sam512bb wrote:Great work! As a suggestion, you may wish to change out your linear regulator to a switcher, as this will reduce the heat dissipation quite dramatically... With the liner regulator you are dropping 13.8v - 5 v, or 8.8V which is a hefty amount. I would suggest you look at the LM2675 switcher IC (+ inductor, diode, etc) by National (TI), or the self contained modules by TI (i.e. PT5101 series), etc.
Secondly, you may wish to place a larger cap (47uf or 100uf) on your 5V rail in order to provide some localized charge for the LEDs... 0.1 uf are pretty small and really meant to reduce switching noise, etc.
Happy Holidays and thank you for your ideas. Here are my thoughts:
The 7805 is dissipating 0.15(13.8-5) = 1.32 watts. Turns out this is well within the device's dissipation capability without a heat sink, and there is actually a small heat sink on the PC board. I ran it overnight sealed in the plastic case with 15V on the input side and it was fine in the morning. The choice of the 7805 in this project was predicated on simplicity and low cost. Thereanks was no requirement for efficiency (1.32 watts is lost in the noise in a motor vehicle). And even if heat were to become a problem, a $0.05 1-watt resistor of 10-20 ohms in series with the input power leads will safely move some of the power dissipation out of the 7805 leading to an even wider heat margin than currently exists.
The second change you suggest seems like it may any have some potential for bumping up the pulsed output power a bit. I will put my scope on the 5V rail and switch it on to see if there is any drooping occurring. Certainly the bandwidth of the 7805 is not sufficient to correct any IR droop caused when the IR LEDs are active so if this is more than 50mV or so, I'd consider it a worthwhile change. I believe a precise needed value of charge can be calculated, leading to a specific minimum value of capacitor, but the values you suggest seem reasonable at first glance.