Actions
Forum Thread Details
  • Replies 2 replies
  • Subscribers 285 subscribers
  • Views 346 views
  • Users 0 members are here
Related

Car Alarm Tips

Former Member

Hello, I want to create my own car alarm from scratch. I know a D-Latch memory circuit using a SN7404 and a DM7400. However, there is a problem.. How can I reduce the voltage and current to not fry my D-latch circuit, or any other sensitive components I may use? I was thinking of using LM 317 as a voltage regulator and a current regulator. I did some research on my battery it is a 12V with a capacity of 78Ah. Im not sure if the 317 can even handle the current of the battery. Can anybody give me some tips?

  • Hi Thomas,

    Do not worry about the current capacity of the battery. Your circuit will only take the amount of current that it needs. The LM317 is a good linear regulator if you need a variable voltage but you would be better off using a LM7805 which is a fixed 5 volt linear regulator. The chips that you mention are rated for 7 volts maximum but are usually run at 5 volts.

     

    If you want to understand better how the voltage and current ratings work the water analogy is useful. Your battery is like the city water tower. It has water pressure which is similar to the voltage of the battery. It also has a very large volume of water inside it which is similar to the amperage capacity of the battery. Now even though there are 100,000 gallons of water potentially in the water tower you do not worry about all of it coming out in your sink when you open the faucet. You only get the amount that the resistance in the pipes will allow at the pressure that is present. The battery is the same way. It has a voltage of 12 volts and this means that we can use Ohms Law Current = Voltage divided by Current. Suppose that your proposed circuit including the 7805 regulator has a resistance of 24 Ohms. This means that the current that will flow will be 12 Volts / 24 Ohms which equals 0.5 Amps.

    John

  • Google 'car alarm circuits' to get some ideas.

     

    MK