virtual ground the TVS wouldn't work, etc etc, but I wanted to show off a little for the hours of typing and putting up with the circuit editor. In reality the TVS is even more cleverly placed, but that'd give away some parts I signed away inside the dotted box.