Project Summary
The CITROEN Saxo Vehicle Touch Sensitive switches For operating Hazards and two peripheral devices If you want to change the look of your dash board inside a vehicle, maybe just fancy some of modern electronics This could be the design for you
Project Description
Faced with a challenge last year a young lad came and spoke to me re his vehicle
Of course the challenge was obviously to fit a car stereo and car amplifier but also to illuminate the rear shelf which held his speakers create some under car vehicle lighting , different colors of course and do away with the Hazard light switch such that he had touch sensitive switches to operate Relays
The task had to achieve the following
A: Switch the hazards lights on
B: use switches to ether switch under car lighting on in the form of LED’s and operate the rear Led’s independently
C: If the hazards were on they would flash at regular intervals keeping the front led’s on or the rear led’s on or both
D: Single touch to turn on visa versa to turn off
Nice easy Microchip to develop small applications for anybody that might be interested a good start to Micro Processors, wanting to learn how to handle interrupts and deal with sensing switches as well as Timer interrupts
The circuits comprises A power supply derived from the 13.8 volt Battery of the vehicle A cpu which uses port A as inputs which rely purely on the fact that our body potential is higher thanthat of ground potential i.e Zero volts combined with fet characteristics of PORTA
An output side comprising PORTB which is connected to relay’s 1 to 4 via npn transistors , pulling the relay’s in
Protection to the input i.e PORT A is set to 5V6 this ensures that any static voltages are suppressed and protects PORT A from spikes D1 to D4, D11
Ignition sense is sensed on Porta RA4.
This ensures that we cant turn on the relays except the hazard lights which must be operable even when the car ignition is off
The relay’s simply short out the links on the back of the hazard switch which has been removed
See Diagram in PDF File for explanation
D9 to D13 provide back EMF protection for the transistors Q1 to Q 4 of the relay drive circuitry
R1 to R4 , R17 ensure that PORTA is held at Zero potential when switches are not being touched
D20 ensures reverse polarity protection as does D23 on the 5volt regulator for the cpu which comprises Q5 , R13, D18
D14 to D17 provide luminous indication that a switch has been activated, current limited through the led’s via R9,R10,R11,R12
D5 to D8 ensure that should the transistors break down that we don’t end up destroying PORTB in the case of Collector, Base short circuit fault symptoms
R5 to R8 ensure that we limit the base current to Q1 to Q4
D21 and R15 ensure only 5 volts to PORTA, RA4, which forms the ignition sense line
Finally D24 flashes in accordance with the normal hazard light indicator
Explanation of interrupts and code
An interrupt is an unscheduled input, which interrupts the normal flow of a program, causing it to temporarily suspend what it is doing and perform a specified sequence of instructions required by the interrupting agent before returning to its original point in the program
For more detail: CITROEN Saxo Vehicle Touch Sensitive switches using PIC16F84A
