The circuit is very basic as the PIC (IC1) performs all the complex functions. RG1 provides 5volts for the IC with C1 and C2 providing stability for the regulator. C2 is a tantalum type and provides more than enough smoothing for the IC.
The relay is driven by Q1 through R3 to provide the extra current for the coil with D2 suppressing any EMF generated by it when it switches off.
The ignition input from the keyswitch in the car is buffered by Q2. This is needed because the PIC runs at 5v and the car ignition could be 12-13.5v. R4/5 switch the transistor on pulling the collector low and providing a low signal to pin 1 of the IC. This buffering is better than just a potential divider as it also provides some isolation and protection to the IC from possible hazardous spikes in the car wiring.
The buzzer provides audible confirmation of key presses and operation - it even plays tunes
- When ignition switch is turned on/off
- When the unit is enabled or disabled
- When SET mode entered or new code stored
- well why not? It's easy with a PIC. They were created using the MUSIC FACTORY software and then stored on the chip.
The unit starts up in disabled mode - the ignition +Out is disabled. The LED pulses RED.When the iginition is turned on and off, the unit plays a short tune to acknowledge it, but +Out will stay off.
TO ENABLE & DISABLE...
The code can be up to 15 digits. As the code is typed in the LED flashes faster, to show mid-entry. If there is a pause of longer than 4 seconds between presses, the unit aborts the entry and beeps.
Press # to accept the entry.
The correct entry will change the LED from RED to GREEN or vice versa and activates/deactivate the relay and +Out.
TO SET NEW CODE....
The unit must be enabled to change the code (Green). Press the * key and the LED turns red. Enter a new code up to 15 digits. Press * again to confirm, # to cancel. Any delay greater than 4 seconds aborts the change.
If the ignition is off and the unit still enabled (+Out on) for longer than approx 60 seconds, the unit will automatically switch to disable mode, switching off +Out.