Photon and LM7805


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?


Do you mean a 12v DC power supply or a 12 AC transformer? How much current is your “transformer” rated for?

The capacitors are used on the input and output of the LM7805 to smooth the power and prevent noise. These are recommended by the LM7805 manufacturer.

Sorry - I misspoke. It’s a 12v DC power supply.

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.

@rikonor, you can skip the caps but then you take a risk. Clean power requires capacitors so $10 is a good investment.

You still haven’t said how much current your 12 VDC supply is rated for!

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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.

You still haven’t said how much current your 12 VDC supply is rated for!

Sorry, it’s a simple adapter, rated for 2A (Similar to this).

Ok - that’s fair, I’ll order a few capacitors to make sure this stays healthy. I’ll probably try running it without first, just to check anyhow.

Thank you all.

@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.

@peekay123 Ah, ok. I did not expect actual damage to the Photon. I’ll just order the capacitors then (-:

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i found this after doing a search on oscillation [thanks to peekay123 for the reference to the term]