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Need help on some basics

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verbatim68

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First of all, I'm an electrical engineer that has spent the last 15 years working as a mechanical, so although I'm sure I know a lot of these things in the back of my brain someplace, or could look them up in my old textbooks, I'm also chronically lazy as evidenced by the fact that I pretty much slept through all my classes anyways. Hence the reason I'm here asking some very simple question.

First a description of the circuit I'm trying to build (haven't actually even started yet). I want to make my motorcycle keyless. I have found an RFID system that works by firing a supplied 12v relay when the rfid tag comes in range. They sell it specifically for motorcycles, but I want to add a logic circut rather than just running it in series with the kill switch for a number of reasons that I don't want to bore people here with. Essentially, I want to make it so that the ignition doesn't come on until a certain sequence of events happens. The same goes for shutting the ignition off.

I've sketched out my circuit using the required logic gates and a couple of relays and it's pretty simple, but I think it should do the job. I imagine I'll convert it to a nand gate arrangement.

Okay, now here are my questions:

1. If I use CMOS chips, can I hook up the power directly from the motorcycle battery, or do I have to add a regulating and/or smoothing portion prior to the chips to prevent supply voltage variations from causing problems? If so, what would it look like? Just a 1000 uF capacitor with a 7812 chip and then a couple more smaller capacitors? I've seen that one a few times in my brief searches.

2. I basically have two inputs, one from the RFID and the other from the Kill switch. If the answer to question one is that I need something before the chips, I assume that I will also need one for the kill switch input?

3. I plan on using the RFID input to also power the board, so that once the tag comes into range, the board powers up. I then use a gate in the circuit itself to maintain the power to the board if the RFID shuts off due to inactivity. Is it ok to power the board through a gate like this or do I need a transistor added after the gate?

4. I'm also using this board to control the fuel cutoff and ignition cutoff relays. Will it supply enough power to the relays or do I need to add a transistor or two? Should I just measure the current required and go from there?

I imagine a sketch of the circuit would help immensely, but I just signed up here and haven't got a clue how to go about posting pictures, and mine is just a sketch on a piece of paper.

thanks in advance for all replies.
 
Welcome to the forum.

Automotive supplies are very noisy. Motorcycle systems are the worst of all. The 7812 is not a good way of running things on as it has a drop-out of about 2 - 3 volts so you can expect about 9 - 10 volts with the engine stopped and about 11 - 12 volts with the engine running. It isn't rated for the big surges and spikes that you get, nor is it proof against battery reversal.

Do you have a starter motor on the bike? If so, you need to be able to keep the electrics running at about 6 V. It really isn't at all easy.

You may want to run the electronics at 5V, and use low drop-out regulators with maximum input voltage 15 V or more. You should protect the inputs of your regulator with a zenner regulator and resistor that will give you about 1V drop at normal current. The zenner voltage should be less than the maximum input voltage of the regulator. That will protect you from spikes and surges. A series diode is always a good idea if to protect against battery reversal.

I guess that you want the circuitry to run all the time so that you don't have a key. The RFID device may take too much current for that to work, but you could have a button to press to turn on the RFID.

If you make all of the circuitry in HCMOS or CMOS, it can take very little current, maybe just a few µA. If you do that, you can just use a zenner regulator on its own, and a 1000 µF will keep the circuit running for plenty of time to ride over engine starting.

The input from the kill switch can just be a resistor and a zenner. As you only need nanoamps to switch a CMOS input you can have a high value resistor.

You can only get 10 - 20 mA out of a CMOS or HCMOS gate so if you need any more power than that, you need to add a transistor.

Relays take about 50 - 100 mA so you always need a transistor. Remember a freewheel diode in parallel with the relay coil or you will blow the transistor when it turns off.

I hope this helps.

There is a sticky post about adding schematics to posts at
 
Thanks, yes that does answer a lot of my questions...but I'll keep asking more until you're tired of me if that's ok....

I understand what you're saying and you're right, I believe it will be a better option to run the circuit at 5v. Yes, I do have a starter on the bike. Okay, so say I use a 7805 regulator instead, what is the max voltage rating of one of those? If it's up around 35v, then do I need the zener and resistor to protect it from over voltage? I suppose it wouldn't hurt, just trying to keep this as compact as possible.

The kill switch input idea is excellent...now why didn't I think of that? Oh, right I was sleeping in class.

I don't plan to run all the circuitry all the time. The RFID device works such that when you are in range with the key tag, it fires a 12v relay that supplies plenty of power...enough to start the motorcycle if you want, it's actually designed to be a direct replacement for the starter relay on the bike. However, I'm planning on taking that power and using it to power the add on circuit I'm making so that it only powers up when the tag is in range. However, the tag is battery powered as well and after a period of inactivity, it shuts down, so I have to figure out a way to keep the circuit going even after the RFID shuts down due to inactivity, hence the transisitor circuit on one of the outputs, which will become active once the RFID and kill switch are both on at the same time. Then, I'll be using another switch (actually starter switch) to power off the ignition and the board after I hit the kill switch to shut the bike down.

I did plan on protecting the circuit using freewheel diodes across the relays.

Maybe I'll try to refine the circuit and post it here for review. Can anyone tell me a good free program out there for creating circuit diagrams? I don't think anyone will appreciate my hand sketches.
 
I understand what you're saying and you're right, I believe it will be a better option to run the circuit at 5v. Yes, I do have a starter on the bike. Okay, so say I use a 7805 regulator instead, what is the max voltage rating of one of those? If it's up around 35v, then do I need the zener and resistor to protect it from over voltage? I suppose it wouldn't hurt, just trying to keep this as compact as possible.
I always think that a zener wouldn't hurt. I like to protect against load dump if I can. The 7805 is not a very good regulator. It has quite a high drop-out voltage and quiescent current.


I don't plan to run all the circuitry all the time. The RFID device works such that when you are in range with the key tag, it fires a 12v relay that supplies plenty of power...enough to start the motorcycle if you want, it's actually designed to be a direct replacement for the starter relay on the bike.
That sounds like the RFID reader is running all the time. Have you checked what current it takes?
 
I always think that a zener wouldn't hurt. I like to protect against load dump if I can. The 7805 is not a very good regulator. It has quite a high drop-out voltage and quiescent current.

I've read that too. I've also heard of using two regulators starting with say a 7810 to get a little more stable first and then going to the 7805. This would help with overheating of the 7805, no? I'm trying to keep my components down too, because ultimately this has to be small enough to pack away somewhere on the bike out of sight. Space is limited on motorcycles. If I were to heat sink the 7805, would that be sufficient to run it by itself? The zener isn't a bad idea either...I doubt it will ever get higher than the input rating of the 7805, but I guess it doesn't hurt. Do I need a resistor with it or just shunt it to ground?

That sounds like the RFID reader is running all the time. Have you checked what current it takes?

Yes, it does run all the time. No I haven't checked what current it takes. It's being sold for this exact purpose, just not quite as sophisticated as I'm trying to make it, so I'm hoping and assuming that it doesn't take much. I guess I'll find out.
 
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