Introduction: Back-up Alarm for Cars and Trucks
There seems to be a wealth of information on Instructables about how to use a 555 timer, but relatively little about using a LM386 as an oscillator. In this instructable I will use both ICs to build a back-up alarm for my SUV. This can also be great for any larger vehicle with visibility issues or hybrid/electric cars that are whisper quiet!
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Step 1: Gather Parts and Build!
To build the back-up alarm you will need:
- A speaker (I used a 3w 8ohm speaker salvaged from a tv)
- a PC board (I used a stripboard, but any will work)
- general purpose npn transistor like bc547 or 2n3904
- 2 - IC sockets
- .01uf ceramic capacitor
- .1uf, 1uf, 47uf, 100uf electrolytic capacitors, at least 16v
- 3.9k, 12k, 470r, 3.3k, 27k, 1.5k resistors
- solid and stranded wire
- spray paint, double sided mounting tape, and crimp splices (used later)
- Basic tools (soldering iron, solder, screw drivers, pliers, wire strippers, multimeter)
You may want to build this on a breadboard first to test it, then go ahead and solder it up following the schematic. I used the stranded wire for the connections that go from the board to power and ground, and to the speaker, solid wire for the jumpers on the board, and enamel-coated solid wire to secure the stranded wire to the board for strain relief.
The circuit will work on any voltage from 5 to 12vdc. If you plan on putting it on a vehicle with a 24v system, then add a voltage regulator like an lm7812 to the circuit.
Update: after about a week, the alarm stopped working. I took it out of the vehicle and tested it with a 9v battery. The 555 chip was heating up so i replaced it and the alarm started to work again. I added a bc547 npn transistor and a 470r resistor to the circuit. This prevents the large amount of current that the lm386 was drawing from frying the 555.
Also, the next time you need an oscillator, consider using an LM386. I followed the schematic from the lm386 datasheet and then did a little bit of trial and error with different capacitors and potentiometers before I found the combination that worked for the alarm.
Step 2: Spray Paint the PCB
Since this will ultimately be installed in my SUV, I did not want to put it in an enclosure, but I did want to protect it from shorting and moisture. I used some enamel spray paint I had laying around, and it seems to do the trick.
Step 3: Install in Your Vehicle
Remove one of the tail light assemblies that has a clear reverse lamp. Follow the wires from the reverse lamp and identify the positive and ground with your multimeter (I got lucky, the wires were identified). The ground will have continuity to the body/frame and the positive will have about 12v when the vehicle's transmission is in reverse and the key is in the ignition. after identifying the positive and ground, put the vehicle back into park/neutral. Use the crimp splices to connect the leads from the alarm to the reverse light wires. I find this method much easier than trying to solder inside of a vehicle. Test the alarm by shifting back into reverse. Use the double sided tape to mount the PCB and speaker inside the vehicle and reinstall the tail light assembly.
Enjoy your new back-up alarm!