315Mhz remote receiver from Adafruit - Voltage?

Using Arduino Networking, Protocols, and Devices
#1

Please forgive in advance that I'm mostly a software dude and a tinkerer... Electronics aren't usually my thing.

I have these two paired products (and similar products are available elsewhere):

I am using this to lock and unlock 5 different vehicles, which also have a Unos for other reasons.

My problem is with providing an appropriate power supply.

I would like the key fob to power up the Uno upon unlocking doors, which means I'd like to supply the receiver from something other than the Uno at the "+5V" pin. Currently available I have a nicely regulated 9V supply from a small DC-DC converter (which otherwise becomes the power supply for the Uno). But for greater simplicity, I would prefer to use my vehicle's operating voltage. (Which I know isn't really 12V, but as much as 14.5 "noisy" volts.)

As far as I know there's no documentation for the receiver. Surely it has a voltage range, but what is it?

The Adafruit page only says it's "based on the PT2262", and the spec sheet for that IC says it can operate from 4-15 volts... BUT... I don't know if that applies to the whole board or just the IC.

AND, importantly... Even if it can tolerate more voltage, the outputs can't be more than 5V because I need it to provide a logical high/low direct to the Uno pins.

So, can this work? And if so, will the outputs still be no higher voltage than the Uno can take? (5V I believe.)

On a related question: All of these questions are moot if it is practical to keep my Uno powered constantly and use its own 5V supply. But how much current would the receiver unit plus the Uno draw? I'm thinking the vehicle battery is much less likely to die if only the receiver is powered. But if keeping the Uno always on requires negligible current, please tell me and that'll be my solution.

P.S. Recently I have been "challenged on my judgement" for using these units to lock vehicles: Yes, I am keeping the vehicles secure in other ways and I'm using one of the 6500 randomly-selected codes, matching remote and receiver codes.

#2

5 V regulated are what those receivers need.
The decoder chip will work up to 12V , but the rest of the receiver wont.

#3

Surely there's a range of tolerable voltage? Power supplies are rarely precise, after all.

#4

According to this schematic of the receiver, there are no parts that require 5V. It should work fine on 9V. However, check to make sure that you do not have the LMV358 version of the op amp, which does have a 5.5V maximum.

If operated at 9V, the receiver will output about 9V, which would damage the Arduino input. You would need a voltage divider or series resistor (10K) to protect the Arduino input.

#5

Thank you so much! Karma!

Unfortunately it does have an LM358 onboard. But at least I have answers. I have learned that the Uno plus other components will consume too much power and drain a car battery, so it's not appropriate to have it all constantly powered.

Maybe I can run relays from the outputs, or make a bare bones processor board so it'll digest fewer electrons.

Since I won't be powering it from an Uno, I'll have to at least find a good stable 5V power supply to run in a car, sufficient for this receiver plus a couple intermittent relays. Any recommendations?

#6

Whats determining the frequency that the receiver is receiving?
Its a super regen receiver.
What effect do you think the supply voltage has on the quench frequency?

#7

Unfortunately it does have an LM358 onboard

That is as expected. It should run on 9V, so try it and let us know.
It is the low voltage version of the op amp, the LMV358 that would be a problem.

Edit: I tried two running two different 434 MHz receiver modules on 9V and both worked fine. One is labeled “RXD1” (an older model that also has a 358 op amp) and a newer XY-MK-5V, but both clearly have circuits very similar to that in the link I posted above.

Below is a screenshot from Audacity, showing the receiver output (running on 9V) while one of my temperature sensors reports in.
9v_434.png

#8

Thanks a lot! Yes, voltage dividers sound like the way to go. (Cutting the 9V in half should do so all resistors can be the same.)

I've learned a couple things about how to approach this, which others may find helpful.

To summarize: The goal is to have an Uno and this receiver in a vehicle, where the Uno is NOT powered until a button is pushed on the remote, which also unlocks doors. As the vehicle sits overnight, the only things drawing current should be the receiver and a tiny 9V supply. The drain will be sufficiently low not to cause concern for draining a car battery, but it would be a problem if the Uno was powered overnight.

I understand someone "could" get away with running an Uno constantly in a car using low power modes, or switching to a barebones board, but the drain would be a bit high with a regular Uno, and I can't do it because the Uno also powers a display on the dashboard, serving another purpose.

Anyway, I realized I need to switch to a "latching" version of the receiver. I know that sounds wrong, but it's necessary as I'll explain. The receiver toggles a relay which both unlocks the doors and also delivers power to the Uno. The Uno can immediately "reset" the latched state the receiver by cutting its power momentarily through another relay where the circuit is normally closed. (This will occur after an experimentally-determined number of milliseconds, sufficient for doors to unlock.)

The Uno will also use another relay to "hold" its own power on from the 9V source, which it can release at any time using the normally open connection. (In this case, the Uno will cut its own power some period after all doors are closed and the key is turned off.)

The same relay which unlocks the doors also delivers power to the Uno, and because it is a latching style, it will hold it powered on until the Uno is fully powered up and running code, whereby it can hold its own power, then reset the latched state of the receiver by cycling its power.

WHY THE LATCHING TYPE IS NEEDED:
A momentary receiver won't work because someone may push the button for a very short time, not giving the Uno sufficient time to start running code to activate its own "hold power" relay. The result could be either that the Uno doesn't stay on... Or worse: It could start up and behave unpredictably as one relay delivering power may be switched off just microseconds before another relay delivering power is switched on.

I understand there are similar receivers which are "momentary", but always keep an output pin high for a minimum short time even if the button is already released. If I had that, and if the minimum time was sufficient, it might also work.

To Mauried: It seems the title of this thread may have created a misunderstanding. I had no questions regarding frequency. I only said "315Mhz" to distinguish it from an IR or other remote receiver.

#9