Basic circuit but too rusted :-)

[Yoshimura]

Hi guys,

I want to make a very basic circuit but the last time I did electronic it was like 20 years ago. So I would really appreciate your help.

So this circuit monitors the coach battery tension in my RV and when voltage get too low, it sends a low pulse signal to the remote starter so the coach alternator (there are two alternators) can recharge the coach battery. The remote started automatically stop the engine after 30 minutes. Regarding the electric system of my RV, there are two 12V systems. One for the chassis and one for the coach and each has it own battery and alternator. Beside sharing the same ground, both systems are independent of each other. Finally, the Remote starter is powered by the chassis battery.

At the heart of the circuit, I will use a 8-pins microprocessor (IC2) from Microchip and use its internal ADC to monitor the coach battery voltage. The same IC will be used to generate the low pulse.

So here're a few questions...

- For the voltage divider (R1 / R2), resistors in the range of a few 10 thousand ohms so be good, right? 1% resistors?
- A low pass filter would be a good idea? 1 second for RC? Any capacitor type will be appropriate?
- For the output used to trigger the remote starter, I would like to protect it using diodes (D2, D3) and a resistor (R3). From memory, R3 should be 10k but I don't remember which diodes will be appropriate. Hopefully, I could find an old schematic I still have.

- For the voltage regulator, a LM7805 in a TO-222 package? The datasheet suggest a 0.22uF for C2 (which kind?) and 0.1-μF, ceramic disc for C3. Sounds good? For C1, I really don't remember how to size it. 1N40001 for D1?

- Finally, do I really need a condenser for the μC or C1 will be enough?

Many thanks!!!!

 

[Pommie]

Do you know what voltage the remote starter considers logic zero? If it's half voltage (6V) then the 5V from the pic chip may trigger it all the time - you may need an NPN transistor to convert 0-5V to 0-12V. The protection diodes (D2,D3) are built into the pic chip so not required. C4 probably isn't needed as the battery will act like a huge capacitor but may be needed if the battery drops when a large load is suddenly applied. Actually, C4 will also act to charge the ADCs holding capacitor quickly and therefore reduce the acquisition time. How's you coding? Will you be using asm of C?

Mike.

 

[gophert]

Well, a lot has happened in diy Electronics in the past 20-years.
You should look into Arduino, a microcontroller mounted to a small board, all the pins pulled out to some through-hole pins, an on-board voltage regulator and can be programmed directly with just a USB cable and an Arduino IDE (free software). You can get official Arduino boards for $20 or a knock-off made via open source plans for about $3.

take a google and let us know if you have any questions - if you want to use the PIC, that's fine too.

 

[gophert]

Also, a small transistor (2N3904) will help keep the voltage high until you need it low (microcontroller pin goes high to lower the collector voltage on transistor.

 

[Yoshimura]

Well, a lot has happened in diy Electronics in the past 20-years.

Yeah, this seems the perfect occasion to initiate me to the Arduino!!!! Thanks!

 

[Yoshimura]

Do you know what voltage the remote starter considers logic zero? If it's half voltage (6V) then the 5V from the pic chip may trigger it all the time - you may need an NPN transistor to convert 0-5V to 0-12V.

Yes, very good point!

I though using ASM but I might initiate me to the Arduino instead.

Thanks!