I’m building a circuit where a Photon controls a relay which powers a Siren.
The siren consumes about 500mA and uses a 12V input voltage.
I have a transformer which outputs 12V, which I hook up to the relay as well as to a LM7805. The output of the LM7805 goes to power the Photon.
My question: Is this a reasonable setup? Also, I often see people using a few coupling capacitors with an LM7805. But I don’t have any capacitors at the moment. Do people find that they are absolutely necessary for this kind of application?
I am just wondering if someone has used an LM7805 similarly and found the experience with no capacitors to be fine.
I am worried I will get brownouts and other weird phenomena because of not including them.
The obvious solution to this would be to just include the capacitors but I’m trying to avoid spending $10 on a package of 50 capacitors or something similar.
I wouldn’t skip capacitors either. I can think of all kinds of problems that could result, and most would be worth more than $10 whether in time or in damaged hardware. You should be able to find smaller quantities. Sparkfun, adafruit, or even eBay are good sourced.
By a 12Vdc supply do you mean a package PSU (Live and Neutral AC inputs and +12VDC and GND outputs in an enclosed box)?
I am using a Myrra (47154) 12V 5A AC/DC Converter for a device where a very smooth and stable 5V reference is required alongside a 12Vdc supply to switch relays. This was professionally designed and provides an ultra smooth supply using a few capacitors and inductors - you will not need this just to power the photon which has its own VReg on board.
A LM7805 voltage regulator is used for this circuit - you can find many reference circuit designs online. Try 1uF 50V across the +12VDC and GND and 10uF/50V across the 5V and GND of the voltage regulator.
@rikonor, with a (cheap) switching supply as the source for the 7805, the MORE reason to use capacitors! The classic failure on a 7805 without capacitors is oscillation. Google it before you potentially sacrifice your Photon.